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Oxygen vacancies and their correlation with the electronic structure are crucial to understanding the functionality of TiO2 nanocrystals in material design applications. Here, we report spectroscopic investigations of the electronic structure of anatase TiO2 nanocrystals by employing hard and soft X-ray absorption spectroscopy measurements along with the corresponding model calculations. We show that the oxygen vacancies significantly transform the Ti local symmetry by modulating the covalency of titanium-oxygen bonds. Our results suggest that the altered Ti local symmetry is similar to the C3v, which implies that the Ti exists in two local symmetries (D2d and C3v) at the surface. The findings also indicate that the Ti distortion is a short-range order effect and presumably confined up to the second nearest neighbors. Such distortions modulate the electronic structure and provide a promising approach to structural design of the TiO2 nanocrystals.

The local symmetry of (1-x)Na0.5Bi0.5TiO3-xKNbO3 (NBT-xKN, x=0.01-0.07) ceramics was thoroughly examined using high-energy synchrotron x-ray scattering and analysed using the Pair-Distribution Function (PDF) method. At room temperature, the structure of NBT-xKN (x=0.01-0.07) ceramics was best refined using monoclinic C1c1 structures within the local-range (r 420 °C (above the temperature of the maximum dielectric peak, Tm), as confirmed by in-situ temperature-dependent PDF analysis.

Various alkyl-methylimidazolium ionic liquids (ILs) were inspected using infrared spectroscopy in the middle frequency range. In the 1050-1200 cm-1 range, there is a skeletal vibrational mode accompanied with a large in-plane +C(2)-H bending motion and +C(4)-H and +C(5)-H motions, and in the 1500-1650 cm-1 range, there are two skeletal vibrational modes with in-plane +C(4,5)-H bending motions. Interestingly, in both ranges, we found that skeletal vibrational modes with a large in-plane +C(2)-H bending motion and in-plane +C(4,5)-H bending motions are insensitive to increases in the basicity of anions or the strengthening of hydrogen bond-type interactions, and the behaviors are completely different from those in the +C-H stretching vibrational modes in the 3000-3200 cm-1 range and the skeletal vibrational modes with large out-of-plane +C-H motions in the 700-950 cm-1 range. Furthermore, in alkyl-methylimidazolium tetrafluoroborate [C n mim+][BF4 -] ILs, we found that absorption due to the (threefold) degenerate vibrational mode of [BF4 -] was observed as a broad absorption band with three splitting peaks in the 900-1150 cm-1 range as a result of local symmetry breaking due to the cation-anion interactions.

Exotic domain-wall phenomena make ferroelectrics the candidates for nanoelectronics. The local symmetry and structure anomalies at domain walls have raised interest in the unusual functionalities, such as domain wall chirality and conductivity. Especially, the spontaneous lattice distortion and symmetry breaking at domain walls activate them as the location of structural transformation. However, the routes to achieve ferroelectric-antiferroelectric phase transition via ferroelectric domain walls remain challenging, which are important to develop materials for energy storage and conversion. Here, we have observed stepwise antiferroelectric phase transition in strained pure BiFeO3 ultrathin films derived from ferroelectric domain walls. Aberration-corrected transmission electron microscopy observation reveals that the resultant phase transition is mediated by dense 180° domain walls via providing the antiparallel cation displacement, cooperating with the enhanced interfacial oxygen octahedral clamping. First-principles calculations further confirm the critical role of interfacial oxygen octahedral coupling during this transition. These findings report an alternative route for antiferroelectric phase transition. Besides, our results provide fresh insights into functionalities of ferroelectric domain walls and open a venue for developing energy-storage materials based on domain engineering.

Photoluminescence spectra of Mn2+ in ZnS single crystals were recorded in the range 77–623 K at different activator concentrations and exciting radiation wavelengths. Spectra were decomposed into individual bands, each of which corresponded to radiation of manganese centers with a certain type of local symmetry. The obtained results made it possible to estimate the effect of activator concentration and temperature on changes in the number of radiating manganese centers with different types of local symmetry relative to all luminescent Mn2+ in ZnS single crystals with different excitation wavelengths. The results are explained in terms of the redistribution of direct optical and resonant mechanisms of energy transfer of the exciting light to Mn2+ taking into account losses due to nonradiative interaction with the ZnS single-crystal lattice.

Ti silicates, and in particular, titanium silicalite-1 (TS-1), are nowadays important catalysts for several partial oxidation reactions in the presence of aqueous H2 O2 as an oxidant. Despite the numerous studies dealing with this material, some fundamental aspects are still unclear. In particular, the structure and the catalytic role of defective Ti sites, other than perfect tetrahedral sites recognized as the main active species, has not been quantitatively discussed in the literature. We assess the structural features of defective Ti sites on the basis of outcomes of electronic spectroscopies, as interpreted through quantum mechanical simulation. Strong evidence is disclosed to support the fact that the most common defective Ti sites, often reported in the TS-1 literature, are monomeric Ti centers that are embedded in the zeolite framework, and which have a distorted octahedral local symmetry.

Tremendous effort has been devoted to tailoring structure-correlated properties, especially for the luminescence of lanthanide nanocrystals (NCs). High pressure has been demonstrated as a decent way to tune the performance of lanthanide NCs; however, little attention has been paid to the local structure evolution accompanied by extreme compression and its effect on luminescence. Here, we tailor the local structure around lanthanide ions with pressure in β-NaGdF4 NCs, in which Eu3+ ions were doped as optical probes for local structure for the sensitive electric dipole transition. As the pressure increases, the intensity ratio of the 5D0 → 7F2 to 5D0 → 7F1 transition decreases monotonically from 2.04 to 0.81, implying a higher local symmetry around Eu3+ ions from compression. In situ X-ray diffraction demonstrates that the sample maintains the hexagonal structure up to 33.5 GPa, and density functional theory calculations reveal the tendency of the local structure to vary under high pressure.

