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Pauflerite β-VOSO4 has recently been identified as a one-dimensional S = ½ Heisenberg system, of interest both from a fundamental point of view and a potential material for future spintronics applications. The observation of diffuse scattering in a synthetic β-VOSO4 provides a microscopic interpretation of the underlying correlated disorder, which is linked to the inversion of the short–long V—O distance pairs along VO6 chains, forming a local defect state. Direct Monte Carlo modeling indicates that such defects form thin layers with a positive inter-layer correlation, forming small domains with inverted vanadyl bonding patterns. Two-dimensional defects in anisotropic magnetic systems may perturb, or even destroy, long-range magnetic ordering leading to unusual interactions. In particular, the lack of inversion symmetry in the defect layers opens up the possibility for the Dzyaloshinskii–Moriya interaction (DMI) and, consequently, chiral magnetism localized in the defect planes. The defect β-VOSO4 structure, therefore, opens up new possibilities for the study of low-dimensional magnetic systems.

Nanomaterials can intensively scatter and/or reflect radiation. Such processes and materials are of theoretical and practical interest. Here, we study the quasi-specular reflections (QSRs) of cold neutrons (CNs) and the reflections of very cold neutrons (VCNs) from nanodiamond (ND) powders. The fluorination of ND increased its efficiency by removing/replacing hydrogen, which is otherwise the dominant cause of neutron loss due to incoherent scattering. The probability of the diffuse reflection of VCNs increased for certain neutron wavelengths by using appropriate ND sizes. Based on model concepts of the interaction of CNs with ND, and in reference to our previous work, we assume that the angular distribution of quasi-specularly reflected CNs is narrower, and that the probability of QSRs of longer wavelength neutrons increases if we increase the characteristic sizes of NDs compared to standard detonation nanodiamonds (DNDs). However, the probability of QSRs of CNs with wavelengths below the cutoff of ~4.12 Å decreases due to diffraction scattering on the ND crystal lattice. We experimentally compared the QSRs of CNs from ~4.3 nm and ~15.0 nm ND. Our qualitative conclusions and numerical estimates can help optimize the parameters of ND for specific practical applications based on the QSRs of CNs.

International audience; Aperiodic crystals possess long-range order without translational symmetry. They constitute a state of matter that has forced a profound paradigm shift in solid-state physics. A common feature of aperiodic crystals is that they recover periodicity in higher dimensional spaces, the so-called crystallographic superspaces. Much work has been dedicated to the structural order within these superspaces and also to their specific dynamics. This paper focuses on the phase transitions within crystallographic superspaces. Aperiodic crystals are divided into three families: incommensurately modulated crystals, composite crystals, and quasicrystals. Incommensurately modulated crystals have been studied heavily and appear now to be relatively well understood, since they possess a periodic high-symmetry phase. The other two members of the family are aperiodic by construction. In this paper, we present a comprehensive and systematic study of a prototype composite host-guest family of n-alkane/urea inclusion compounds [$n−$ C$_n$H$_{2n+2}$/CO(NH$_2$)$_2$]. For these materials, which exhibit a rich sequence of phases, the phase transitions are described in terms of group/subgroup symmetry breaking within crystallographic superspaces. Such phase transitions may decrease, increase, or maintain the dimension of the crystallographic superspace. These results highlight the multiplicity of specific structural solutions thataperiodicity offers.

Using X-ray scattering, we measured detailed maps of the diffuse scattering intensity distribution and a number of phonon dispersion branches for a single crystal of inorganically formed natural calcite and for high-quality mesocrystals of biogenic calcite from a Mediterranean sea urchin spine. A comparison shows that the known differences in the mechanical properties between the `strong' biogenic and `brittle' abiotic material should be attributed to the mesoscopic architecture of the crystal rather than to a modification of the calcite crystal structure. The data are rationalized by comparing them to the results of ab initio model calculations of lattice dynamics. For the mesocrystal, they are augmented by the evaluation of the faceting of the constituent nanocrystals.

The nature of the charge density wave phases in the kagome metal compound AV3Sb5 has raised many questions and their origin is still under debate. Here, we combine diffuse scattering (DS) and inelastic x-ray scattering (IXS) to identify a 3-dimensional (3D) precursor of the charge order with propagation vector (0.5 0 0.5), which condenses into a 3D-CDW through a first order phase transition. The quasi-elastic critical scattering indicates that the dominant contribution to the diffuse precursor is the elastic central peak without phonon softening. However, the inelastic spectra show a small broadening of the Einstein-type phonon mode on approaching the phase transition. Our results point to the situation where the Fermi surface instability at the L point is of order-disorder type with a critical growth of quasi-static domains. The results go beyond the classical weak-coupling Peierls transition dynamics and are discussed within models including strong-electron phonon coupling and non-adiabaticity.

