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This work addresses the study of the high-temperature phase sequence of ${\mathrm{Bi}}_{0.7}{\mathrm{La}}_{0.3}{\mathrm{FeO}}_{3}$ by undertaking temperature-dependent high-resolution neutron powder diffraction (NPD) and Raman spectroscopy measurements. A determination of lattice parameters, phase fractions, and modulation wave vector was performed by Pawley refinement of the NPD data. The analysis revealed that ${\mathrm{Bi}}_{0.7}{\mathrm{La}}_{0.3}{\mathrm{FeO}}_{3}$ exhibits an incommensurate modulated orthorhombic $Pn{2}_{1}a(00\ensuremath{\gamma})000$ structure at room temperature, with a weak ferromagnetic behavior, likely arising from a canted antiferromagnetic ordering. Above ${T}_{1}=543\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the low-temperature modulated $Pn{2}_{1}a(00\ensuremath{\gamma})000$ evolves monotonically into a fractionally growing Pnma structure up to ${T}_{\mathrm{N}}=663\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. At 663 K, the low-temperature canted antiferromagnetic phase is suppressed concurrently with the switching of the former into a nonmodulated $Pn{2}_{1}a$ structure that continues to coexist with the Pnma one, until the latter is expected to reach the 100% fraction of the sample volume at high temperatures above 733 K. The $Pn{2}_{1}a$ space group is obtained from the Pnma one through the ${\mathrm{\ensuremath{\Gamma}}}_{4}^{\ensuremath{-}}$ polar distortion. Neutron diffraction and Raman spectroscopy results provide evidence for the emergence of noteworthy linear spin-phonon coupling. In this regard, magnetostructural coupling is observed below ${T}_{\mathrm{N}}$, revealed by the relation between the weak ferromagnetism of the canted iron spins and the ${\mathrm{FeO}}_{6}$ octahedra symmetric stretching mode. The correlation between magnetization and structural results from NPD provides definite evidence for the magnetic origin of the structural modulation. The analysis of the temperature-dependent magnetization and the magnetic peak intensity as well yields a critical exponent (\ensuremath{\beta}) value of 0.38. The lower limit of the phase coexistence temperature ${T}_{1}=543\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, marking the emergence of the Pnma phase, is also associated with the temperature whereupon the modulation magnitude starts to decrease.

A spark-plasma sintered ${\mathrm{GdCrO}}_{3}$ (SPS-GCO) is found to stabilize in its ferroelectric phase beyond room temperature. The intrinsic nature of this room-temperature ferroelectricity is established using ferroelectric positive-up--negative-down measurements and supported through piezoresponce force microscopy measurements. The SPS-GCO undergoes antiferromagnetic ordering at much lower temperatures, only below $\ensuremath{\approx}170\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Thus, any role of magnetism to the observed room temperature ferroelectricity in SPS-GCO can be ruled out. This is contrast to the concomitant antiferromagnetic and ferroelectric ordering observed below $\ensuremath{\approx}170\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ in ${\mathrm{GdCrO}}_{3}$ (GCO) (prepared using standard solid-state synthesis technique). Using detailed Rietveld refinements of room-temperature x-ray diffraction patterns, SPS-GCO is found to stabilize in the noncentrosymmetric orthorhombic $Pna{2}_{1}$ space group (the reported low-temperature ferroelectric phase in GCO), while GCO stabilizes in the centrosymmetric $Pbnm$ space group at room temperature. Using first-principles calculations, we investigated the relative energies among various possible structures of ${\mathrm{GdCrO}}_{3}$ and found that the orthorhombic $Pna{2}_{1}$ and $Pbnm$ space groups are the most stable structures. The ferroelectric $Pna{2}_{1}$ phase of SPS-GCO (stabilized at room temperature using the high-pressure and high-temperature spark-plasma sintering process) undergoes transition to the paraelectric centrosymmetric phase upon heating beyond $\ensuremath{\approx}450\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ (as confirmed using dielectric and calorimetric measurements), which on subsequent cooling to room temperature does not undergo a transition back to the ferroelectric phase and remains in the centrosymmetric $Pbnm$ phase.

Structural domains in ferroic materials can be manipulated to achieve novel properties and functionalities of devices with multiple tunabilities. Herein, we report a study of tunable domain structures in ferroelastic ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_{3}$ (LSMO) films via interfacial engineering. Distinct domain structures are formed in rhombohedral LSMO films depending on the crystallographic orientations of the orthorhombic ${\mathrm{NdGaO}}_{3}$ (NGO) substrates. A unidirectional lattice modulation is observed in LSMO films grown on $\mathrm{NGO}{(110)}_{\mathrm{Or}}$ substrates, whereas a unidirectional twinned-domain structure is generated by $\mathrm{NGO}{(001)}_{\mathrm{Or}}$ substrates (where Or in the subscript denotes orthorhombic notation). The orientation-dependent domain structures in LSMO films are controlled by anisotropic strain as well as interfacial oxygen octahedral coupling at the heterointerface between LSMO and NGO. The orbital occupancy and in-plane magnetic anisotropy are markedly affected by strain relief induced by the unidirectional structural domains in LSMO films. In addition, the structural results observed in LSMO/$\mathrm{NGO}{(001)}_{\mathrm{Or}}$ films further demonstrate that anisotropic strain is capable to disturb octahedral connectivity in epitaxial films. Our findings provide future directions to understand and control the domain structures in ferroelastic heterostructures of perovskite materials and open a pathway towards tailoring the desirable physical properties.

The representative cuprate ${\mathrm{La}}_{2\ensuremath{-}x}{M}_{x}{\mathrm{CuO}}_{4}$ with $M=\mathrm{Sr}$ and $x=1/8$ is studied via first-principles calculations in the high-temperature tetragonal (HTT), low-temperature orthorhombic (LTO), and low-temperature less-orthorhombic (LTLO) structures. By suppressing the magnetism and superconductivity, the LTLO phase, which has rarely been observed in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$, is found to be the ground state where the structural phase transitions $\mathrm{HTT}\ensuremath{\rightarrow}\mathrm{LTO}\ensuremath{\rightarrow}\mathrm{LTLO}$ can be understood via phonon instability. Although the La-O composition is identified to be responsible for the phonon softening, the superconducting ${\mathrm{CuO}}_{2}$ layer is dynamically stable. The LTLO phase, which can exhibit an $\ensuremath{\sim}20\text{\ensuremath{-}}\mathrm{meV}$ splitting in the density of states, is proposed to have an intimate relationship with the observed pseudogap and the charge-density wave giving the stripe. We argue that at low temperatures, the superconducting LTO ${\mathrm{La}}_{1.875}{\mathrm{Sr}}_{0.125}{\mathrm{CuO}}_{4}$ competes with the phonon-preferred LTLO phase by spontaneously forming the Cooper pairs, resulting in suppressing the stripe. Therefore, the revealed LTLO phase is indispensable for understanding ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$.

Among the transition-metal oxides, bismuth nickelate is particular in that, under normal conditions, it crystallizes in a triclinic structure with charge ordering in the Bi sites. Upon increasing pressure, a phase transition takes the material to the $\mathrm{Gd}\mathrm{Fe}{\mathrm{O}}_{3}$-type orthorhombic phase commonly found in perovskite oxides; recent attention to $\mathrm{Bi}\mathrm{Ni}{\mathrm{O}}_{3}$ arises, in part, because this transition can be engineered to be accompanied by a colossal negative thermal expansion. On the other hand, the closely related rare-earth nickelates also show the orthorhombic phase, but as temperature decreases they undergo a metal-insulator transition to a monoclinic phase where charge ordering appears instead in the Ni sites. Since bismuth is similar in valence and size to some of the rare-earth atoms, it has been conjectured that a $\mathrm{Bi}\mathrm{Ni}{\mathrm{O}}_{3}$ phase with charge ordering in the Ni sites might also exist---this would make this perovskite unique in that it can harbor charge ordering in either set of cation species, while the other set contains identical nominal ionic charges. In this paper we show that such a phase indeed corresponds to a special point of the energy surface of $\mathrm{Bi}\mathrm{Ni}{\mathrm{O}}_{3}$ when studied with methods based on density-functional theory. The existence of this phase could explain the somewhat conflicting experimental reports regarding the valence state of the Ni ions in the high-pressure phase of $\mathrm{Bi}\mathrm{Ni}{\mathrm{O}}_{3}$.