The K4Ag18Te11, Tl4Ag18Te11, and BaHg11 cubic structures have been crystallographically analyzed within the study of the “symmetry–stability of crystalline materials” problem. The high symmetry of these structures is initiated by the local symmetry of templates, which are cuboctahedra composed of twelve Ag polyhedra around a Te anion (for the former two compounds) and twelve Hg polyhedra around a central Hg atom (for the latter compound). Similar cuboctahedra, consisting of K, Tl, and Ba cations, are formed in the second coordination sphere. The role of symmetry in the reduction of atomic degrees of freedom, which provides the structure stability, has been estimated. The qualitative stability criteria are formulated.

Ab initio molecular dynamics (MD) simulations and NMR GIPAW (gauge including projector augmented wave) calculations have been used to analyze the coordination and mobility of Li ions in LiTi2 (PO4 )3 (rhombohedral), LiZr2 (PO4 )3 (triclinic), and LiZr2 (PO4 )3 (rhombohedral) phases. Significant discrepancies are observed between static calculations of 7 Li quadrupolar parameters and experimental values. The dynamical origin of this disagreement is demonstrated by incorporating in the calculations thermal vibrations and local motion of atoms with MD simulations. For LiTi2 (PO4 )3 , the quadrupolar constant associated with Li ions grows with temperature because the local symmetry of the system decreases, whereas for the Zr phases, the quadrupolar constant decreases because thermal vibrations reduce the anisotropy of the interaction. Finally, for both Zr phases, MD yields Li distributions that compare well with disorder reported from diffraction studies.

Systematic and strong rare-earth photoluminescence (PL) color flips that are highly sensitive to structural phase transformation in KLaF4:RE3+ (RE3+ = Eu3+, Er3+/Yb3+) nanocrystals are demonstrated. Room-temperature wet chemical synthesis at various reaction times involves a systematic conversion from cubic (α, space group Fm3[combining macron]m) to hexagonal (β, space group P6[combining macron]2m) polymorph nanocrystals of 4 to 8 nm sizes. The unusual down-conversion photoluminescence (DC-PL) asymmetric ratio of the hypersensitive transition (5D0→7F2, Red) to that of nearly invariant transition (5D0→7F1, Orange) (R/O from 1.2 to 3.0) of KLaF4:Eu3+ is substantially affected by the site-selective occupancy and local symmetry around the Eu3+ ions, according to crystal packing transformation. The NIR excited up-conversion photoluminescence (UC-PL) of Er3+ ions produces a strong color flip from green (2H11/2 and 4S3/2) to red (4F9/2) dominated emissions based on their cubic or hexagonal crystal packing. The site occupancy and phonon energies strongly influence various nonlinear energy transfer mechanisms within RE3+ ion energies and the results are explained accordingly. The present study substantially reveals the local host effects and these two distinct polymorph nanoparticles can be potentially utilized for color-specific studies related to applications such as color-specific biological in vitro and in vivo imaging and other optoelectronic device applications.

The V1−xMoxO2 phase diagram has high structural and electronic complexity that is driven by strong, short-range correlations that compete with the long-range rutile crystal structure. The substitution regime near 50% Mo occupancy is no exception, but there has so far been no significant progress in determining the actual structure. Reported here is a combined study using single crystal X-ray diffraction, powder X-ray diffraction, and representational analysis to examine both the local and crystallographically averaged atomic structures simultaneously near x = 0.50. Between about x = 0.50 and 0.60, the average structure of V1−xMoxO2 is the parent rutile phase, but the local symmetry is broken by atomic displacements that are best described using the orthorhombic subgroup Fmmm. This model is locally similar to the two-dimensionally ordered 2D-M2 phase recently reported in the compositional range 0.19 ≤ x ≤ 0.30, except the correlation length is much shorter in the 2D plane, and longer in the frustrated one, making it more isotropic. This work also extends the 2D-M2 phase regime up to x = 0.43, and suggests that the local-Fmmm phase observed here can be seen as the end result of the continued suppression of the 2D-M2 phase through enhanced geometric frustration between the intrinsic order parameters. This suggests that other doped-rutile phases with elusive structures may also be dominated by similar short-range correlations that are hidden in the diffuse scattering.

The atomic buckling in 2D “Xenes” (such as silicene) fosters a plethora of exotic electronic properties such as a quantum spin Hall effect and could be engineered by external strain. Quantifying the buckling magnitude with subangstrom precision is, however, challenging, since epitaxially grown 2D layers exhibit complex restructurings coexisting on the surface. Here, we characterize using low-temperature (5 K) atomic force microscopy (AFM) with CO-terminated tips assisted by density functional theory (DFT) the structure and local symmetry of each prototypical silicene phase on Ag(111) as well as extended defects. Using force spectroscopy, we directly quantify the atomic buckling of these phases within 0.1-Å precision, obtaining corrugations in the 0.8- to 1.1-Å range. The derived band structures further confirm the absence of Dirac cones in any of the silicene phases due to the strong Ag-Si hybridization. Our method paves the way for future atomic-scale analysis of the interplay between structural and electronic properties in other emerging 2D Xenes.

On the example of synthetic single crystal diamond with negatively charged nitrogen-vacancy center (NV)—center, we have experimentally reviled the changes in the polarized spectrum of the first-order Raman scattering due to the presence of nitrogen impurities. It has been found the appearance of rather intense spectra of the Brillouin zone-center optical phonons in an unpredictable crossed polarization of the incident and scattered light, while in the previously reported Raman data it was observed only in parallel polarization of the incident and scattered light. Such a recovery of Raman intensity is prescribed to the change of the local symmetry from the point group Oh to C3v in accordance with the results of Raman scattering polarization rules based on conclusions of the group theory. The data make obvious also the observation of the strong in-coming resonance with the nitrogen NV 0 center electronic transition.