Controversy in the description/identification of so-called intermediate phase(s) in PbHfO3, stable in the range ∼420–480 K, has existed for a few decades. A synchrotron diffraction experiment on a partially detwinned crystal allowed the structure to be solved in the superspace groupImma(00γ)s00 (No. 74.2). In contrast to some previously published reports, in the pure compound only one distinct phase was observed betweenPbamPbZrO3-like antiferroelectric andPm3mparaelectric phases. The modulation vector depends only slightly on temperature. The major structure modulation is associated with the displacement of lead ions, which is accompanied by a smaller amplitude modulation for the surrounding O atoms and tilting of HfO6octahedra. Tilting of the octahedra results in a doubling of the unit cell compared with the parent structure.

The question of the microscopic origin of the M-superstructure and additional satellite peaks in the Zr-rich lead zirconate-titanate is discussed for nearly 50 years. Clear contradiction between the selection rules of the critical scattering and the superstructure was found preventing unambiguous attributing of the observed superstructure either to the rotation of the oxygen octahedra or to the antiparallel displacements of the lead cations. Detailed analysis of the satellite pattern explained it as the result of the incommensurate phase transition rather than antiphase domains. Critical dynamics is the key point for the formulated problems. Recently, the oxygen tilt soft mode in the PbZr0.976Ti0.024O3 (PZT2.4) was found. But this does not resolve the extinction rules contradiction. The results of the inelastic X-ray scattering study of the phonon spectra of PZT2.4 around M-point are reported. Strong coupling between the lead and oxygen modes resulting in mode anticrossing and creation of the wide flat part in the lowest phonon dispersion curves is identified. This flat part corresponds to the mixture of the displacements of the lead and oxygen ions and can be an explanation of the extinction rules contradiction. Moreover, a flat dispersion surface is a typical prerequisite for the incommensurate phase transition.

Despite many years of research, the full complexity of the electron-lattice interactions in ${\mathrm{UO}}_{2}$ is not fully understood. We present x-ray inelastic scattering at low temperature showing that the interaction between electronic degrees of freedom and transverse acoustic phonons is strong only along the reciprocal space direction [100]. The anisotropy is reflected in the phonon-linewidth broadening, which persists also well above the N\'eel temperature. This intrinsic effect infers an anisotropy in the thermal conductivity, which has been observed, but which is formally forbidden in a cubic material. We have no model capable of connecting our experimental observations with the low thermal conductivity of ${\mathrm{UO}}_{2}$ below room temperature.

The development of useful structure-function relationships for materials that exhibit correlated nanoscale disorder requires adequately large atomistic models which today are obtained mainly via theoretical simulations. Here, we exploit our recent advances in structure-refinement methodology to demonstrate how such models can be derived directly from simultaneous fitting of 3D diffuse- and total-scattering data, and we use this approach to elucidate the complex nanoscale atomic correlations in the classical relaxor ferroelectric PbMg1/3Nb2/3O3 (PMN). Our results uncover details of ordering of Mg and Nb and reveal a hierarchical structure of polar nanoregions associated with the Pb and Nb displacements. The magnitudes of these displacements and their alignment vary smoothly across the nanoregion boundaries. No spatial correlations were found between the chemical ordering and the polar nanoregions. This work highlights a broadly applicable nanoscale structure-refinement method and provides insights into the structure of PMN that require rethinking its existing contentious models., The understanding of relaxor ferroelectrics is hindered by the complexity of nanoscale perturbations of their structure. Here, a data set of independent techniques treated on common footing provides a multiscale description of atomic order which reconciles conflicting models derived from single methods.

A new diffractometer is now available to the general user community at the ESRF. The new diffractometer is a side station of the high-resolution inelastic X-ray scattering spectrometer on beamline ID28 and is located in the same experimental hutch. Both instruments can be operated simultaneously. The new diffractometer combines a fast and low-noise hybrid pixel detector with a variable diffraction geometry. The beam spot on the sample is 50 µm × 50 µm, where focusing is achieved by a combination of Be lenses and a KB mirror. Wavelengths from 0.5 to 0.8 Å can be used for the diffraction experiments. The setup is compatible with a variety of sample environments, allowing studies under non-ambient conditions. The diffractometer is optimized to allow a rapid survey of reciprocal space and diffuse scattering for the identification of regions of interest for subsequent inelastic scattering studies, but can also be employed as a fully independent station for structural studies from both powder and single-crystal diffraction experiments. Several software packages for the transformation and visualization of diffraction data are available. An analysis of data collected with the new diffractometer shows that the ID28 side station is a state-of-the-art instrument for structural investigations using diffraction and diffuse scattering experiments.