Spin reorientation and magnetization reversal are two important features of the rare-earth orthorhombic perovskites ($RM{\mathrm{O}}_{3}$) that have attracted a lot of attention, though their exact microscopic origin has eluded researchers. Here, using density functional theory and classical atomistic spin dynamics we build a general Heisenberg magnetic model that allows to explore the whole phase diagram of the chromite and ferrite compounds and to scrutinize the microscopic mechanism responsible for spin reorientations and magnetization reversals. We show that the occurrence of a magnetization reversal transition depends on the relative strength and sign of two interactions between rare-earth and transition-metal atoms: superexchange and Dzyaloshinskii-Moriya. We also conclude that the presence of a smooth spin reorientation transition between the so-called ${\mathrm{\ensuremath{\Gamma}}}_{4}$ and the ${\mathrm{\ensuremath{\Gamma}}}_{2}$ phases through a coexisting region, and the temperature range in which it occurs, depends on subtle balance of metal-metal (superexchange and Dzyaloshinskii-Moriya) and metal--rare-earth (Dzyaloshinsky-Moriya) couplings. In particular, we show that the intermediate coexistence region occurs because the spin sublattices rotate at different rates.

Recently, theoretical work predicted that the cobalt mono-antimonide (CoSb) with a NiAs-type structure could be stabilized in the form of a monolayer PbO-type FeSe structure on a $\mathrm{SrTi}{\mathrm{O}}_{3}$ substrate [Ding et al., Phys. Rev. Lett. 124, 027002 (2020)], which may host high-temperature superconductivity comparable with monolayer FeSe [Wang et al., Chin. Phys. Lett. 29, 037402 (2012)]. Motivated to explore the possible pressure-induced superconductivity in bulk CoSb associated with structural instability, utilizing the diamond anvil cell technique, we herein report a comprehensive study of the electrical transport, crystalline structure, and electronic band structure of the bulk CoSb with electrical conductivity, synchrotron x-ray diffraction, and first-principles calculations. No pressure-induced superconductivity is detected down to 2 K with pressure up to 42.0 GPa, but a structural phase transition occurs between 11.7 and 16.2 GPa. Instead, an electronic transition, featured by a carrier-type switch from an electron $l10\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ to a hole ($n\ensuremath{\rightarrow}p$) dominant one at higher pressure, is observed. The $n\text{\ensuremath{-}}p$ switch at room temperature is ascribed to a pressure-induced structural phase transition from hexagonal ($P{6}_{3}/mmc$, No. 194) to orthorhombic ($Pnma$, No. 62), which is confirmed by a combination of first-principles calculations and structural refinement. Interestingly, the temperature-driven $p\text{\ensuremath{-}}n$ switch also occurs in the orthorhombic phase at \ensuremath{\sim}90 K, emphasizing the tunable multiband structure (with charge neutrality point) of the semimetal CoSb by pressure or temperature. Moreover, the electronic transition is assigned as a semimetal-semimetal transition. The resistivity shows a robust ${T}^{3}$ dependence trend $l30\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for both semimetal phases. At high temperature, all the $\ensuremath{\rho}(T)$ curves become saturated, as described by the parallel-resistor model. The results support that the $s\text{\ensuremath{-}}d$ interband scattering dominates the conductivity of both the hexagonal and orthorhombic phases under pressure. The superconductivity absence implications are discussed, assuming the electron-phonon interaction in CoSb drives the superconductivity.

The ${\mathrm{Pr}}_{0.50}{\mathrm{Sr}}_{0.50}{\mathrm{CoO}}_{3}$ perovskite exhibits unique magnetostructural properties among the rest of ferromagnetic (FM)/metallic $L{n}_{0.50}{\mathrm{Sr}}_{0.50}{\mathrm{CoO}}_{3}$ compounds. The sudden orthorhombic-tetragonal $(Imma\ensuremath{\rightarrow}I4/mcm)$ structural transition produces an unusual magnetic behavior vs temperature and external magnetic fields. The symmetry change is responsible for a spontaneous spin rotation in this metallic oxide. We have studied half-doped $L{n}_{0.50}{({\mathrm{Sr}}_{1\ensuremath{-}x}{A}_{x})}_{0.50}{\mathrm{CoO}}_{3}$ cobaltites varying the ionic radius ${r}_{A}$ of A-site cations (divalent cations and lanthanides) to complete the T-${r}_{A}$ phase diagram. The influence of the structural distortion and the A-cation size for the occurrence of a spontaneous spin reorientation in the metallic state has been investigated. As the magnetization reorientation is driven by the temperature-induced collapse of the orthorhombic distortion, a careful investigation of the structural symmetry is presented, varying the structural distortion of the Sr-rich half-doped cobaltites by means of both compositional and temperature changes. The region in the phase diagram of these FM/metallic cobaltites where $F{m}^{\ensuremath{'}}{m}^{\ensuremath{'}}m$ magnetic symmetry replaces $I{m}^{\ensuremath{'}}{m}^{\ensuremath{'}}a$ was determined. In that region, the magnetization direction has rotated 45 \ifmmode^\circ\else\textdegree\fi{} within the $a\text{\ensuremath{-}}b$ plane from the former to the latter.

We report a combined experimental and theoretical study of the ${\mathrm{PdSe}}_{2\ensuremath{-}x}{\mathrm{Te}}_{x}$ system. With increasing Te fraction, structural evolutions, first from an orthorhombic phase (space group Pbca) to a monoclinic phase (space group $C2/c)$ and then to a trigonal phase (space group $P\text{\ensuremath{-}}3m1)$, are observed accompanied with clearly distinct electrical transport behavior. The new monoclinic phase $(C2/c)$ belongs to the very rare Verbeekite polymorphism and is discovered within a narrow range of Te composition $(0.3\ensuremath{\le}x\ensuremath{\le}0.8)$. Electronic calculations and detailed transport analysis of the Verbeekite polymorphic ${\mathrm{PdSe}}_{1.3}{\mathrm{Te}}_{0.7}$ phase are presented. In the trigonal phase region, superconductivity with enhanced critical temperature is also observed within a narrow range of Te content $(1.0\ensuremath{\le}x\ensuremath{\le}1.2)$. The rich phase diagram, new polymorphic structure, and abnormally enhanced superconductivity could further stimulate interest to explore new types of polymorphs and investigate their transport and electronic properties in the family of transition metal dichalcogenides, which are of significant interest.

We report a pervasive phenomenon of gradual emergence of negative magnetization in typical ${R}_{1\ensuremath{-}x}{D}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ orthorhombic perovskite manganites in nanoform (with ${R}^{3+}$ being a trivalent high-magnetic-moment ion from the heavier rare-earth elements Gd, Tb, Dy and ${D}^{2+}$ refers to divalent alkaline elements Sr, Ca, etc.), while the bulk form manifests no negative magnetization. Extensive magnetization studies have been carried out in ${\mathrm{Gd}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ around half doping. We demonstrate experimentally that particle size reduction in nano-form manganites enhances the propensity of the system to exhibit negative magnetization and propose a theoretical explanation for the phenomenon. To test the universality of our findings, we have extended the measurement to ${\mathrm{Dy}}_{0.6}{\mathrm{Ca}}_{0.4}\mathrm{Mn}{\mathrm{O}}_{3}$ and obtained similar results.

Polarization rotation is considered one of the most important intrinsic origins of high piezoelectricity in perovskite ferroelectrics. Such rotation strongly increases the lateral polarization, thus giving rise to the ultrahigh-shear piezoelectric coefficient. The evolution of polarization vector as a function of shear strain in a lead-free perovskite ferroelectric, ${\mathrm{K}}_{0.5}{\mathrm{Na}}_{0.5}{\mathrm{NbO}}_{3}$ (KNN), is demonstrated using the first-principle calculations. The polarization rotation is found to correlate directly with the shear piezoelectric constant. The larger the rotation angle, the higher the shear piezoelectric constant. The atomic insight reveals the polarization rotation mainly relies on the reorientation of Nb and O atoms along low potential-energy surfaces. We propose the polarization vector is more likely to rotate along the ${[111]}_{\mathrm{PC}}\ensuremath{\rightarrow}{[101]}_{\mathrm{PC}}\ensuremath{\rightarrow}{[001]}_{\mathrm{PC}}$ path instead of direct ${[111]}_{\mathrm{PC}}\ensuremath{\rightarrow}{[001]}_{\mathrm{PC}}$ transition in high-performance lead-free piezoelectric materials with broad thermotropic phase boundaries; thus, the critical role of orthorhombic structure should be emphasized. We hope that this work will inspire future experimental studies for further developing lead-free piezoelectric materials.