The crystallographic analysis of six SiO2 phases (α-quartz, β-tridymite, α- and β-cristobalite, coesite, and stishovite) is performed. The effect of the local symmetry of SiO4 tetrahedra on the symmetry of phases tending either to highly symmetrical cubic P-cristobalite or hexagonal P-tridymite is shown. The parameters of cation, anion, and joint pseudotranslational sublattices characterizing the atomic configurations of the structures and their individual features are calculated. The relative stability of the phases is considered.

The acoustic and vibrational properties of Mn-doped (Na1/2Bi1/2)TiO3-xBaTiO3 with x = 0.05 were investigated by using Brillouin and Raman spectroscopies and compared with those of undoped crystals. The mode frequency and the half width of the Brillouin doublet of the longitudinal acoustic mode showed more drastic changes near the dielectric maximum temperature compared to the broad and diffused changes of the undoped crystals, which indicates that Mn-doping induces polar domains larger than the typical polar nanoregions formed in undoped counterpart. The acoustic anomalies were most pronounced along the direction suggesting that the polarization fluctuations are anisotropic and largest along this direction. Temperature dependence of Raman spectra showed that there are two characteristics temperatures where mode splitting occurs due to local symmetry changes. The high-temperature anomaly was associated with the changes in the acoustic properties, while the low-temperature mode splitting was related to the dielectric anomaly near depolarization temperature.

Crystallographic analysis of three TiO2 structures shows that all of them have a rhombohedral anion sublattice close to their cubic closest packing. In brookite it competes with the hexagonal packing, and the packing of TiO6 octahedra is less regular. High symmetry of anatase and rutile is initiated by the local (cubic) symmetry of TiO6 octahedra. Cations occupy most symmetrical special positions and provide a coherent assembly of these “building units” by spreading the local symmetry over the entire structure.

The obstacle for efficient electrochemical water splitting lies in the kinetically sluggish oxygen evolution reaction. Despite the various efforts that have been made to understand and tune the act...

The introduction of acceptor dopants into perovskites is a common method for designing hard piezoelectric ceramics by forming defect dipoles. It is interesting to find that the doping effect of a few amount of MnO2 in (Bi0.5Na0.5)TiO3–BaTiO3 lead-free ceramics significantly depends on the local symmetry. A conventional hardening effect corresponding to an obviously enhanced mechanical quality factor and a decreased piezoelectric coefficient can be generated in the tetragonal phase rich zone, rather than in the rhombohedral-rich zone, where a typically amphoteric characteristic can be observed unexpectedly. The result was well interpreted by means of the formation of a local random field as a result of the local symmetry-breaking effect of point defects after doping MnO2 in addition to a traditional internal bias field as a result of the pinning effect of defect dipoles. Compared with tetragonal-rich ones, the obviously enhanced random field in rhombohedral-rich compositions tends to induce a weakening ferroelectricity, as evidenced by composition dependent normal ferroelectric-relaxor ferroelectric phase transition. This work provides a new understanding of how charged defects affect the domain stability in the matrix with different symmetries through the competition between random electric fields and internal bias fields.

Symmetry aspects of the periodic model of a crystal with a point defect (a supercell model or a extended unit cell (EUC) model) are considered; splitting of the Wyckoff positions in a primitive crystal cell with the introduction of supercell and the change of the one-electron states classification over k-vector (Brillouin zone folding) are discussed. When considering a point defect in a crystal in the supercell model, it is necessary to take into account the symmetry of the one-electron states of the original crystal at the top of the valence band and at the bottom of the conduction band. The selected supercell should reproduce these states. Each specific selection of a supercell corresponds to a specific splitting of the Wyckoff positions of the original crystal and, as a result, the possibility to place a defect in positions with different point symmetry and to perform a calculation without taking into account the point symmetry of the crystal with a defect at all (site symmetry method). By the results of the calculation of a crystal with a defect in the supercell model, taking into account site symmetry, it is possible to determine the real symmetry of the crystal with a defect, which is essential for the interpretation of experimental data. A copper impurity in a lithium node in a LiCl crystal retains the cubic symmetry of the atom being replaced; the impurity of an iron atom in a titanium site with cubic local symmetry in a SrTiO3 crystal lowers the symmetry to tetragonal; and the H site in the CsPbI3 crystal is characterized by the complete removal of point symmetry with the formation of an $${\text{I}}_{2}^{ - }$$ ion.

The electron paramagnetic resonance(EPR) parameters (g factors gi and hyperfine structure constants Ai, where i = x, z) for Cu2+ ions embedded in beryllium oxide (BeO) crystal at Be2+ sites with C3v local symmetry have been calculated by the complete diagonalization method (CDM) and high-order perturbation theory method (PTM). The calculated results from the two methods are in reasonable agreement with the observed data available. The defect structural data of the trigonal Cu2+ center in beryllium oxide are determined from the calculations and analyses. The calculated results show that the Be2+ ions are replaced by Cu2+ ions in BeO crystal and the ligands undergo outward movement.

In 2016, our group first reported a single-ion magnet (SIM) [Dy(bbpen)Br] (H2bbpen = N,N′‑bis(2‑hydroxybenzyl)‑N,N′‑bis(2‑methylpyridyl)ethylenediamine) with the highest effective energy barrier (Ueff) over 1000 K. To explore further for the high performance single-ion magnets (SIMs), we used functional ligand to replace the Br− anion. The H2bbpen and 4′‑(1,2,2‑triphenylvinyl)benzoic acid (tpe-COOH) ligand with aggregation-induced-emission-active (AIE) property, formed a triangular dodecahedral SIM, [Dy(bbpen)(tpe-COOH)]⋅2MeOH. The structural, magnetic and luminescent properties of the SIM have been investigated. Related to its lower local symmetry around Dy(III) ion, the Ueff was sharply reduced to 77 K under an external dc field of 0.15 T. Therefore, it proved the importance of local symmetry to get the high performance SIMs from the opposite side.