Understanding the role of disorder and the correlations that exist within it, is one of the defining challenges in contemporary materials science. However, there are few material systems, devoid of other complex interactions, which can be used to systematically study the effects of crystallographic conflict on correlated disorder. Here, we report extensive diffuse x-ray scattering studies on the epitaxially stabilised alloy $\mbox{U}_{1-x}\mbox{Mo}_x$, showing that a new form of intrinsically tuneable correlated disorder arises from a mismatch between the preferred symmetry of a crystallographic basis and the lattice upon which it is arranged. Furthermore, combining grazing incidence inelastic x-ray scattering and state-of-the-art ab initio molecular dynamics simulations we discover strong disorder-phonon coupling. This breaks global symmetry and dramatically suppresses phonon-lifetimes compared to alloying alone, providing an additional design strategy for phonon engineering. These findings have implications wherever crystallographic conflict can be accommodated and may be exploited in the development of future functional materials., Comment: V3: Some supplementary content moved into main text and manuscript structure reformatted to accommodate. No substantive change to transferred material but linking statements added. Section III discussion expanded by one paragraph. No change to results, figures, or conclusions in either the main manuscript or supplementary material. Main text: 10 pg, 7 figs. SM: 14 pg, 5 figs, 1 table

Journal of applied crystallography 54(1), 287 - 294 (2021). doi:10.1107/S1600576720016039, The complete elastic stiffness tensor of thiourea has been determined fromthermal diffuse scattering (TDS) using high-energy photons (100 keV).Comparison with earlier data confirms a very good agreement of the tensorcoefficients. In contrast with established methods to obtain elastic stiffnesscoefficients (e.g.Brillouin spectroscopy, inelastic X-ray or neutron scattering,ultrasound spectroscopy), their determination from TDS is faster, does notrequire large samples or intricate sample preparation, and is applicable toopaque crystals. Using high-energy photons extends the applicability of theTDS-based approach to organic compounds which would suffer from radiationdamage at lower photon energies., Published by Wiley-Blackwell, [S.l.]

We report an investigation of the combined structural and electronic properties of the bronze ${\mathrm{Na}}_{1.5}{({\mathrm{PO}}_{2})}_{4}{({\mathrm{WO}}_{3})}_{20}$. Its low-dimensional structure and possible large reconstruction of the Fermi surface due to charge density wave instability make this bulk material a natural superlattice with a reduced number of carriers and Fermi energy. Signatures of multilayered two-dimensional (2D) electron weak localization are consequently reported, with an enhanced influence of quantum oscillations. A crossover between these two antagonistic entities, previously observed only in genuine low-dimensional materials and devices, is shown to occur in a bulk crystal due to its hidden 2D nature.

Thanks to the remarkable developments of ultrafast science, one of today's challenges is to modify material state by controlling with a light pulse the coherent motions that connect two different phases. Here we show how strain waves, launched by electronic and structural precursor phenomena, determine a macroscopic transformation pathway for the semiconducting-to-metal transition with large volume change in bistable Ti$_3$O$_5$ nanocrystals. Femtosecond powder X-ray diffraction allowed us to quantify the structural deformations associated with the photoinduced phase transition on relevant time scales. We monitored the early intra-cell distortions around absorbing metal dimers, but also long range crystalline deformations dynamically governed by acoustic waves launched at the laser-exposed Ti$_3$O$_5$ surface. We rationalize these observations with a simplified elastic model, demonstrating that a macroscopic transformation occurs concomitantly with the propagating acoustic wavefront on the picosecond timescale, several decades earlier than the subsequent thermal processes governed by heat diffusion., 30 pages (including supplementary text), 5 main figures, 9 supplementary figures; corrected author list

Model materials are precious test cases for elementary theories and provide building blocks for the understanding of more complex cases. Here, we describe the lattice dynamics of the structural phase transition in francisite ${\mathrm{Cu}}_{3}\mathrm{Bi}({\mathrm{SeO}}_{3}{)}_{2}{\mathrm{O}}_{2}\mathrm{Cl}$ at 115 K and show that it provides a rare archetype of a transition driven by a soft antipolar phonon mode. In the high-symmetry phase at high temperatures, the soft mode is found at (0,0,0.5) at the Brillouin zone boundary and is measured by inelastic x-ray scattering and thermal diffuse scattering. In the low-symmetry phase, this soft-mode is folded back onto the center of the Brillouin zone as a result of the doubling of the unit cell, and appears as a fully symmetric mode that can be tracked by Raman spectroscopy. On both sides of the transition, the mode energy squared follows a linear behavior over a large temperature range. First-principles calculations reveal that, surprisingly, the flat phonon band calculated for the high-symmetry phase seems incompatible with the displacive character found experimentally. We discuss this unusual behavior in the context of an ideal Kittel model of an antiferroelectric transition.

International audience; The understanding of the symmetry breakings within crystals that are aperiodic by construction is actually very limited. Quasicrystals and incommensurate composite crystals may potentially allow such studies. We focus on the phase transitions of the aperiodic n-nonadecane/urea which recovers a translational symmetry within a four-dimensional space at room temperature. High-resolution neutron and synchrotron studies are reported as a function of the temperature on this organic crystal which presents an exceptional mosaicity. They reveal the richness of such approach, showing the appearance of very long wavelength supplementary intermodulations. This work generalizes the Landau theory to incommensurate composite crystals.