Here, we report the possible origin of natural and magnetic field induced polar order near room temperature (RT) in $\mathrm{Pr}{\mathrm{Fe}}_{1/2}{\mathrm{Cr}}_{1/2}{\mathrm{O}}_{3}$. The temperature-dependent Raman spectroscopy (RS) and synchrotron x-ray diffraction experiments disregard any change in the structure at low temperatures, which is not compatible with the emergence of natural ferroelectricity in this sample. Further, inelastic neutron scattering, neutron Compton scattering, neutron resonance transmission analysis and magnetic field-dependent RS experiments, suggest an increase in the delocalization of ${\mathrm{Pr}}^{3+}$ ions in $\mathrm{Pr}{\mathrm{Fe}}_{1/2}{\mathrm{Cr}}_{1/2}{\mathrm{O}}_{3}$ with respect to the parent compound $\mathrm{PrFe}{\mathrm{O}}_{3}$. The evaluation of the present results reveals that more delocalized ${\mathrm{Pr}}^{3+}$ ions and strong spin-phonon coupling in $\mathrm{Pr}{\mathrm{Fe}}_{1/2}{\mathrm{Cr}}_{1/2}{\mathrm{O}}_{3}$ seem to be a driving force for the observed natural and magnetic field induced switchable polar order in $\mathrm{Pr}{\mathrm{Fe}}_{1/2}{\mathrm{Cr}}_{1/2}{\mathrm{O}}_{3}$. The present results provide a significant contribution in understanding the ferroelectric polarization and progress in the search for RT magnetodielectric materials.

We present an extensive powder and single-crystal neutron scattering investigation of the crystallographic structure and magnetic order of the frustrated metallic $f$-electron magnet $\mathrm{U}{\mathrm{Ni}}_{4}\mathrm{B}$. We carry out a full refinement of the crystallographic structure and conclude that the low-temperature lattice symmetry is orthorhombic [space group Pmm2; cell parameters: $a=6.963(4)$, $b=14.793(9)$, $c=17.126(8)$ \AA{}]. We determine the magnetically ordered structure, concluding that below ${T}_{\mathrm{N}}=19.5\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, the material undergoes a transition into a partially ordered antiferromagnetic state. The magnetic structure is consistent with the existence of toroidal order in this material. We further test the proposal of a second magnetic transition occurring at $330\phantom{\rule{0.28em}{0ex}}\mathrm{mK}$, concluding that the thermodynamic anomalies observed at these temperatures do not reflect modifications of the magnetic structure. Our study provides a consistent picture of the interrelationship of structural and magnetic properties in the frustrated magnet $\mathrm{U}{\mathrm{Ni}}_{4}\mathrm{B}$ previously unresolved.

In this work we revisit the phase diagram of Co-doped LaFeAsO using single crystals and thermodynamic methods. From magnetic susceptibility studies, we track the doping evolution of the antiferromagnetic phase to reveal a continuous suppression of ${T}_{\mathrm{N}}$ up to 5% Co doping. To study the evolution of the so-called nematic phase, the temperature dependence of the length changes along the $a$ and $b$ orthorhombic directions, $\mathrm{\ensuremath{\Delta}}L/{L}_{0}$, was determined by high-resolution capacitance dilatometry. The results clearly show a gradual reduction of the orthorhombic distortion $\ensuremath{\delta}$ and of ${T}_{\mathrm{S}}$ with increasing Co content up to 4.5%, while it is completely suppressed for 7.5% Co. Bulk superconductivity was found in a small doping region around 6% Co content, while both ${T}_{\mathrm{c}}$ and the superconducting volume fraction rapidly drop in the neighboring doping regime. Ultimately, no microscopic coexistence between the superconducting and magnetic phases can be assessed within our resolution limit, in sharp contrast with other iron-pnictide families, e.g., electron- and hole-doped ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$.

We report the magnetic ordering and structural distortion in PrFeAsO crystals, the basis compound for one of the oxypnictide superconductors, using high-resolution x-ray diffraction, neutron diffraction, and x-ray resonant magnetic scattering (XRMS). We find the structural tetragonal-to-orthorhombic phase transition at ${T}_{\mathrm{S}}=147\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the AFM phase transition of the Fe moments at ${T}_{\mathrm{Fe}}=72\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and the Pr AFM phase transition at ${T}_{\mathrm{Pr}}=21\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Combined high-resolution neutron diffraction and XRMS show unambiguously that the Pr moments point parallel to the longer orthorhombic $\mathbit{a}$ axis and order antiferromagnetically along the $\mathbit{a}$ axis but ferromagnetically along the $\mathbit{b}$ and $\mathbit{c}$ directions in the stripelike AFM order. The temperature-dependent magnetic order parameter of the Pr moments shows no evidence for a reorientation of moments.

Determining the correct electronic structure of ${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ remains a formidable experimental and theoretical challenge. In the low-temperature phase, two crystallographic U sites are separated into a distinct 2U(V)$+1\mathrm{U}$(VI) oxidation configuration. At low temperatures, the U(V) sites form a distorted honeycomb lattice, but the U(VI) sit on a triangular sublattice, suggesting potential for magnetic frustration effects. The spin configuration of the unpaired $f$ electrons on the U(V) sites is likely antiferromagnetic (AFM) from susceptibility measurements, but this has not been confirmed. Here, we present a neutron scattering investigation of the structure and dynamics of ${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ from 1.7 to 600 K. We confirm static AFM ordering onset at between 22 and 25 K, which is present down to at least 1.7 K with AFM peaks corresponding to [0.5 1 1] and [0.5 2 2] in the orthorhombic phase. These measurements rule out static AFM order along the $a$ axis of the $Amm2$ phase, a configuration previously suggested by theory. Above 100 K a quasielastic scattering channel opens that we speculate arises from a lattice relaxation response to thermally activated electron hopping. This term does not conform to a magnetic form factor, so it is not related to spin relaxations. If correct, this mechanism stabilizes a continuous valence transition from $2\mathrm{U}(\mathrm{V})+1\mathrm{U}(\mathrm{VI})$ in the low-temperature ($Tl600$ K) orthorhombic phase to the hexagonal phase that contains only one degenerate U site, wherein the U valence can be dynamically stabilized between $\mathrm{U}(\mathrm{V})\ensuremath{\leftrightarrow}\mathrm{U}(\mathrm{VI})$ by phonon-assisted electron hopping.

Copper monotelluride CuTe has a layered orthorhombic structure (space group Pmmn, No. 59) but exhibits a quasi-one-dimensional charge density wave (CDW) order. Here, we present a high-pressure study of CuTe in a diamond anvil cell at pressures up to 41.8--49.8 GPa, through a combination of electrical transport, synchrotron x-ray diffraction (XRD), and Raman spectroscopy measurements as well as theoretical calculations. Our transport experiments show that the CDW transition is manifested as a hump in resistivity; with the application of external pressure, its transition temperature ${T}_{\mathrm{CDW}}$ decreases gradually and the amplitude of the resistivity hump decays rapidly. The CDW gets suppressed completely above \ensuremath{\sim}10 GPa as estimated by a linear extrapolation of the ${T}_{\mathrm{CDW}}$ versus $P$ trend. Upon further compression, a sudden drop in resistivity is initially observed at \ensuremath{\sim}2.4 K and \ensuremath{\sim}20 GPa, and zero resistance is established above \ensuremath{\sim}37 GPa, suggesting the occurrence of superconductivity (SC). Combined XRD and Raman data as well as theoretical calculations together evidence that the emergence of SC at \ensuremath{\sim}20 GPa is accompanied by an orthorhombic to monoclinic (space group $\mathit{Cm}$, No. 8) structural transition.

First-principles calculations are performed to investigate and analyze properties of (001) thin films made of the most complex perovskite system, namely, $\mathrm{Na}\mathrm{Nb}{\mathrm{O}}_{3}$ (NNO), and are subject to epitaxial strain. In particular, an energy-versus-misfit strain phase diagram is constructed and reveals the existence of three different ground states for different strain regimes. For large compressive strain and up to moderate tensile strain, a monoclinic $Cc$ phase occurred, with its polarization and axis of antiphase tilting both rotating within a ($\overline{1}10$) plane with the magnitude of the strain. For large tensile strain, a ferroelectric orthorhombic state of $Pmc{2}_{1}$ symmetry emerges with polarization lying along the [110] in-plane direction together with an octahedral tilting adopting the ${a}^{\ensuremath{-}}{a}^{\ensuremath{-}}{c}^{+}$ pattern and an antiferroelectric vector associated with the reciprocal zone-border $X$ point. Finally, in between and for a narrow region of strain, a complex ground state is found. It has orthorhombic $Pca{2}_{1}$ symmetry, for which a complex tilting pattern coexists with polarization pointing along the [001] out-of-plane direction and antiferroelectric displacement associated with the $\mathrm{\ensuremath{\Delta}} k$ point located halfway between the zone-center $\mathrm{\ensuremath{\Gamma}}$ and zone-border $X$ point. Ferroelectric, antiferroelectric, and antiferrodistortive properties are also reported and discussed as a function of misfit strain.