Single crystals of new non-centrosymmetric iodate Ba(IO3)OH are synthesized hydrothermally and structurally investigated. OD topology-symmetry analysis of layers and their alternation led to conclusions about the similarity with the Bi(IO3)O nonlinear-optical crystal. Both structures are members of a general family with differences in structures and properties. AgBi(IO3)4 and AgBi(SO4)(IO3)2 belong to another family with the two types of layers giving different local symmetry. The properties are estimated semi-quantitatively based on structural data. Such an approach is new and promising. Hypothetical structural variants are predicted.

Indium and bismuth are technologically important elements, in particular as oxides for optoelectronic applications. 115In and 209Bi are both I = 9/2 nuclei with high natural abundances and moderately high frequencies but large nuclear electric quadrupole moments. Leveraging the quadrupolar interaction as a measure of local symmetry and polyhedral distortions for these nuclei could provide powerful insights on a range of applied materials. However, the absence of reported NMR parameters on these nuclei, particularly in oxides, hinders their use by the broader materials community. In this contribution, solid-state 115In and 209Bi NMR of three recently discovered quaternary bismuth or indium oxides are reported, supported by density functional theory calculations, numerical simulations, diffraction, and additional multinuclear (27Al, 69,71Ga, 121Sb) solid-state NMR measurements. The compounds LiIn2SbO6, BiAlTeO6, and BiGaTeO6 are measured without special equipment at 9.4 T, demonstrating that wideline techniques such as the QCPMG pulse sequence and frequency-stepped acquisition can enable straightforward extraction of quadrupolar tensor information in I = 9/2 115In and 209Bi even in sites with large quadrupolar coupling constants. Relationships are described between the NMR observables and local site symmetry. These are amongst the first reports of the NMR parameters of 115In, 121Sb, and 209Bi in oxides.

In functional materials, the local environment around active species that may contain just a few nearest-neighboring atomic shells often changes in response to external conditions. Strong disorder in the local environment poses a challenge to commonly used extended X-ray absorption fine structure (EXAFS) analysis. Furthermore, the dilute concentrations of absorbing atoms, small sample size and the constraints of the experimental setup often limit the utility of EXAFS for structural analysis. X-ray absorption near-edge structure (XANES) has been established as a good alternative method to provide local electronic and geometric information of materials. The pre-edge region in the XANES spectra of metal compounds is a useful but relatively under-utilized resource of information of the chemical composition and structural disorder in nano-materials. This study explores two examples of materials in which the transition metal environment is either relatively symmetric or strongly asymmetric. In the former case, EXAFS results agree with those obtained from the pre-edge XANES analysis, whereas in the latter case they are in a seeming contradiction. The two observations are reconciled by revisiting the limitations of EXAFS in the case of a strong, asymmetric bond length disorder, expected for mixed-valence oxides, and emphasize the utility of the pre-edge XANES analysis for detecting local heterogeneities in structural and compositional motifs.

International audience; The motion of active colloids is generally achieved through their anisotropy, as exemplified by Janus colloids. Recently, there was a growing interest in the propulsion of isotropic colloids, which requires some local symmetry breaking. Although several mechanisms for such propulsion were proposed, little is known about the role played by the interactions within the environment of the colloid, which can have a dramatic effect on its propulsion. Here, we propose a minimal model of an isotropic colloid in a bath of solute particles that interact with each other. These interactions lead to a spontaneous phase transition close to the colloid, to directed motion of the colloid over very long timescales and to significantly enhanced diffusion, in spite of the crowding induced by solute particles. We determine the range of parameters where this effect is observable in the model, and we propose an effective Langevin equation that accounts for it and allows one to determine the different contributions at stake in self-propulsion and enhanced diffusion.

International audience; Edge-bridged halide tantalum clusters based on the {Ta6Br12}4+ core have been the topic of many physico-structural investigations both in solution and in the solid-state. Despite a large number of studies, the fundamental correlations between compositions, local symmetry, electronic structures of [{Ta6Bri12}La6]m+/n- cluster units (L = Br or H2O, in solution and at the solid-state), redox states, and vibrational and absorption properties are still not well established. Using K4[{Ta6Bri12}Bra6] as starting precursor (i: inner, a: apical), we have investigated the behavior of the [{Ta6Bri12}Bra6]4- cluster unit in terms of oxidation properties and chemical modifications both in solution (water and organic solvent) and after recrystallization. A wide range of experimental techniques in combination with quantum chemical simulations afford new data that allow to reveal the puzzling behavior the cluster units in response to changes in their environment. Apical ligands undergo changes like modifications of interatomic distances to complete substitutions in solution that modify noticeably the cluster physical properties. Changes of oxidation state of the cluster units also occur, which modifies significantly their physical properties, including optical properties, which can thus be used as fingerprint. A subtle balance exists between the number of substituted apical ligands and the cluster oxidation state. This study provides new information about the exact nature of the species formed during the transition from solid-state to solutions and vice-versa. This shines new perspectives on optimization protocols for the design of Ta6 cluster-based materials

Calcium aluminate Ca12Al14O33 initially known as cement compound after a series of structural refinements presented a complex cubic structure. It demonstrates a surprising feature consisting of the occurrence of spherical cavities named as cages. After this, mayenite is regarded as a kind of anti-zeolite. An alternative representation of such compounds formulated as highly non-stoichiometric garnets is highlighted in the present study. Mayenite is described as non-stoichiometric garnet {Ca3–x□x}1[Ca, Al]2Al3O12–x using the Raman spectroscopy. The dependence of phase diagram Al2O3–CaO in the region near the mayenite composition on the oxygen activity is summarized regarding its structural deficiency. It is pointed that cage is the result of defects association whereas structural defects tend to interact with each other. The main consequence of such interaction is a spontaneous formation of a core–shell structure. It is evidenced by the presence of a near-surface layer of small thickness over a compact oxide. The local symmetry of the mayenite structure at room temperature in oxidized and reduced (electride) state was studied by Raman spectroscopy which revealed the presence of luminescence without the rare-earths’ doping.