International audience; Lattice dynamics of low-dimensional BaVS3 is reported across the metal-insulator Peierls transition occurring at TP=69 K using a combination of the thermal diffuse scattering of x rays, inelastic x-ray scattering, and density-functional theory calculations. The nondetection of a Kohn anomaly points to a unique situation of an order-disorder Peierls instability with a quasielastic critical scattering which has been fully characterized. These observations are discussed in the scope of a Peierls instability dominated by strong electron-phonon coupling and/or nonadiabatic effects.

Model materials are precious test cases for elementary theories and provide building blocks for the understanding of more complex cases. Here, we describe the lattice dynamics of the structural phase transition in francisite Cu_{3}Bi(SeO_{3})_{2}O_{2}Cl at 115 K and show that it provides a rare archetype of a transition driven by a soft antipolar phonon mode. In the high-symmetry phase at high temperatures, the soft mode is found at (0,0,0.5) at the Brillouin zone boundary and is measured by inelastic x-ray scattering and thermal diffuse scattering. In the low-symmetry phase, this soft-mode is folded back onto the center of the Brillouin zone as a result of the doubling of the unit cell, and appears as a fully symmetric mode that can be tracked by Raman spectroscopy. On both sides of the transition, the mode energy squared follows a linear behavior over a large temperature range. First-principles calculations reveal that, surprisingly, the flat phonon band calculated for the high-symmetry phase seems incompatible with the displacive character found experimentally. We discuss this unusual behavior in the context of an ideal Kittel model of an antiferroelectric transition.

The anharmonic lattice dynamics of MgO has been studied at ambient conditions and at high temperatures by infrared spectroscopy combined with density functional perturbation theory calculations. The agreement between the measured phonon energies and widths with ab-initio calculated values provides a direct and pertinent test of the validity of advanced theoretical methods. Long observed anharmonic features in the infrared reflectivity find a clear explanation in terms of well-defined multiphonons scattering processes and lattice dynamics peculiarities, also responsible of a significant and sharp reduction of the longitudinal optical phonon lifetime at critical finite wave vectors. Our work highlights the importance of multi-phonons scattering processes on collective dynamics and related materials properties.

We report a type of structural phase transitions in dielectric materials: the triggered incommensurate (IC) transition. We demonstrate evidence for such a transition in the perovskite antiferroelectric ${\mathrm{PbHfO}}_{3}$ by means of single-crystal x-ray diffraction, diffuse, and inelastic-scattering experiments, which we interpret using the Landau theory of phase transitions. This transition is not driven by an IC soft mode, as is the case for the majority of IC dielectrics, but by the soft mode associated with a different, coexisting order parameter related to antiferrodistortive (AFD) tilts of the oxygen octahedra. When cooling from the high-temperature cubic phase these two order parameters are established simultaneously and discontinuously at ${T}_{\text{IC}}\ensuremath{\approx}468$ K. Two lattice instabilities are present in the cubic phase: AFD instability with critical temperature ${T}_{0}\ensuremath{\approx}441$ K and ferroelectric instability with ${T}_{0}\ensuremath{\approx}408$ K. The IC instability is absent. The analysis of the transition mechanism within the Landau theory of coupled order parameters indicates that the transition is of triggered character, conditioned upon the attractive biquadratic coupling between the IC modulation and the AFD octahedral tilts, and is driven by the AFD soft mode.