The structural and electronic properties of the rutile-type oxide $\ensuremath{\beta}\ensuremath{-}\mathrm{Pb}{\mathrm{O}}_{2}$ (plattnerite) are studied by neutron and synchrotron radiation diffraction and first-principles density functional theory (DFT) calculations. Both diffraction measurements and DFT calculations show that $\ensuremath{\beta}\ensuremath{-}\mathrm{Pb}{\mathrm{O}}_{2}$ has a $\mathrm{Ca}{\mathrm{Cl}}_{2}$-type orthorhombic structure (space group $Pnnm$) instead of the widely accepted $\ensuremath{\beta}\ensuremath{-}\mathrm{Pb}{\mathrm{O}}_{2}$ rutile-type tetragonal structure (space group $P{4}_{2}/mnm$). This symmetry lowering in $\ensuremath{\beta}\ensuremath{-}\mathrm{Pb}{\mathrm{O}}_{2}$ is a robust effect observed at ambient pressure at temperatures between 100 and 400 K. The orthorhombic symmetry rules out the possibility of a semimetallic symmetry-protected state in $\ensuremath{\beta}\ensuremath{-}\mathrm{Pb}{\mathrm{O}}_{2}$. Both diffraction measurements and DFT calculations show an anisotropy of thermal expansion, atomic vibrations, and elastic constants of $\ensuremath{\beta}\ensuremath{-}\mathrm{Pb}{\mathrm{O}}_{2}$ along the [100] and [010] directions.

In this paper, we study the magnetic properties of orthorhombic (Pbnm) perovskites $\mathrm{Lu}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{3}$ with $x=0.25$, 0.45, 0.55, and 0.75 by magnetization vs temperature and neutron powder diffraction measurements at room temperature. The magnetic moments are oriented along the $x$ direction with a G-type antiferromagnetic (AFM) arrangement that corresponds to irreducible representation ${\mathrm{\ensuremath{\Gamma}}}_{4}$ (${G}_{x}{A}_{y}{F}_{z}$). Magnetization reversal (MR) is observed for $0.45\ensuremath{\le}x\ensuremath{\le}0.75$. The MR phenomenon was modeled using Monte Carlo simulations, modeling both homogeneous and inhomogeneous cation distributions. For $\mathrm{Lu}{\mathrm{Fe}}_{0.25}{\mathrm{Cr}}_{0.75}{\mathrm{O}}_{3}$, the presence of MR could only be explained using an inhomogeneous distribution with well-defined magnetic clusters. Looking at the staggered magnetization of each cluster, we propose a superparamagnetic regime of ${\mathrm{Fe}}^{3+}$ clusters in a ${\mathrm{Cr}}^{3+}$-rich matrix for $\mathrm{Lu}{\mathrm{Fe}}_{0.25}{\mathrm{Cr}}_{0.75}{\mathrm{O}}_{3}$. By calculating the specific heat, we found that, in $\mathrm{Lu}{\mathrm{Fe}}_{0.45}{\mathrm{Cr}}_{0.55}{\mathrm{O}}_{3}$ and $\mathrm{Lu}{\mathrm{Fe}}_{0.25}{\mathrm{Cr}}_{0.75}{\mathrm{O}}_{3}$ the clusters order at nearly the same temperature ${T}_{N}^{\mathrm{Clus}}\ensuremath{\sim}535$ and 520 K, respectively, while the matrix orders at ${T}_{N}^{\mathrm{Mat}}\ensuremath{\sim}115\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. There is considerable reduction in the cluster ordering temperature as compared with the bulk, as ${T}_{N}\ensuremath{\sim}628\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for $\mathrm{LuFe}{\mathrm{O}}_{3}$, while the matrix orders at the expected temperature as ${T}_{N}\ensuremath{\sim}115\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for $\mathrm{LuCr}{\mathrm{O}}_{3}$.

Funding: School of Chemistry, University of St Andrews for funding of a studentship to GWH through the EPSRC doctoral training grant (Grant No. EP/N509759/1). This work was also facilitated by funding provided by the EPSRC (Grant No. EP/P024637/1). K.K.M. and R.S.K. acknowledge financial support from the Department of Defense, USA (DoD Grant No. FA9550-20-1-0064). The RUS component of this work was funded by EPSRC Grant No. EP/P024904/1. The recent report of an intermediate incommensurately modulated orthorhombic phase in LaTaO4 has prompted a re-examination of the phase transition sequence in LaTaO4 as a function of temperature. With falling temperature, the sequence of phases examined is (orthorhombic) Cmc21(C)↔Cmc21(IC)↔(monoclinic)P21/c, with C and IC denoting commensurate and incommensurate phases, respectively. The orthorhombic to monoclinic transition, Tm-o, is a first order reconstructive transition occurring at 440 K and TIC-C is a first-order displacive transition occurring at 500-530 K. Strain and elasticity data confirm a first-order transition between the basic and modulated Cmc21 phases, with similarities to the isostructural fluoride BaMnF4. A Raman spectroscopic study of the LaTaO4 phase transition indicates that the IC-C phase transition is driven by a soft zone-boundary phonon (unstable) of the commensurate orthorhombic (Cmc21) phase. The soft phonon is found to appear (underdamped) above 443 K and vanishes (overdamped) around 528 K. A large supercell of the monoclinic phase below Tm-o is proposed based on the Raman spectroscopic results. Publisher PDF

Selected members of the $\mathrm{La}(\mathrm{Fe},\mathrm{Mn})\mathrm{As}({\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11})$ system were analyzed using high-resolution synchrotron x-ray powder diffraction. The tetragonal to orthorhombic structural transition is progressively recovered in the optimally electron-doped $\mathrm{LaFeAs}({\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11})$ phase by very light Mn substitution; at the same time, superconductivity is suppressed whereas magnetic ordering is restored. Distinct incommensurate satellite peaks develop within different thermal ranges and mark the occurrence of charge density waves characterized by distinct propagation wave vectors, as well as multiple incommensurate structural transitions; in particular, some of them arise in conjunction with the structural transformation process, disappearing after the completion of the dissymmetrization. The thermal evolution of satellite reflections observed at $\mathbf{Q}\ensuremath{\sim}1.93\phantom{\rule{0.16em}{0ex}}{\AA{}}^{--1}$ indicates a strong competition between the charge density waves and the superconductive state. A phase diagram of the $\mathrm{La}(\mathrm{Fe},\mathrm{Mn})\mathrm{As}({\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11})$ system is drawn on the basis of the structural, magnetic, and electronic properties of the analyzed samples.

$\mathrm{Tm}\mathrm{Fe}{\mathrm{O}}_{3}$ (TFO) is a canted antiferromagnet that undergoes a spin reorientation transition (SRT) with temperature between 82 and 94 K in single crystals. In this temperature region, the N\'eel vector continuously rotates from the crystallographic $c$ axis (below 82 K) to the $a$ axis (above 94 K). The SRT allows for a temperature control of distinct antiferromagnetic states without the need for a magnetic field, making it apt for applications working at terahertz frequencies. For device applications, thin films of TFO are required as well as an electrical technique for read-out of the magnetic state. Here, we demonstrate that orthorhombic TFO thin films can be grown by pulsed laser deposition and the detection of the SRT in TFO thin films can be accessed by making use of the all-electrical spin Hall magnetoresistance, in good agreement for the temperature range where the SRT occurs in bulk crystals. Our results demonstrate that one can electrically detect the SRT in insulators.

We report a study based on first-principles calculations complemented by a tight-binding approach and a k⋅p model of the spin-orbit effects in epitaxially strained PbTiO3. The imposed biaxial tensile strain allows us to rotate the electrical polarization of PbTiO3 from out-of-plane to in-plane directions through structural phase transitions between a tetragonal, monoclinic, and orthorhombic phase. It is found that Ti d-state conduction bands exhibit two strong Rashba-like spin-splitting parameters of (1) 0.39 eV A at a tensile strain η=1.55% due to a polarization rotation in the monoclinic phase and (2) 1.08 eV A under large tensile strain of 3.8% due to band anticrossing in the orthorhombic phase. Remarkably, we also identified the presence of a quasipersistent spin texture for the band with a dyz−zx character in the orthorhombic phase. We then conclude that using strain could be an interesting way to tune the spin-orbit effects in ferroelectric materials with technological interests.