We explore the link between the melting scenarios of two-dimensional systems of hard disks and squares through replica-exchange Monte Carlo simulations of hard superdisks. The well-known melting scenarios are observed in the disk and square limits, while we observe an unusual three-step scenario for dual shapes. We find that two mesophases mediate the melting: a hexatic phase and another fluid phase with a ${D}_{2}$ local symmetry, we call it rhombatic, where both bond and particle orientational orders are quasi-long-range. Our results show that not only can the melting process of liquid-crystal forming molecules be complicated, where elongated shapes stabilize several mesophases, but also that of anisotropic quasispherical molecules.

We introduce a new image contrast mechanism for scanning transmission electron microscopy (STEM) that derives from the local symmetry within the specimen. For a given position of the electron probe on the specimen, the image intensity is determined by the degree of similarity between the exit electron intensity distribution and a chosen symmetry operation applied to that distribution. The contrast mechanism detects both light and heavy atomic columns and is robust with respect to specimen thickness, electron probe energy and defocus. Atomic columns appear as sharp peaks that can be significantly narrower than for STEM images using conventional disc and annular detectors. This fundamentally different contrast mechanism complements conventional imaging modes and can be acquired simultaneously with them, expanding the power of STEM for materials characterisation., 7 pages, 5 figures

Efficiency of solar cells can be improved by introducing intermediate bands. However, width and occupation requirements on the intermediate bands bring challenges for materials design. In this study, we systematically investigate the electronic structure of doped CsPbX3 (X = Cl, Br, or I). A screening in transition metals suggests that Cr and Mo doped perovskites have dispersive and half-filled intermediate bands, which is desirable for building solar cells. However, intermediate bands originated from degenerated d orbitals are easily split when the octahedral symmetry of the local chemical environment around the dopant is broken. To avoid this problem, we further perform a screening of non-transition metals aiming to generate sp orbital-based intermediate bands. It turns out that In and Ga can generate a half-filled intermediate band, which is not sensitive to the local symmetry. The estimated efficiency of In and Ga doped CsPbCl3 at a doping concentration of 8.3% is 57.97% and 54.21%, notably higher than the Shockley–Queisser limit (40.7%). Results presented here demonstrate the potential of intermediate band engineering in photovoltaic applications.

Defect pyrochlore oxides have attracted great interest as a promising luminescent material due to their flexible composition and high electron/hole mobility. In this paper, we investigate the structural and electronic properties of lanthanide-doped (Ln) defect pyrochlore oxides ${\mathrm{KNbWO}}_{6}:0.125{\mathrm{Ln}}^{3+}$ by using first-principles calculations. We perform structural optimizations of various defect pyrochlore models and calculate their electronic structures, revealing that hydration has a significant influence on both the local symmetry around the ${\mathrm{Eu}}^{3+}$ ion and the band structure with an alteration of their luminescent behavior. In the hydrated compounds, the electric-dipole $^{5}D_{0}\text{\ensuremath{-}}^{7}F_{2}$ transition is found to be partially suppressed by a raised local symmetry, and the water molecules in the compounds can mediate the nonradiative energy transfer between the activator ${\mathrm{Eu}}^{3+}$ ions and the host. It turns out that the oxygen vacancies are detrimental to luminescence as they reduce the ${\mathrm{Eu}}^{3+}$ ion in its vicinity to the ${\mathrm{Eu}}^{2+}$ ion and also serve as traps for conduction electrons excited by incident light. Our calculations for ${\mathrm{KNbWO}}_{6}:0.125{\mathrm{Ln}}^{3+}$ (Ln=Ce, Pr, Nd, Pm, Sm) support that the defect pyrochlore oxide ${\mathrm{KNbWO}}_{6}$ can also be used as a luminescence host for ${\mathrm{Ln}}^{3+}$ ion doping, giving valuable insight into a variation trend in luminescent properties of these materials at the atomic level.

The common “template” of datolite and okayamalite structures is a column-ribbon formed by alternating empty and Si occupied oxygen tetrahedra interconnected by common edges. The columns consolidate with their environment into crystalline structures largely due to the influence of the local symmetry of SiO4 and BO4 tetrahedra. As a result, mirror symmetry pseudo-planes of monoclinic datolite transform into okayamalite m planes; the pseudo-axis $\bar 4$ becomes the principal axis, whereas both structures contain glide planes and cation sublattices close in size.

This paper describes a systematic study of the spectra and local structures of lanthanide (Ln) L-edge XANES. We found that Ln L1 and L3 -edge XANES spectra exhibit characteristic features correlated to their local symmetry through experimental and theoretical simulations. We also propose a simple local structure index criterion for a combination of XANES study and theoretical simulation. Possible solutions of intrinsic problems such as low resolution of characteristic features in the Ln L-edge XANES and site distributions are also discussed.