International audience; The anharmonic lattice dynamics of MgO has been studied at ambient conditions and at high temperatures by infrared spectroscopy and inelastic x-ray scattering measurements combined with density functional perturbation theory calculations. The agreement between the measured phonon energies and widths with ab initio calculated values provides a direct and pertinent test of the validity of advanced theoretical methods. Long observed anharmonic features in the infrared reflectivity find a clear explanation in terms of well-defined multiphonon scattering processes and lattice dynamics peculiarities, also responsible for a significant and sharp reduction of the longitudinal optical phonon lifetime at critical finite wave vectors. Our work highlights the importance of multiphonon scattering processes on the collective dynamics and related material properties. The importance and effectiveness of a synergetic approach combining experiments and ab initio simulations to understand the complex phenomena that control the properties of materials under extreme thermodynamic conditions cannot be overstated. This joint effort becomes essential for acquiring knowledge of, and correctly modeling, the anharmonic effects on the collective dynamics of crystalline solids. Phonon an-harmonicity is a very important topic of fundamental interest, also crucial on an applied basis, as phonons play a major role in many physical properties such as thermal and electrical conductivity, or superconductivity. During the past two decades theoretical and numerical tools have improved dramatically (e.g., Refs. [1-3]), allowing for the simulation of anharmonic phenomena, such as phonon lifetimes and lattice thermal conductivity. This led to important applications in the study of thermoelectricity [4,5]. Indeed, knowing how the heat flows, or dissipates, is of primary importance in the conception of new and improved thermoelectric devices. More generically, the influence of multiparticle coupling has been largely highlighted in a variety of materials going from those of technological interest such as the thermoelectrics Bi 2 Te 3 , PbTe, to the alkali halides or simple metals such as aluminum [6-9]. In a similar manner , calculations of the lattice thermal conductivity at high pressures and high temperatures have been carried out for many compounds of geophysical interest such as MgO [10] and MgSiO 3 [11], as the relative importance of heat transport mechanisms (conduction versus radiation versus convection) strongly influences the planet internal dynamics [12]. However, the pertinence of such an approach has been challenged, in the absence of an advanced description (and the consequent modeling) of the normal mode anharmonic interactions responsible for the thermodynamic behavior of crystals [13]. The way in which phonons interact has a non-negligible impact on the physical properties of the material, and the concept of the phonon itself finds its limits. Noteworthy, such intense and diversified research is mostly based on computational studies. Only a few are experiments and even fewer are studies in which the theoretical predictions and measurements are directly compared [9,14-19]. Macroscopic quantities, such as thermal conductivity or the thermoelectric figure of merit ZT, depend on the entire phonon spectrum and are very sensitive to the approximations used in the ab initio simulations and to the specific characteristics of individual samples (degree of crystallinity, presence and nature of defects, etc.), making any comparison difficult to interpret. From an experimental standpoint several spectro-scopic techniques are necessary to probe phonons over the entire dispersion: visible (Raman, Brillouin) and infrared spectroscopies at vanishing crystal momentum, and inelastic neutrons or x-ray scattering at finite wave vectors. In this context we undertook a complete experimental and computational study on the rocksalt compound MgO, so to directly and unambiguously assess the quality of theoretical simulations of the anharmonic phonon dynamics. The choice of MgO is based on the availability of high-quality single crystals, jointly to a major geophysical interest. Moreover, anharmonic effects have been observed in the dielectric function of MgO and other cubic ionic crystals [20-22]. Specifically in these compounds, a pure harmonic scenario fails to describe the infrared reflectivity in the reststrahlen region, where an unexpected shoulder is systematically observed. This feature translates into an excess of spectral weight in the imaginary part of the dielectric function that is explained 2469-9950/2019/99(22)/220304(5) 220304-1

International audience; The lattice dynamics of low-dimensional BaVS3 is reported accross the hexagonal-orthorhombic phase transition occuring at TS = 250 K using a combination of Thermal Diffuse Scattering of X-rays, Inelastic X-ray Scattering and DFT calculations. We observed the occurence of strongly temperature-dependent diffuse scattering upon approaching the transition, centered at the Γ points associated with weak Bragg reflections with odd L. Inelastic scattering experiment related this observation to the condensation of a low-lying overdamped optical phonon at the Brillouin zone center, evidenced by significant variations of quasi-elastic scattering. These results unravel the dynamical origin of the zigzag distortion of the V chains at TS, which is probably a prerequisite of the Peierls anomaly driving the metal-insulator transition at T MI = 70 K.

The high-pressure structural properties of bismuth oxide ${\mathrm{Bi}}_{2}{\mathrm{SiO}}_{5}$ have been investigated up to 29 GPa using in situ powder synchrotron x-ray diffraction (XRD) and up to 50 GPa with density functional theory (DFT) calculations. We found a compressibility anomaly at about 20 GPa that is observed both in our XRD and DFT results. The rotation of the corner-connected ${\mathrm{SiO}}_{4}$ tetrahedra produces a straightening of the silicate chains, yielding a lower compressibility above 20 GPa for the structure with straight chains. We analyzed the stereochemical activity of the ${\mathrm{Bi}}^{3+}$ lone electron pair, which is found to decrease with increasing pressure, but it can still be identified in the calculated electron density difference maps at 50 GPa.

In order to resolve inconsistencies encountered in published data for SrSO[Formula: see text], the elasticity and the phase stability of celestite has been studied using thermal diffuse scattering, high pressure powder synchrotron x-ray diffraction, Raman scattering and DFT calculations. The structure of SrSO[Formula: see text] is found to be stable up to 62 GPa at ambient temperature. The preferred values for the components of the elastic stiffness tensor have been determined using x-ray thermal diffuse scattering and are (in GPa): [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text]. The preferred value for the bulk modulus is [Formula: see text] GPa. This work shows that thermal diffuse scattering collected at two temperatures allows the determination of the full elastic tensor of crystals with low space group symmetry.