Crystal and electronic structure of ${\mathrm{Co}}_{3}{\mathrm{O}}_{4}$ spinel have been investigated by x-ray absorption near edge structure (XANES) at the Co $K$ edge up to 58.5 GPa and Raman scattering up to 65 GPa. Several transitions have been observed upon pressurization, and the original structure was recovered on decompression. Experimental and theoretical XANES and Raman data are compatible with the occurrence of a monoclinic $P{2}_{1}/c$ phase above $\ensuremath{\sim}52.7$ GPa. Vibrational modes analyzed in details by Raman scattering indicate that two other subtle transitions take place above $\ensuremath{\sim}21.9$ GPa (orthorhombic $Fddd$) and $\ensuremath{\sim}43.0$ GPa (monoclinic $C2/m$), in agreement with the previous x-ray diffraction experiments. Our combined experimental XANES and multiple scattering calculations indicate clear evidence of a tetrahedral to octahedral coordination crossover at the ${\mathrm{Co}}^{2+}$ sites, being completed upon transition to the monoclinic $P{2}_{1}/c$ phase. The valence and spin states at two different $\mathrm{Co}$ sites remained unchanged at least up to the transition onset to the monoclinic $P{2}_{1}/c$ phase ruling out the possibility of charge transfer and spin crossover in the intermediate phases as proposed previously.

Quasiparticle self-consistent $GW$ calculations are presented for the band structures of $\mathrm{Li}\mathrm{Ga}{\mathrm{O}}_{2}$ and $\mathrm{Na}\mathrm{Ga}{\mathrm{O}}_{2}$ in the orthorhombic $Pna{2}_{1}$ tetrahedrally coordinated crystal structures, which are closely related to the wurtzite structure of ZnO. Symmetry labeling of the bands near the gap is carried out, and effective-mass tensors are extracted for the conduction-band minimum and crystal-field split valence-band maxima (VBM) at $\mathrm{\ensuremath{\Gamma}}$. The gap is found to be direct at $\mathrm{\ensuremath{\Gamma}}$ and is 5.81 eV in $\mathrm{Li}\mathrm{Ga}{\mathrm{O}}_{2}$ and 5.46 eV in $\mathrm{Na}\mathrm{Ga}{\mathrm{O}}_{2}$. Electron-phonon coupling zero-point normalization is estimated to lower these gaps by about $0.2\ifmmode\pm\else\textpm\fi{}0.1$ eV. Optical response functions are calculated within the independent-particle long-wavelength limit, and they show the expected anisotropy of the absorption onsets due to the crystal-field splitting of the VBM. The results show that both materials are promising candidates as ultrawide-gap semiconductors with wurtzite-based tetrahedrally bonded crystal structures. Direct transitions from the lowest conduction band to higher bands, relevant to $n$-type doped material and transparent conduction applications, are found to start only above 3.9 eV and are allowed for only one polarization, and several higher band transitions are forbidden by symmetry. Alternative crystal structures, such as $R\overline{3}m$ and a rocksalt-type phase with a tetragonally distorted $P4/mmm$ spacegroup, both with octahedral coordination of the cations, are also investigated. They are found to have higher energy but about 20% smaller volume per formula unit. The transition pressures to these phases are determined, and for $\mathrm{Li}\mathrm{Ga}{\mathrm{O}}_{2}$ they are found to be in good agreement with experimental studies. The $R\overline{3}m$ phase also has a comparably high but slightly indirect band gap, while the rocksalt-type phase is found to have a considerably smaller gap of about 3.1 eV in $\mathrm{Li}\mathrm{Ga}{\mathrm{O}}_{2}$ and 1.0 eV in $\mathrm{Na}\mathrm{Ga}{\mathrm{O}}_{2}$.

Orthorhombic $\mathrm{HoMn}{\mathrm{O}}_{3}$ is a multiferroic in which Mn antiferromagnetic order induces ferroelectricity. A second transition occurs within the multiferroic phase, in which a strong enhancement of the ferroelectric polarization occurs concomitantly to antiferromagnetic ordering of Ho $4f$ magnetic moments. Using the element selectivity of resonant x-ray diffraction, we study the magnetic order of the Mn $3d$ and Ho $4f$ moments. We explicitly show that the Mn magnetic order is affected by the Ho $4f$ magnetic ordering transition. Based on the azimuthal dependence of the $(0\phantom{\rule{0.16em}{0ex}}q\phantom{\rule{0.16em}{0ex}}0)$ and $(0\phantom{\rule{0.16em}{0ex}}1\ensuremath{-}q\phantom{\rule{0.16em}{0ex}}0)$ magnetic reflections, we suggest that the Ho $4f$ order resembles an $ac$ cycloid. Using nonresonant diffraction, we show that the magnetically induced polar lattice distortion is unaffected by the Ho ordering, suggesting a mechanism through which the Ho order affects ferroelectric polarization without affecting the lattice in the same manner as the Mn order.

We perform redundant x-ray diffraction versus temperature experiments in bulk transparent KTN. We find a violation of the standard perovskite cubic-to-tetragonal symmetry breaking at the Curie point in the form of an orthorhombic cell distortion. The lattice distortion spans coherently macroscopic volumes of the sample and is characterized by a negative-to-zero thermal volume expansion. Dielectric measurements and calorimetry indicate that the anomalous behavior is not associated to a distinct thermodynamic phase. The comparison of linear and nonlinear optical scattering experiments with structural data suggests that the lattice distortion is a consequence of strain relaxation to a 3D superlattice of polarization vortices.

The electronic structure of a pulsed laser deposited epitaxial $\mathrm{La}\mathrm{V}{\mathrm{O}}_{3}$ (LVO) thin film grown on a $\mathrm{LaAl}{\mathrm{O}}_{3}$ (001) (LAO) substrate has been studied at room temperature and at 130 K, which is below the structural (141 K) and magnetic transition temperatures (143 K) of single-crystal LVO. Significant modification in spectral intensity, largely around the $\mathrm{V}\phantom{\rule{0.16em}{0ex}}3d\text{\ensuremath{-}}\mathrm{O}\phantom{\rule{0.16em}{0ex}}2p$ hybridized region, is observed in the valence band spectrum (VBS) of a LVO film at 130 K. Resonant photoemission study at 130 K confirms the presence of a charge-transfer screened $3{d}^{n}\underline{\mathrm{L}}$ ($\underline{\mathrm{L}}$: hole in the $\mathrm{O}\phantom{\rule{0.16em}{0ex}}2p$) final state along with the dominant $3{d}^{n\ensuremath{-}1}$ final state at 1.5 eV binding energy in the valence band. On the contrary, in the room temperature VBS, dominant $\mathrm{V}\phantom{\rule{0.16em}{0ex}}3d$ states with only the $3{d}^{n\ensuremath{-}1}$ kind of final state are accentuated. To understand this difference, density functional theory calculations are employed. The changes in crystal structure from room temperature orthorhombic to low temperature monoclinic ($M$ LVO) symmetry leads to remarkable change in the electronic structure around the Fermi level, including transition from the direct to the indirect nature of the band gap. Our calculations also confirm an enhanced $\mathrm{O}\phantom{\rule{0.16em}{0ex}}2p$ character in the valence band edge of $M$ LVO that is hybridized with $\mathrm{V}\phantom{\rule{0.16em}{0ex}}3d$. These results are further corroborated with octahedral distortions associated with the structural transition.

We used resonant x-ray scattering at the $\mathrm{Cu}\phantom{\rule{4pt}{0ex}}{L}_{3}$ and $\mathrm{Dy}\phantom{\rule{4pt}{0ex}}{M}_{5}$ edges to investigate charge order in thin films of underdoped ${\mathrm{DyBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ (DyBCO) epitaxially grown on ${\mathrm{NdGaO}}_{3}\phantom{\rule{0.16em}{0ex}}(110)$ substrates. The films show an orthorhombic crystal structure with short-range ortho-II oxygen order in the charge-reservoir layers. At the Dy ${M}_{5}$ edge we observe diffraction peaks with the same planar wave vectors as those of the two-dimensional charge density wave in the ${\mathrm{CuO}}_{2}$ planes and of the ortho-II oxygen order, indicating the formation of induced ordered states on the rare-earth sublattice. The intensity of the resonant diffraction peaks exhibits a nonmonotonic dependence on an external magnetic field. Model calculations on the modulation of the crystalline electric field at the Dy sites by charge and oxygen order capture the salient features of the magnetic field, temperature, and photon energy dependence of the scattering intensity.