Lowering the local symmetry around the lanthanide dopant represents a well-established strategy to enhance the quantum yield and colour selectivity of nanoparticle upconversion (UPC) emission. Here, we show that, under pulsed excitation at two near-infrared (NIR) wavelengths, the UPC emission of the Er activator (no Yb sensitizer) is comparable and more than two orders of magnitude (110 times) stronger in higher (tetragonal, D2d) compared to in lower (monoclinic, C1) local symmetry, a result that contrasts with the general trend observed for cw excited UPC. The much longer-lived reservoir levels of Er in the higher local symmetry host determine a higher contribution of the energy transfer mechanism (ETU) over excited state absorption (ESA), despite the fact that the two types of nanoparticles have an identical Er concentration and share similar structural properties. Time-resolved UPC excitation spectra (TREXS), which correlate the absorption in the spectral regions around 980 and 1530 nm and UPC emission during the timescale of Er green (around 550 nm), red (around 680 nm) and NIR (around 810 nm) emission, emerge as a valuable investigation tool. Using TREXS, we show that the local symmetry influences not only the emission intensity but also the relative contribution of the UPC mechanisms, which can be spectrally tuned by the excitation wavelength. Considering the advantages of pulsed over cw excitation of lanthanide based UPC nanoparticles, our results are relevant for bioimaging, thermometry and lifetime multiplexing applications.

International audience; Using electron paramagnetic resonance (EPR), we investigate the electronic and nuclear magnetic properties of ZnO:Co single crystals, grown by the optical furnace method. The high crystal quality of the studied samples allows for the determination of the full hyperfine and g tensors. We explain how the local magnetic anisotropy of the Co 2+ impurities is used as a very fine probe for the local symmetry and crystal quality of the host. The temperature-and power-study of EPR intensities recorded in three static-and microwave-field configurations give a qualitative insight into the dynamics of spin-lattice and spin-spin relaxations. In addition, in the context of nanostructures, we explain how a detailed analysis of the intensities anisotropy can reveal the proportion of ordered and disordered phases.

Efficient bifunctional electrocatalysts of V- and Co-substituted α-MnO2 materials can be synthesized by one-pot hydrothermal reaction of ion-adducts V3+/Co2+–MnO4−, leading to the successful substitution of both V and Co ions in the α-MnO2 lattice. According to extended X-ray absorption fine structure analysis, the substituted V ion is stabilized in highly distorted local symmetry whereas the substituted Co ion has regular octahedral local environment. Both the V- and Co-substituted α-MnO2 materials exhibit excellent bifunctional electrocatalytic functionalities with lowered overpotentials and enhanced current densities for oxygen evolution (OER) and oxygen reduction reactions (ORR), underscoring the beneficial role of cation substitution in enhancing the bifunctional electrocatalytic activity of α-MnO2. The Co-substituted α-MnO2 shows higher bifunctional oxygen electrocatalyst functionality than does the V-substituted homologue, which is attributable to the greater improvement of charge transfer kinetics upon the Co substitution than the V one. The poorer charge transfer property of V-substituted α-MnO2 can be ascribed to highly distorted local symmetry of V ion, which is unfavorable for increasing band width and depressing bandgap energy. Taking into account many advantages of the oxides of V, Mn, and Co elements such as low price, rich abundance, and environmentally benignity, the present study underscores that the partial substitution of neighboring 3d metal ion having regular octahedral symmetry can provide an effective and environmental friendly way of optimizing the oxygen electrocatalytic activity of manganese oxide.

Linearly polarized absorption spectra and magnetic circular dichroism (MCD) spectra of ErFe3(BO3)4 single crystal were measured in the range of 9000–23000 cm−1 at 90 K. The absorption spectra of f-f transitions were decomposed into the Lorentz shape components and intensities of the components were found. MCD spectra permitted us to measure the Zeeman splitting of some transitions and so to determine changes of the Lande factor along the C3 axis of the crystal during these transitions. Optical and magneto-optical properties of f-f transitions in the ErFe3(BO3)4 crystal were compared with those in the ErAl3(BO3)4 crystal. Substantial difference of the properties connected with the difference of the Er3+ ions local symmetry in two crystals was revealed. Large splitting of one of the f-f transitions without magnetic field, which is not possible for the Kramers doublets, was observed. It was explained by appearance of two absorbing centers due to the local decrease of symmetry in the excited state. Appreciable difference of the local vibrations energy in some excited states was revealed.

Temperature and pressure dependent Raman spectroscopic studies are presented which showed signatures of spin-reorientation transition in SmFeO3. The x-ray diffraction study as a function of pressure is also presented that shows the variation in structural parameters like inter-atomic distances, bond angles and degree of octahedral tilt. The electron density plots obtained from Rietveld refinement of the high pressure x-ray diffraction provides a direct visualization of the deformation at the Fe-site, while there is more symmetric structure at the Sm site. The variation in the FeO6 octahedral tilt at high pressures results in large variations in the phonon spectra at the respective pressures indicating modification in local symmetry leading to changes in magnetic structure. The observations in Raman spectroscopy studies are supplemented by density functional theoretical studies. The Raman frequencies calculated from the DFT matched well with the observed values and both showed a subtle modulation between 8 and 12 GPa.

Ab initio molecular dynamics simulations were performed to study the atomistic structures of Ni48Nb52, Ni59.5Nb40.5 and Ni75Nb25 liquids near their melting temperatures. It is found that the total coordination number only changes slightly across the compositions. However, there is significant difference in the chemical ordering. The Cluster Alignment method was used to identify the dominant local structure motifs. It is found that distorted and perfect icosahedral clusters are the most popular motifs around Ni atoms, while Z14, Z15, and Z16 Frank-Kasper polyhedra are dominant around Nb atoms. Among the three compositions, the eutectic composition (Ni59.5Nb40.5) has the largest population of these dominant motifs and the highest degree of five-fold local symmetry. Chemical order in the distorted icosahedral motif is studied and the occupation probability of Nb in the distorted region is found to be much different from other sites. The network formed by distorted and perfect icosahedral clusters and that by the interconnection of the perfect icosahedra, Z14, Z15, and Z16 motifs are the strongest at the eutectic composition. The high percentage of perfect and distorted icosahedral clusters and their strong network structures in this system (especially around the eutectic composition) correlate well with the excellent glass forming ability in this system.