International audience

International audience; Despite a large number of studies [2,3] over the years since the first discovery [7] and a couple of comprehensive reviews [8,9] the actual mechanism for PLD/CDW formation is still under debate. The most recent experimental [10-13] and theoretical [14] works focus on the large area growth of the CDW phase [13] the thickness dependence , and the possible unconventional behavior in the ultimate 2D limit of a single layer TiSe 2. [10-12,14] On the other hand, the other Ti dichalcogenides namely TiS 2 and TiTe 2 did not show any clear evidence until very recently when a CDW state was reported only for 1 monolayer (ML)-thin TiTe 2 at temperatures lower than 92 K. [15] It is surprising that the CDW in TiTe 2 was found to be totally suppressed for films thicker than 1 ML, [15] unlike the case of other TMDs where 1 ML and bulk-like films both make the transition to a CDW at nearly the same temperature. The interest about TiTe 2 is continuously increasing in view of theoretical predictions [16] and more recent experimental evidence [17] about pressure induced topological phase transitions in TiTe 2. The possibility to also manipulate superconduc-tivity by external pressure as predicted [18] and more recently evidenced [19] in bulk TiTe 2 creates the prospect to explore the emergence of topological superconductivity in this material. In the latter work [19] it has been shown that under nonhydro-static pressure, a CDW-like state with estimated transition temperature above room temperature (RT) appears in bulk TiTe 2 at around 0.5-1.8 GPa. These results call for a re-examination of the possibility to obtain a CDW in multilayer TiTe 2 and indeed at RT with good potential for real world applications utilizing the properties of the CDW state. These applications include a voltage-controlled oscillator device operating at room temperature , [20] fast electronic resistance switching for nonvolatile memories, [21,22] and field-effect transistor devices potentially suitable for implementation of non-Boolean logic. [23] In this paper it is shown that multilayer films (50 ML ≈ 32 nm), as well as single layer TiTe 2 epitaxially grown on InAs(111)/ Si(111) substrates by molecular beam epitaxy exhibit, in ambient pressure conditions, a CDW distortion at room temperature which is sustained up to higher temperatures, at least 400 °C, as evidenced by reflection high energy electron diffrac-tion (RHEED) (Figure S1, Supporting Information). The results are explained in terms of anisotropic strain imposed by the substrate. The group IVB 2D transition metal dichalcogenides are considered to be stable in the high symmetry trigonal octahedral structure due to the lack of unpaired d-electrons on the metal site. It is found that multilayer epitaxial TiTe 2 is an exception adopting a commensurate 2 × 2 × 2 charge density wave (CDW) structure at room temperature with an ABA type of stacking as evidenced by direct lattice imaging and reciprocal space mapping. The CDW is stabilized by highly anisotropic strain imposed by the substrate with an out-off-plane compression which reduces the interlayer van der Waals gap increasing the coupling between TiTe 2 layers.

Controversy in the description/identification of so-called intermediate phase(s) in PbHfO

The high pressure lattice dynamics of rhombohedral antimony have been studied by a combination of diffuse scattering and inelastic x-ray scattering. The evolution of the phonon behavior as function of pressure was analyzed by means of two theoretical approaches: density functional perturbation theory and symmetry-based phenomenological phase transition analysis. This paper focuses on the first structural phase transition, SbI-SbIV, and the role of vibrations in leading the transition. The phonon dispersion exhibits complex behaviour as one approaches the structural transition, with the branches, corresponding to the two transitions happening at high pressure in the Va elements (A7-to-BCC and A7-to-PC) both showing softening., Comment: 8 pages and 8 figures

International audience; Neutrons can be an instrument or an object in many fields of research. Major efforts all over the world are devoted to improving the intensity of neutron sources and the efficiency of neutron delivery for experimental installations. In this context, neutron reflectors play a key role because they allow significant improvement of both economy and efficiency. For slow neutrons, Detonation NanoDiamond (DND) powders provide exceptionally good reflecting performance due to the combination of enhanced coherent scattering and low neutron absorption. The enhancement is at maximum when the nanoparticle diameter is close to the neutron wavelength. Therefore, the mean nanoparticle diameter and the diameter distribution are important. In addition, DNDs show clustering, which increases their effective diameters. Here, we report on how breaking agglomerates affects clustering of DNDs and the overall reflector performance. We characterize DNDs using small-angle neutron scattering, X-ray diffraction, scanning and transmission electron microscopy, neutron activation analysis, dynamical light scattering, infra-red light spectroscopy, and others. Based on the results of these tests, we discuss the calculated size distribution of DNDs, the absolute cross-section of neutron scattering, the neutron albedo, and the neutron intensity gain for neutron traps with DND walls.

In order to resolve inconsistencies encountered in published data for SrSO[Formula: see text], the elasticity and the phase stability of celestite has been studied using thermal diffuse scattering, high pressure powder synchrotron x-ray diffraction, Raman scattering and DFT calculations. The structure of SrSO[Formula: see text] is found to be stable up to 62 GPa at ambient temperature. The preferred values for the components of the elastic stiffness tensor have been determined using x-ray thermal diffuse scattering and are (in GPa): [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text]. The preferred value for the bulk modulus is [Formula: see text] GPa. This work shows that thermal diffuse scattering collected at two temperatures allows the determination of the full elastic tensor of crystals with low space group symmetry.