The role of nematic order for the mechanism of high-temperature superconductivity is highly debated. In most iron-based superconductors (IBSs) the tetragonal symmetry is broken already in the normal state, resulting in orthorhombic lattice distortions, static stripe magnetic order, or both. Superconductivity then emerges, at least at weak doping, already from the state with broken ${C}_{4}$ rotational symmetry. One of the few stoichiometric IBSs, lithium iron arsenide superconducts below 18 K and does not display either structural or magnetic transition in the normal state. Here we demonstrate, using angle-resolved photoemission spectroscopy, that even the superconducting state in LiFeAs is also a nematic one. We observe spontaneous breaking of the rotational symmetry in the gap amplitude on all Fermi surfaces, as well as unidirectional distortion of the Fermi pockets. Remarkably, these deformations are hardly visible above superconducting ${T}_{c}$. Our results demonstrate the realization of the phenomenon of superconductivity-induced nematicity in IBSs, emphasizing the intimate relation between them. We suggest a theoretical explanation based on the emergence of a secondary instability inside the superconducting state, which leads to the nematic order and $s\ensuremath{-}d$ mixing in the gap function.

Many positive electrode materials in lithium ion batteries include transition metals which are difficult to describe by electronic structure methods like density functional theory (DFT) due to the presence of multiple oxidation states. A prominent example is the lithium manganese oxide spinel Li$_x$Mn$_2$O$_4$ with $0\leq x\leq2$. While DFT, employing the local hybrid functional PBE0r, provides a reliable description, the need for extended computer simulations of large structural models remains a significant challenge. Here, we close this gap by constructing a DFT-based high-dimensional neural network potential (HDNNP) providing accurate energies and forces at a fraction of the computational costs. As different oxidation states and the resulting Jahn-Teller distortions represent a new level of complexity for HDNNPs, the potential is carefully validated by performing X-ray diffraction experiments. We demonstrate that the HDNNP provides atomic level details and is able to predict a series of properties like the lattice parameters and expansion with increasing Li content or temperature, the orthorhombic to cubic transition, the lithium diffusion barrier, and the phonon frequencies. We show that for understanding these properties access to large time and length scales as enabled by the HDNNP is essential to close the gap between theory and experiment., 20 pages, 13 figures

We report inelastic x-ray scattering measurements of the in-plane polarized transverse acoustic phonon mode propagating along $q\ensuremath{\parallel}[100]$ in various hole-doped compounds belonging to the 122 family of iron-based superconductors. The slope of the dispersion of this phonon mode is proportional to the square root of the shear modulus ${C}_{66}$ in the $q\ensuremath{\rightarrow}0$ limit and, hence, sensitive to the tetragonal to orthorhombic structural phase transition occurring in these compounds. In contrast to a recent report for $\mathrm{Ba}{({\mathrm{Fe}}_{0.94}{\mathrm{Co}}_{0.06})}_{2}{\mathrm{As}}_{2}$ [Weber et al., Phys. Rev. B 98, 014516 (2018)], we find qualitative agreement between values of ${C}_{66}$ deduced from our experiments and those derived from measurements of the Young modulus in ${\mathrm{Ba}}_{1\ensuremath{-}x}{(\mathrm{K},\mathrm{Na})}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ at optimal doping. These results provide an upper limit of about 50 \AA{} for the nematic correlation length for the investigated optimally hole-doped compounds. Furthermore, we also studied compounds at lower doping levels exhibiting the orthorhombic magnetic phase, where ${C}_{66}$ is not accessible by volume probes, as well as the ${C}_{4}$ tetragonal magnetic phase.

${\mathrm{SrPt}}_{2}{\mathrm{As}}_{2}$ undergoes a structural transition around 470 K (from a high-temperature tetragonal to a low-temperature orthorhombic phase) accompanied by a charge density wave transition and followed by a superconducting transition at much lower temperature (5 K). The superconducting gap symmetry of this noncentrosymmetric compound was investigated in detail by means of muon-spin rotation or relaxation $(\ensuremath{\mu}\mathrm{SR})$ experiments in transverse-field (TF) configuration. The temperature-dependent magnetic penetration depth obtained from the $\mathrm{TF}\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{SR}$ spectra suggests an isotropic $s$-wave-type superconducting gap. No observable differences are seen in the asymmetry time spectra collected in zero field above and below the superconducting transition temperature ${T}_{c}$. This strongly suggests the absence of spontaneous magnetic fields in the superconducting state, hence pointing to a preserved time-reversal symmetry below ${T}_{c}$.

In-plane anisotropy of electrical resistivity was studied in samples of the hole-doped ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ in the composition range $0.21\ensuremath{\le}x\ensuremath{\le}0.26$ where anisotropy changes sign. Low-temperature ($\ensuremath{\sim}20$ K) irradiation with relativistic 2.5 MeV electrons was used to control the level of disorder and residual resistivity of the samples. Modification of the stress-detwinning technique enabled measurements of the same samples before and after irradiation, leading to the conclusion of anisotropic character of predominantly inelastic scattering processes. Our main finding is that the resistivity anisotropy is of the same sign irrespective of residual resistivity, and remains the same in the orthorhombic ${C}_{2}$ phase above the reentrant tetragonal transition. Unusual $T$-linear dependence of the anisotropy $\mathrm{\ensuremath{\Delta}}\ensuremath{\rho}\ensuremath{\equiv}{\ensuremath{\rho}}_{a}(T)\ensuremath{-}{\ensuremath{\rho}}_{b}(T)$ is found in pristine samples with $x=0.213$ and $x=0.219$, without similar signatures in either ${\ensuremath{\rho}}_{a}(T)$ or ${\ensuremath{\rho}}_{b}(T)$. We show that this feature can be reproduced by a phenomenological model of R. M. Fernandes et al. [Phys. Rev. Lett. 107, 217002 (2011)]. We speculate that onset of fluctuations of nematic order on approaching the instability towards the reentrant tetragonal phase contributes to this unusual dependence.

A complex orthorhombic carbon allotrope in $Pbam$ symmetry with 32 atoms in its unit cell, thus termed $Pbam$-32 carbon, was recently predicted [C. Y. He et al., Phys. Rev. Lett. 121, 175701 (2018)]. Its crystal structure comprises alternating fivefold, sixfold, and sevenfold carbon rings and exhibits reduced bonding anisotropy compared to diamond, raising the prospects of finding a superstrong material with distinct and favorable mechanical properties. Here we report findings from first-principles calculations that reveal peculiar stress-strain relations in $Pbam$-32 carbon. The obtained stress responses under various tensile and shear strains display outstanding characteristics contrasting those of traditional superhard materials like diamond and cubic boron nitride ($c$-BN). The $Pbam$-32 carbon undergoes structural deformations that produce highly isotropic stress responses under a wide variety of large tensile and shear strains, showcasing unprecedented nearly degenerate stress-strain curves along multiple deformation paths extended over ultralarge, including full-range, strains up to the bond-breaking points. These deformation modes impede or even suppress the graphitization process commonly seen in highly strained diamond and $c$-BN crystals while still sustaining large peak stresses comparable to those in diamond. Most notably, we find conspicuous bond-weakening and -breaking mechanisms stemming from bonding symmetry reduction in $Pbam$-32 carbon. At large tensile strains, a sequential bond elongation process occurs, generating a more ductile deformation past the peak stress; at large shear strains, the crystal structure goes through a similar sequential bond elongation process and, interestingly, transforms into a distinct three-dimensional network containing mixed $s{p}^{2}$ and $s{p}^{3}$ bonding states, suppressing the usual graphitization process. These more gradual bonding-state changes in the severely strained $Pbam$-32 carbon improve ductility and toughness in this superstrong carbon crystal. These insights elucidate mechanisms for toughening superstrong covalent crystals via microstructural arrangements, which shed light on rational design and development of a distinct class of superstrong materials that exhibit more isotropic mechanical responses with improved toughness under diverse loading conditions.