The crystallographic analysis of the garnet structure type (Al2Ca3(SiO4)3, Ia3d symmetry) establishes the determining role of the [AlCa6Si6] cation group with the local symmetry $$\bar 3$$ in the structure formation. The glide planes of three directions coherently assemble these groups. High structural stability (three not symmetry-fixed coordinates per 80 atoms, the fundamental volume V*~17 A3) promotes the occurrence of various atomic compositions in it. The relation with the β-W structure type and the structure of menezesite mineral is shown.

Using ab initio calculations, here, we identified inorganic aromaticity of Mn6-ring cluster in layered Li(Ni0.5Mn0.5)O2 with Ni/Li mixing. The comparison between delocalized orbitals Mn-dz2 in Mn6-ring and benzene-pz uncovers the fundamental similarity between a local cluster and benzene in electronic behavior. We also demonstrate that the delocalized interactions between transition metals (TMs) play a great role in the formation of inorganic aromaticity of Mn6-ring in layered TM Li(Ni0.5Mn0.5)O2 (with Ni/Li mixing) via a two-level hierarchy: (i) the same TM ions (here manganese) can interact with each other directly through delocalized orbitals Mn-d in Mn6-ring, and the local symmetry can impose restrictions on this delocalization. The direct exchange interactions play an important role at this level. (ii) The anions, here oxygen, can match with these delocalized orbitals of d-manifold via bridged bonds to share their electrons with TM ions to create the superexchange interactions, which enable the forma...

New noncentrosymmetric double borate Rb3EuB6O12 was designed and synthesized by the solid state reaction method, and its crystallographic parameters were obtained by Rietveld analysis. This borate crystallizes in the trigonal space group R32 with cell parameters a = 13.4604(2) Å, c = 30.7981(5) Å, Z = 15. Its structure features a three-dimensional framework composed of the [B5O10]5− groups that are bridged by Eu-O polyhedra. The existence of B5O10 group in the structure was confirmed by vibrational spectroscopy. Rb3EuB6O12 melts incongruently at 1101 K. The second harmonic generation effect of Rb3EuB6O12 is 16 times higher than that of the α-quartz standard. In the luminescence spectrum, the domination of a single prominent narrow line from the hypersensitive 5D0 - 7F2 manifold of Eu3+ ions is observed, while the 5D0 - 7F1 manifold and ultranarrow 5D0 - 7F0 line are of comparable peak intensity. These features are explained by a specific local symmetry of the Eu3+ ion within the crystal structure of Rb3EuB6O12. Keywords: Rubidium rare earth borate, Solid state reaction, Rietveld refinement, Raman, Infrared spectroscopy, Luminescence

Two dinuclear dysprosium complexes of formulae [Dy2(L1)2(NO3)2(MeOH)2]·2MeOH (1) and Dy2(L2)2(NO3)2(MeOH)2 (2), (H2L1 = 4-chloro-2-(((2-hydroxy-3-methoxybenzyl)imino)methyl)phenol, H2L2 = 2-(((2-hydroxybenzylidene)amino)methyl)-6-methoxyphenol) have been synthesized under solvothermal conditions. The two ligands differ in the substituents at the phenol moieties of the ligands. The purposeful exclusion of the functional group –Cl at the phenol backbone significantly changed the geometries of the metal centers, as evidenced by the X-ray crystallographic analysis. In complex 1, two DyIII ions are in D2d local symmetry, while the DyIII ions of 2 are in D4d local symmetry. Consequently, these two complexes demonstrate distinct magnetic properties. Complexes 1 and 2 both exhibit SMM behavior with energy barriers of 51.97 K for 1 and 87.16 K for 2 under a zero direct-current field. Complete-active-space self-consistent field (CASSCF) calculations were performed on the two Dy2 complexes to rationalize the remarkable observed discrepancy in their magnetic behavior. Theoretical calculations reveal that the angle θ between the magnetic axis and the vector connecting two dysprosium(III) ions is the vital factor for the energy barriers of SMMs. The smaller θ angle of 2 renders it as an SMM with a higher energy barrier. This work demonstrates that tuning the functional groups on the bridging ligands is an effective strategy in modulating the energy barriers of SMMs.

β-NaREF4 (RE: rare earth metals) is a peculiar crystal due to its intriguing disordered structure. This crystal is usually considered to have an average structure with a space group of P-6 (174). However, a long-term contradiction existing in this system is that the luminescence characteristics do not match the corresponding theoretical predictions based on the average structure. To resolve this contradiction, we proposed a reasonable local structure model clearly showing the local symmetry of the cations based on the measurement of the extended X-ray absorption fine structure of β-NaGdF4. We also observed an intrinsic distortion in the disordered structure that resulted from the disordered arrangement of the Na+ and Gd3+ cations. Furthermore, using Eu3+ as a photoluminescence probe, we concluded that the breakdown in the crystallographic site symmetry originated from the intrinsic distortion of these crystals. This work provides a comprehensive explanation for the contradiction observed in this system and a foundation for further investigations to understand such disordered structures.