Cuprates exhibit antiferromagnetic, charge density wave (CDW), and high-temperature superconducting ground states that can be tuned by means of doping and external magnetic fields. However, disorder generated by these tuning methods complicates the interpretation of such experiments. Here, we report a high-resolution inelastic x-ray scattering study of the high-temperature superconductor YBa₂Cu₃O6.67 under uniaxial stress, and we show that a three-dimensional long-range-ordered CDW state can be induced through pressure along the a axis, in the absence of magnetic fields. A pronounced softening of an optical phonon mode is associated with the CDW transition. The amplitude of the CDW is suppressed below the superconducting transition temperature, indicating competition with superconductivity. The results provide insights into the normal-state properties of cuprates and illustrate the potential of uniaxial-pressure control of competing orders in quantum materials., Facultad de Ciencias Exactas, Instituto de Física de Líquidos y Sistemas Biológicos

Bi${}_{2}$SiO${}_{5}$ recently emerged as a promising ambient-temperature lead-free ferroelectric with a Curie temperature of 663 K, where the ferroelectricity arises from the tilt of quasi-one-dimensional SiO${}_{4}$ chains. Here, the authors analyze the competition between ferroelectric and antiferroelectric instabilities by investigating the characteristic temperature-dependent phonon energies at the center and the edge of the Brillouin zone, respectively. The direct evidence for this competition opens the way towards tuning the properties of Bi${}_{2}$SiO${}_{5}$ by application of pressure or electrical field.

To observe, by microscopic probe, how low Zn doping in $\mathrm{F}{\mathrm{e}}_{3\ensuremath{-}x}\mathrm{Z}{\mathrm{n}}_{x}{\mathrm{O}}_{4}$ ($x$ is below 1%) changes the Verwey transition, we have performed M\"ossbauer spectroscopy measurements on three single crystalline samples with various Zn doping. In spectra analysis we used the recently published model of M\"ossbauer data treatment formulated as a result of ab initio calculations for a low-temperature monoclinic structure (of Cc symmetry) of magnetite. It was suggested there that the hyperfine parameters for all 24 Fe distinct positions in the lattice can be grouped into four major components with very similar hyperfine parameters within each set. Using these parameters as starting values, very good fits were obtained for magnetite with low doping level, while for higher doping, $x=0.03$, where the Verwey transition changes its character, one component is significantly different. In particular, low hyperfine field ${B}_{\mathrm{eff}}=36\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, considered as a characteristic feature of the Cc phase spectrum, is absent here. Also, in this case, the high-temperature spectra are different from those for lower doped magnetite showing more pronounced continuous alteration with temperature. This might be due to crystal structure of lower than Fd-3m symmetry, a fact suggested by our x-ray synchrotron studies. All this triggered a discussion about an experimental fingerprint for the difference between these two classes of magnetite, frequently referred to as magnetite of first- and second-order Verwey transition, and about the electronic structure of both kinds of systems.

Author(s): He, Y; Wu, S; Song, Y; Lee, WS; Said, AH; Alatas, A; Bosak, A; Girard, A; Souliou, SM; Ruiz, A; Hepting, M; Bluschke, M; Schierle, E; Weschke, E; Lee, JS; Jang, H; Huang, H; Hashimoto, M; Lu, DH; Song, D; Yoshida, Y; Eisaki, H; Shen, ZX; Birgeneau, RJ; Yi, M; Frano, A | Abstract: We report a persistent low-energy phonon broadening around qB∼0.28 reciprocal lattice unit (r.l.u.) along the Cu-O bond direction in the high-Tc cuprate Bi2Sr2CaCu2O8+δ (Bi-2212). We show that such broadening exists both inside and outside the conventional charge-density wave (CDW) phase, via temperature-dependent measurements in both underdoped and heavily overdoped samples. Combining inelastic hard x-ray scattering, diffuse scattering, angle-resolved photoemission spectroscopy, and resonant soft x-ray scattering at the Cu L3-edge, we exclude the presence of a CDW in the heavily overdoped Bi-2212 similar to that observed in the underdoped systems. Finally, we discuss the origin of such anisotropic low-energy phonon broadening and its potential precursory role to the CDW phase in the underdoped region.

We report experiments to determine the effect of radiation damage on the phonon spectra of the most common nuclear fuel, UO$_2$. We have irradiated thin ($\sim$ 300 nm) epitaxial films of UO$_2$ with 2.1 MeV He$^{2+}$ ions to 0.15 dpa and a lattice swelling of $\Delta$a/a $\sim$ 0.6 %, and then used grazing-incidence inelastic X-ray scattering to measure the phonon spectrum. We succeeded to observe the acoustic modes, both transverse and longitudinal, across the Brillouin zone. The phonon energies, in both the pristine and irradiated samples, are unchanged from those observed in bulk material. On the other hand, the phonon linewidths (inversely proportional to the phonon lifetimes), show a significant broadening when comparing the pristine and irradiated samples. This effect is shown to increase with phonon energy across the Brillouin zone. The decreases in the phonon lifetimes of the acoustic modes are roughly consistent with a 50 % reduction in the thermal conductivity., Comment: 11 pages, 7 figures