In this work we report the temperature and doping-level evolution of the crystallographic and magnetic properties of the Sr-doped cobaltates ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{BaCo}}_{4}{\mathrm{O}}_{7}$. The noncollinear ferrimagnetic phase known for the magnetoelectric parent compound is found to persist only for a small amount of Sr doping ($x=0.02$) and is accompanied by a strong unit cell distortion. In turn, further Sr doping blurs this distortion of lattice parameters and favors other magnetic arrangements. In particular, this work focuses on an intermediate temperature region $62\phantom{\rule{0.28em}{0ex}}\mathrm{K}lTl82\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ that shows a plateau in the magnetization. Using neutron powder diffraction at selected temperatures and compositions, we have solved the magnetic structure of this intermediate phase. By means of the superspace group theory and its implementation in the Rietveld refinement, we found that the phase belongs to the superspace group $Pna{2}_{1}{1}^{\ensuremath{'}}(1/2,1/2,g)qq0s$ with a basic orthorhombic structure and a modulation of the magnetic moments along the $c$ axis. The propagation vector is $\mathbf{k}=(\frac{1}{2},\frac{1}{2},g)$ with $g\ensuremath{\sim}0.02$ and possibly dependent on temperature. A modulated spin structure with distinct behaviors of the triangular and kagome cobalt sites is reported in ``114'' cobaltates.

Exotic multifold topological excitations have been predicted and were recently observed in transition metal silicides like $\ensuremath{\beta}$-RhSi. Herein, we report that interesting topological features of RhSi are also observed in its orthorhombic $\ensuremath{\alpha}$ phase, which displays multiple types of Dirac nodes very close to the Fermi level ${\ensuremath{\varepsilon}}_{F}$. We discuss the symmetry analysis, band connectivity along high-symmetry lines using group representations, band structure, the nature of the Dirac points and of a nodal line occurring near ${\ensuremath{\varepsilon}}_{F}$ which is protected by the crystalline symmetry. The de Haas--van Alphen effect indicates a Fermi surface in agreement with the calculations. We find an elliptically shaped nodal line very close to ${\ensuremath{\varepsilon}}_{F}$ around and near the $S$ point on the ${k}_{y}\text{\ensuremath{-}}{k}_{z}$ plane that results from the intersection of two upside-down Dirac cones. Both Dirac points of the participating Kramers degenerate bands are only 5 meV apart; hence, an accessible magnetic field might induce a crossing between the spin-up partner of the upper Dirac cone and the spin-down partner of the lower Dirac cone, possibly explaining the anomalies observed in the magnetic torque. $\ensuremath{\alpha}$-RhSi is a unique system since all bands crossing ${\ensuremath{\varepsilon}}_{F}$ emerge from Dirac nodes.

${\mathrm{PbZrO}}_{3}$ is regarded as the first antiferroelectric and currently is under intense reexamination. We propose a multiscale approach that combines classical and first-principles density functional theory based simulations to explore polar phases in this material. Application of this approach to ${\mathrm{PbZrO}}_{3}$ predicts three polar phases, none of which is the common $R3c$. The lowest-energy polar phase $Cc$ is metastable at 0 K but can be stabilized by application of electric field. Its structural and polarization response to the electric field is in good agreement with experimental data from the literature. The other two phases are $Ima2$ and $I4cm$ of orthorhombic and tetragonal symmetry, respectively, and predicted to stabilize at finite temperatures and under application of larger electric fields. These two phases exhibit very large strain which is of technological significance.

We report the synthesis and systematic studies of a new layered ternary telluride ${\mathrm{TaPdTe}}_{5}$ with quasi-one-dimensional ${\mathrm{PdTe}}_{2}$ chains. This compound crystalizes in a layered orthorhombic structure with space group $Cmcm$. Analysis of its curved field-dependent Hall resistivity, using the two-band model, indicates the hole-dominated transport with a high mobility ${\ensuremath{\mu}}_{h}=2.38\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at low temperatures. The in-plane magnetoresistance (MR) displays significant anisotropy with field applied along the crystallographic $b$ axis. The MR with the current applied along the $c$ axis is also measured in high magnetic fields up to 51.7 T. Remarkably, it follows a power-law dependence and reaches $(9.5\ifmmode\times\else\texttimes\fi{}{10}^{3})%$ at 2.1 K without any signature of saturation. The de Haas--van Alphen oscillations show a small Fermi surface pocket with a nontrivial Berry phase. The Shubnikov--de Haas (SdH) oscillations are detected at low temperatures and under magnetic fields above 28.5 T. Two effective masses ${m}^{*}$ ($0.26{m}_{e}$ and $0.41{m}_{e}$) are extracted from the oscillatory SdH data. Our first-principles calculations unveil a topological Dirac cone in its surface states, and, in particular, the topological index indicates that ${\mathrm{TaPdTe}}_{5}$ is a topologically nontrivial material.

The composition dependence of the structural transition between the monoclinic $1{T}^{\ensuremath{'}}$ and orthorhombic ${T}_{d}$ phases in the ${\mathrm{Mo}}_{1\ensuremath{-}x}{\mathrm{W}}_{x}{\mathrm{Te}}_{2}$ Weyl semimetal was investigated by elastic neutron scattering on single crystals up to $x\ensuremath{\approx}0.54$. First observed in ${\mathrm{MoTe}}_{2}$, the transition from ${T}_{d}$ to $1{T}^{\ensuremath{'}}$ is accompanied by an intermediate pseudo-orthorhombic phase, ${T}_{d}^{*}$. Upon doping with W, the ${T}_{d}^{*}$ phase vanishes by $x\ensuremath{\approx}0.34$. Above this concentration, a phase coexistence behavior with both ${T}_{d}$ and $1{T}^{\ensuremath{'}}$ is observed instead. The interlayer in-plane positioning parameter $\ensuremath{\delta}$, which relates to the $1{T}^{\ensuremath{'}}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ angle, decreases with temperature as well as with W substitution, likely due to strong anharmonicity in the interlayer interactions. The temperature width of the phase coexistence remains almost constant up to $x\ensuremath{\approx}0.54$, in contrast to the broadening reported under pressure.

Elastic neutron scattering on a single crystal combined with powder x-ray diffraction measurements were carried out to investigate how the crystal structure evolves as a function of temperature in the Weyl semimetal $\mathrm{W}{\mathrm{Te}}_{2}$. A sharp transition from the low-temperature orthorhombic phase (${T}_{d}$) to the high-temperature monoclinic phase ($1{T}^{\ensuremath{'}}$) was observed at ambient pressure in the single crystal near $\ensuremath{\sim}565$ K. Unlike in ${\mathrm{MoTe}}_{2}$, the solid-solid transition from ${T}_{d}$ to $1{T}^{\ensuremath{'}}$ occurs without the cell doubling of the intermediate ${T}_{d}^{*}$ phase with AABB (or ABBA) layer stacking. In powders, however, the thermal transition from the ${T}_{d}$ to the $1{T}^{\ensuremath{'}}$ phase was broadened and a two-phase coexistence was observed until 700 K, well above the structural transition.

The recent discovery of $\mathrm{Fe}{\mathrm{O}}_{2}$ under high pressures has aroused great interest. With the fully anisotropic density-functional theory+U method, we present a predictive study of structural and magnetic transitions of rutile-type $\mathrm{Fe}{\mathrm{O}}_{2}$ after reproducing the experimental spiral wave vector in isostructural $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Mn}{\mathrm{O}}_{2}$. A second-order structural distortion (tetragonal to orthorhombic) involving octahedral rotation occurs at a critical pressure of $3\ensuremath{\sim}4\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. From a global search in the Brillouin zone, the ground-state spin order of rutile-type $\mathrm{Fe}{\mathrm{O}}_{2}$ is found to be collinear below 22 GPa and transforms to a helix at higher pressures. The phases remain insulating throughout the whole pressure range, with a change from an indirect gap in the high-spin state to a direct gap in the low-spin state. Our work extends the fundamental understanding of iron oxides and provides schemes that treat strongly correlated magnetic systems in a proper and effective way.