A series of four β-diketone mononuclear Dy(iii) complexes, namely Dy(Hthd)3(MeOH)·2,5-Py (1), Dy(Hthd)3(Tppo) (2), Dy(Hthd)3(PyNO) (3), and Dy(Hthd)3(4-PyNO) (4) (Hthd = 2,2,6,6-tetramethyl-3,5-heptanedione, 2,5-Py = 2,5-pyridine, Tppo = triphenylphosphine oxide, PyNO = pyridine N-oxide, 4-PyNO = 4-methylpyridine N-oxide), have been isolated by reactions of Hthd, DyCl3·6H2O and an auxiliary ligand. X-ray crystallographic analysis revealed that complexes 1-4 are all seven-coordinated mononuclear structures with C2v or C3v local symmetry. The magnetic measurements of complexes 1-4 revealed their field-induced single-ion magnet (SIM) behavior, due to mixed low-lying states. The crystal field parameters and orientations of the magnetic easy axes were obtained from simulations of the DC magnetic data and an electrostatic model calculation. The magneto-structural correlation of seven-coordinated Dy(iii) SIMs with low-symmetry coordination environments was further discussed.

In this work, a large excess of electrostatic repulsion, arising from the axial ligands, over that from the equatorial ligands is taken as the design strategy for high performance pentagonal bipyramidal (PBP) DyIII single-ion magnets (SIMs). In this strategy, two PBP DyIII-SIMs (1 and 2) [Dy(bbpen-CH3)X] (X = Cl, 1; Br, 2; H2bbpen = N,N′-bis(2-hydroxybenzyl)-N,N′-bis(2-methylpyridyl)ethylenediamine) were synthesized and structurally characterized on the basis of a highly symmetrical ligand H2bbpen-CH3 in which an electron-donating group (–CH3) was installed to favor the conformation necessary for the axial oxygen atom coordinating to DyIII. Dynamic magnetic measurements verify the value of our design strategy since complex 2 exhibits high performance with large Ueff (above 1000 K) and a high magnetic hysteresis temperature (15 K). Ab initio calculations further verified the importance of the high excess of axial interaction which eventually leads to the special electronic structure possessing the desired magnetic properties. The search for excessive axial repulsion is not incompatible with the strategy based on local symmetry around the central ion since various high-performance PBP DyIII-SIMs of both clearly distorted and nearly ideal geometries successfully acquire such a kind of excess. Apparently this study presents an alternative to the designing strategy for promising SIMs.

Many fundamental studies have been conducted on the electrochemical and electronic structures in transition metal cation-substituted LiNi0.5Mn1.5O4 systems. These systems have potential use as 5 V-level high voltage cathode materials for lithium ion batteries, but there are only a few reports regarding the control of their symmetry transitions which contribute the electronic structures and Li+ transport efficiency. We address this solid chemistry and the corresponding electrochemical characteristics using both systematic experimental and theoretical approaches. Trace substitution of Cu2+ with Mn4+ (CuMn) can promote the symmetry transition from Fdm to P4332 phase in oxygen-deficient LiNi0.5Mn1.5O4−δ. This behavior is detectable by Fourier-transform infrared and Raman spectroscopies but undetectable by X-ray diffraction, suggesting that the symmetry transition was localized in the space near the point of extrinsic defects CuMn in the spinel framework. Notably, a very small amount of Cu2+ substitution not only affects the local atomic arrangement but also remarkably affects the macroscopic electrochemical redox responsiveness, including the inactivation of Mn3+/Mn4+ redox couples, suppression of Mn ion dissolution, and enhancing the C rate capability (increasing the electron conductivity, and reducing the activation barrier for lithium ion hopping along the most energetically preferable 8a–16c–8a route) and cyclability.

The coordination geometries of the Co(II) site in the two complexes [Co(imidazole)6][BPh4]2·0.3CH3CN (1) and [Co(imidazole)6][NO3]2 (2) were observed to display the ideal symmetries Ci and D3d, respectively. Both complexes were shown to be field-induced single-ion magnets. The effective energy barrier was found to decrease as the local symmetry changed from low-symmetry Ci to high-symmetry D3d.

The structural evolution and kinetic properties of Ti110Al14V4 alloy under two constant cooling rates are comprehensively studied by ab initio molecular dynamics simulations. A variety of statistical methods are used to progressively analyze the changes in local atomic structure with temperature under different cooling rates. It is revealed that for slow cooling, the icosahedron and face-centered cubic (FCC) cluster dominate in the melt before the system transforms into a body-centered cubic (BCC) crystalline phase at a temperature of about 1600 K. For fast cooling, the system melts from 2600 to 1600 K, then turns into solid glass at 1400 K, and finally forms a crystalline and amorphous mixture. This is a process in which the icosahedron competes with the FCC structure, indicating that the Ti110Al14V4 alloy has a very poor glass-forming ability because of the fragility of Ti110Al14V4 liquids at high temperature. Furthermore, under a fast cooling rate, the self-diffusion coefficient obeys the Arrhenius function at high temperature but conforms to the mode coupling theory when approaching the critical temperature. Finally, through the five-fold local symmetry parameters and viscosity, a direct relationship between structure and dynamics is established, confirming that structural evolution is the root cause of slow dynamics. This discovery provides a new perspective on the structural mechanism of dynamic arrest in the Ti110Al14V4 melt.

We employed the computational modelling technique with crystal field models to indicate the local symmetry of the Eu3+ ion in AF2 (A = Ca, Sr and Ba) crystals. Using a set of Buckingham potential parameters, the lattice parameters and the unit cell volume for all these crystals were reproduced satisfactorily with good fit in comparison with experimental data. Bound extrinsic defects calculations through low solution energy revealed that the Eu3+ occupies the A2+ site compensated by cationic vacancies. The spectroscopic study using the SOM with the MENN showed that the Eu3+ occupies a distorted inversion centre being a distorted S6 symmetry. In this indication, we used a set of phenomenological charge factors that reproduced the 7F1 level splitting and the 7F1, 7F2 and 5D1 energy sublevels.