We report on the effects of hydrostatic pressure (HP) on the charge density wave observed in underdoped cuprates. We studied ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.6}\phantom{\rule{4pt}{0ex}}({T}_{c}=61$ K) using high-resolution inelastic x-ray scattering (IXS), and reveal an extreme sensitivity of the phonon anomalies related to the charge density wave (CDW) order to HP. The amplitudes of the normal state broadening and superconductivity-induced phonon softening at ${\mathbf{Q}}_{\text{CDW}}$ rapidly decrease as HP is applied, resulting in the complete suppression of signatures of the CDW below $\ensuremath{\sim}10$ kbar. Additional IXS measurements on ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.75}$ demonstrate that this very rapid effect cannot be explained by pressure-induced modification of the doping level and highlight the different role of external pressure and doping in tuning the phase diagram of the cuprates. Our results provide insights into the mechanisms underlying the CDW formation and its interplay with superconductivity.

Among Transition Metal Dichalcogenides (TMD), NbS$_2$ is often considered as the archetypal compound that does not have a Charge Density Wave (CDW) in any of its polytypes. By comparison, close iso-electronic compounds such as NbSe$_2$, TaS$_2$ and TaSe$_2$ all have CDW in at least one polytype. Here, we report traces of CDW in the 2H polytype of NbS$_2$, using diffuse x-ray scattering measurements at 77\,K and room temperature. We observe 12 extremely weak satellite peaks located at $\pm$13$^\mathrm{o}$ 54' from $\vec{a}^*$ and $\vec{b}^*$ around each Bragg peaks in the $(h,k,0)$ plane. These satellite peaks are commensurate with the lattice via $3\vec{q}-\vec{q}\,'=\vec{a}^*$, where $\vec{q}\,'$ is the 120$^{\circ}$ rotation of $\vec{q}$, and define two chiral $\sqrt{13}\,a\times\sqrt{13}\,a$ superlattices in real space. These commensurate wavevectors and tilt angle are identical to those of the CDW observed in the 1T polytype of TaS$_2$ and TaSe$_2$. To understand this similarity and the faintness of the peaks, we discuss possible sources of local 1T polytype environment in bulk 2H-NbS$_2$ crystals.

We carried out sound velocity and density measurements on solid hcp-Fe and an hcp-Fe–Si alloy with 9 wt.% Si at 300 K up to ∼170 and ∼140 GPa, respectively. The results allow us to assess the density (ρ) dependence of the compressional sound velocity ( V P ) and of the shear sound velocity ( V S ) for pure Fe and the Fe–Si alloy. The established V P –ρ and V S –ρ relations are used to address the effect of Si on the velocities in the Fe–FeSi system in the range of Si concentrations 0 to 9 wt.% applicable to the Earth's core. Assuming an ideal linear mixing model, velocities vary with respect to those of pure Fe by ∼ + 80 m / s for V P and ∼ − 80 m / s for V S for each wt.% of Si at the inner core density of 13 000 kg/m3. The possible presence of Si in the inner core and the quantification of its amount strongly depend on anharmonic effects at high temperature and on actual core temperature.

International audience; Fluorinated nanodiamonds are considered for the fabrication of diffusive slow-neutron reflectors of a new kind. Hard fluorination of nanodiamonds drastically reduces the neutron absorption, thus providing an unprecedented albedo for the reflection of slow neutrons, bridging the so-called reflectivity gap in the neutron spectrum. The resulting reflectors can dramatically improve the performance of neutron sources, the efficiency of neutron delivery, and thus fluxes at neutron instruments. They open the way to the design of a new generation of neutron sources and experiments.

Structural disorder is a core of the relaxor behavior and can be probed by measuring diffuse scattering. Here we focus on the experimentation and discuss diffuse scattering data for the PbMg1/3Nb2/3O3 relaxor collected as a function of temperature. Experimental pitfalls such as a resolution in reciprocal space and heating of the crystal by an intense synchrotron beam are discussed together with different presentation of diffuse scattering data. The diffuse scattering is found to be very anisotropic at all temperatures with the main component following q−2 dependence. A new component with a similar anisotropy and q−4 decay emerges below the room temperature and in the close neighborhood of the Bragg nodes. A two-term model is proposed for parameterization of the observed patterns.

32nd European Crystallography Meeting, ECM 32, Vienna, Austria, 18 Aug 2019 - 23 Aug 2019; Acta crystallographica / A Foundations and advances 75(a2), MS27-04 (2019). doi:10.1107/S2053273319091289 special issue: "August 2019 issue Abstracts of the 32nd European Crystallography Meeting Vienna, Austria, 18-23 August 2019", Published by Blackwell, Oxford [u.a.]