Single crystals of solid solutions of ferromagnetic superconductors UCoGe and URhGe were investigated by magnetization and specific-heat measurements. These compounds behave as anisotropic ferromagnets with the easy-magnetization direction along the orthorhombic $c$ axis. The maximum Curie temperature ${T}_{C}=\phantom{\rule{0.16em}{0ex}}20\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ has been observed in $\mathrm{UC}{\mathrm{o}}_{1\ensuremath{-}x}\mathrm{R}{\mathrm{h}}_{x}\mathrm{Ge}$ for $x=0.4$. The main interest of the study was the intriguing behavior in magnetic fields applied along the $b$ axis. The temperature dependence of the $b$-axis magnetic susceptibility exhibits a maximum at a characteristic temperature ${T}_{\mathrm{max}}$. For low Rh concentrations $(xl0.4){T}_{\mathrm{max}}g{T}_{C}$ and the magnetic entropy ${S}_{\mathrm{mag}}$ at ${T}_{C}$ is very low, whereas ${T}_{\mathrm{max}}={T}_{C}$ and the ${S}_{\mathrm{mag}}$ at ${T}_{C}$ is much higher for $x\ensuremath{\ge}0.4$. A metamagneticlike anomaly in magnetization is observed in all compounds, at a critical field which appears on a similar energy scale with the value of the corresponding ${T}_{\mathrm{max}}$. The character of three paramagnetic regimes which can be distinguished in the T-$x$ and H-$x$ phase diagrams, normal, correlated, and field polarized, is discussed in relation to the field-induced reentrant superconductivity in the parent compounds.

X-ray absorption measurements at the U ${L}_{\mathrm{III}}$, Ru $K$, and Fe $K$ edges are reported for the hidden order (HO) material ${\mathrm{URu}}_{2\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{Si}}_{2}$ ($x=0$, 0.05, 0.08, 0.10, 0.12, 0.15, and 0.20) as a function of $x$ and temperature $T$. When Fe is substituted for Ru, the local structure about Fe shrinks slightly and the first neighbor Fe-Si bond length decreases by $\ensuremath{\approx}0.05\phantom{\rule{0.16em}{0ex}}\AA{}$. More importantly excess disorder is observed below 80--100 K (the coherence temperature ${T}^{*}$) in plots of the Debye-Waller factor ${\ensuremath{\sigma}}^{2}$ ($\ensuremath{\sigma}$ is the width of the pair distribution function); at low $T$ the data deviate from the usual Einstein or correlated-Debye model plots. This excess disorder is most prominent for the Ru-Si bond, and ${\ensuremath{\sigma}}^{2}$ actually increases below 80 K. These results suggest a local orthorhombic distortion with ${B}_{1g}$-like symmetry that develops below 80--100 K. A model that describes these local distortions is presented, and discussed in terms of other measurements that indicate a breaking of fourfold symmetry at low $T$. In addition, the square root of the difference between ${\ensuremath{\sigma}}^{2}(T)$ for the Ru-Si pair and a Debye fit to these data serves as an order parameter for this orthorhombic distortion, in the temperature range below 100 K. This quantity is a length related to $a\text{\ensuremath{-}}b$, the difference between the $a$ and $b$ lattice constants in the orthorhombic phase, and provides a connection between this distortion and ${T}^{*}$. X-ray absorption near edge structure (XANES) measurements also show that there are no changes in the edge positions down to 0.1 eV for any edge as a function of $x$, for $T$ in the HO regime.

The dynamic magnetic properties of the Kondo-insulator state in $\mathrm{Ce}{\mathrm{Fe}}_{2}{\mathrm{Al}}_{10}$ (spin gap, resonance mode) have been investigated using polarized neutrons on a single crystal of pure $\mathrm{Ce}{\mathrm{Fe}}_{2}{\mathrm{Al}}_{10}$. The results indicate that the magnetic excitations are polarized mainly along the orthorhombic $a$ axis and their dispersion along the orthorhombic $c$ direction could be determined. Polycrystalline samples of Nd- and Rh-doped $\mathrm{Ce}{\mathrm{Fe}}_{2}{\mathrm{Al}}_{10}$ were also studied by the time-of-flight technique, with the aim of finding out how the low-energy magnetic excitation spectra change upon isoelectronic substitution of a rare-earth element (Nd) on the magnetic Ce site or electron doping (Rh) on the transition-element Fe sublattice. The introduction of magnetic Nd impurities strongly modifies the spin gap in the Ce dynamic magnetic response and causes the appearance of a quasielastic signal. The crystal-field excitations of Nd, studied in both $\mathrm{La}{\mathrm{Fe}}_{2}{\mathrm{Al}}_{10}$ and $\mathrm{Ce}{\mathrm{Fe}}_{2}{\mathrm{Al}}_{10}$, also reveal a significant influence of $f$-electron hybridization (largest in the case of Ce) on the crystal-field potential. As a function of the Rh concentration, a gradual change is observed from a Kondo-insulator to a metallic Kondo-lattice response, likely reflecting the decrease in the hybridization energy.

We study the coupling of the fully symmetric vibration mode of arsenic atoms to magnetism in a $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Au}}_{x})}_{2}{\mathrm{As}}_{2}$ system by polarization-resolved Raman spectroscopy and neutron diffraction. In this system, there are two phase transitions: a tetragonal-to-orthorhombic structural phase transition at temperature ${T}_{S}$ and a magnetic phase transition into collinear spin-density wave (SDW) state at temperature ${T}_{N}$ ($\ensuremath{\le}{T}_{S}$). ${T}_{S}$ and ${T}_{N}$ almost coincide in the pristine compound, whereas they differ by as much as 8 K for compounds with dilute gold substitution for iron. Raman coupling to the ${A}_{g}$(As) phonon is forbidden for the $XY$ scattering geometry in the tetragonal phase above ${T}_{S}$, whereas it becomes allowed in the orthorhombic phase below ${T}_{S}$: The emerging mode's intensity indicates the lattice orthorhombicity. We find that upon cooling below ${T}_{S}$, first, weak ${A}_{g}$(As) phonon mode intensity appears in the $XY$ scattering geometry spectra; however, the mode's intensity is significantly enhanced in the magnetic phase below ${T}_{N}$. The ${A}_{g}$(As) phonon also shows an asymmetric line shape below ${T}_{N}$ and an anomalous linewidth broadening upon Au doping. We describe the anomalous behavior of the ${A}_{g}$(As) mode in the $XY$ scattering geometry using a Fano model involving the ${A}_{g}$(As) phonon interacting with the ${B}_{2g}({D}_{4h}$)-symmetry-like electron-hole continuum. We conclude that the temperature dependence of light coupling amplitude to the ${A}_{g}$(As) phonon follows the evolution of the magnetic order parameter $M(T)$. We propose that the intensity enhancement of the ${A}_{g}$(As) phonon in the $XY$ scattering geometry below ${T}_{N}$ is due to electronic anisotropy induced by the collinear SDW order parameter.

Authors acknowledge Diamond Light Source for time on beamline I05-ARPES under Proposal SI23890. L.C.R. acknowledges funding from the Royal Commission for the Exhibition of 1851. We measure the electronic structure of FeSe from within individual orthorhombic domains. Enabled by an angle-resolved photoemission spectroscopy beamline with a highly focused beam spot (nano-ARPES), we identify clear stripelike orthorhombic domains in FeSe with a length scale of approximately 1-5 μm. Our photoemission measurements of the Fermi surface and band structure within individual domains reveal a single electron pocket at the Brillouin zone corner. This result provides clear evidence for a one-electron-pocket electronic structure of FeSe, observed without the application of uniaxial strain, and calls for further theoretical insight into this unusual Fermi surface topology. Our results also showcase the potential of nano-ARPES for the study of correlated materials with local domain structures. Publisher PDF

Owing to their structural complexity and wide range of possible chemical combinations, perovskite oxides exhibit many technically important physical properties. Pressure is a thermodynamic parameter which is useful for tuning physical properties; however, the response of the complex crystal structure to high pressure has not been thoroughly studied and rationalized. This study focuses on in situ high-pressure x-ray diffraction of the orthorhombic perovskite oxides ${A}^{3+}{B}^{3+}{\mathrm{O}}_{3}$, commonly found for the rare-earth transition-metal oxides of the $RM{\mathrm{O}}_{3}$ formula. Each of the four families of $RM{\mathrm{O}}_{3}$ ($M=\mathrm{Ti}$, Cr, Mn, Fe) perovskites in this study all crystallize in the same orthorhombic perovskite structure with the Pbnm space group. The lanthanide contraction in these materials leads to varying degrees of orthorhombic distortions that are primarily associated with octahedral site rotations. The pressure-induced change of the lattice parameters demonstrates an evolution from a suppression to an enlargement of the orthorhombic distortion for substitution of the rare-earth element from $R=\mathrm{La}$ to Lu in $RM{\mathrm{O}}_{3}$ perovskites. This unusual crossover of the lattice parameters' dependence on pressure contradict the results from first-principles calculation but can be rationalized by the intrinsic distortion of the perovskite structure.