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International audience; Data of quasi-specular reflection of cold neutrons, prompt-γ neutron analysis, X-ray Raman scattering (XRS), and neutron pair distribution function (PDF) analysis with powder of detonation nanodiamonds are analyzed to collect their structural properties and chemical composition. Both as-synthesized and purified samples were studied using fluorination samples. Removal of both the sp2 amorphous carbon shell and the hydrogen atoms is evidenced respectively by the change of neutron-nuclei optical potentials of nanoparticles and the increase of their neutron reflectivity. Moreover, sp3 diamond cores of nanoparticles stay intact during the fluorination as revealed by similar scattering patterns, PDF, and XRS data. Quasi-specular reflection, PDF, and XRS data are complementary for the study of nanomaterials and in good agreement with conventional characterization techniques (infrared spectroscopy and solid-state NMR).

We report a joint experimental and theoretical study of the structural, vibrational, and electronic properties of layered monoclinic arsenic sulfide crystals (α-As2S3), aka mineral orpiment, under compression. X-ray diffraction and Raman scattering measurements performed on orpiment samples at high pressure and combined with ab initio calculations have allowed us to determine the equation of state and the tentative assignment of the symmetry of many Raman-active modes of orpiment. From our results, we conclude that no first-order phase transition occurs up to 25 GPa at room temperature; however, compression leads to an isostructural phase transition above 20 GPa. In fact, the As coordination increases from threefold at room pressure to more than fivefold above 20 GPa. This increase in coordination can be understood as the transformation from a solid with covalent bonding to a solid with metavalent bonding at high pressure, which results in a progressive decrease of the electronic and optical bandgap, an increase of the dielectric tensor components and Born effective charges, and a considerable softening of many high-frequency optical modes with increasing pressure. Moreover, we propose that the formation of metavalent bonding at high pressures may also explain the behavior of other group-15 sesquichalcogenides under compression. In fact, our results suggest that group-15 sesquichalcogenides either show metavalent bonding at room pressure or undergo a transition from p-type covalent bonding at room pressure towards metavalent bonding at high pressure, as a precursor towards metallic bonding at very high pressure., Authors thank the financial support from Spanish Ministerio de Economia y Competitividad (MINECO) through MAT2016-75586-C4-2/3-P and FIS2017-83295-P and from Generalitat Valenciana under project PROMETEO/2018/123-EFIMAT. ELDS acknowledges the European Union Horizon 2020 research and innovation programme under Marie Sklodowska-Curie for grant agreement No. 785789-COMEX. JAS also acknowledges Ramón y Cajal program for funding support through RYC-2015-17482. AM, SR and ELDS thank interesting discussions with J. Contreras-García who taught them how to analyze the ELF. Finally, authors thank ALBA Light Source for beam allocation at beamline MSPD (Experiment No. 2013110699) and acknowledge computing time provided by MALTA-Cluster and Red Española de Supercomputación (RES) through computer resources at MareNostrum with technical support provided by the Barcelona Supercomputing Center (QCM-2018-3-0032).

International audience; Fluorinated single-layer diamond (“F-diamond”) is a new form of two-dimensional (2D) carbon allotrope. Herein, poly(dicarbon monofluoride) (C2F)n which is essentially made of stacked layers of “F-diamane” has been synthesized and exfoliated in a variety of solvents to yield well-dispersed ultrathin sheets. Microscopic and spectroscopic analyses revealed that the exfoliated nanosheets retained the “F-diamane”-like structure. The experimental results are also supported by density functional theory (DFT) calculations.

International audience; C 2 F graphite fluoride has a unique mutable structure when subjected to external applied pressure. In the present study, we examine C 2 F using X-ray Raman scattering (XRS) up to 6.5 GPa coupled with theoretical simulations. Using the XRS technique, we follow the in situ high pressure evolution of the energy loss corresponding to the C and F K-edge. Significant variations occur at 2.9 GPa, remain up to 6.5 GPa and persist at ambient conditions after decompression. The permanent changes are related to an increased planarity of the graphitic layers and a modulation of the fluorine configuration. The all-electron density of a C 2 F-sp 3 slab obtained from the DFT simulation and the Quantum Theory of Atoms in Molecules (QTAIM), reveals the appearance of bond-critical points between in-plane and out of plane F-F, suggesting an increasingly ionic character of the structure under biaxial isotropic strain. Pressure can be used as an alternative to chemical synthesis for tuning C-F bonding and metastabilizing new intercalated fluorine compounds with potentially improved electrochemical properties.

International audience; Combination of porosity and hydrostaticity during compression is used with a view to explore the energy landscape of germanium. In this work, pressure-induced phase transformations in mesoporous crystalline Ge has been investigated by in situ Raman spectroscopy. A pressure-induced amorphization to a low-density amorphous (LDA) state was observed prior to a reversible polyamorphic transformation between LDA and high-density amorphous states. These pressure-induced transformations show some similarities with the behavior previously reported in nanoparticles. Thermodynamics models developed in the case of nanoparticles are successfully used indicating that, in both cases, the large surface-to-volume ratio leads to an increase of the system energy and that mesoporous materials may be considered as the negative image of a collection of nanoparticles. However, an inhomogeneous stress distribution is expected in porous materials because of it being a network with hyperbolic geometry. A control experiment is presented using a reference bulk germanium sample. The diamond-to-β-tin transformation is observed starting at around 8.0 GPa. On decompression, the metastable ST12 (Ge-III) phase is observed. Ab initio simulations are used to assign and interpret Raman spectra of this phase.

We present a study of the different high-pressure polymorphs of $\mathrm{GdV}{\mathrm{O}}_{4}$ and its stability. Powder x-ray diffraction and Raman experiments show a phase transition from a zircon- to a scheelite-type structure taking place at 6.8(4) GPa. Ab initio density functional theory calculations support this conclusion. The equations of state of these two phases are reported. In addition, we studied the pressure evolution of the Raman modes for the zircon and scheelite phases, showing good agreement between calculations and experiments. For the sake of completeness, we performed optical-absorption measurements up to 16 GPa, showing a band-gap collapse at the transition point. Beyond 20 GPa a second phase transition to a monoclinic fergusonite structure takes place as a consequence of a mechanical instability. A third transition is observed at around 29.3 GPa in Raman experiments. According to our calculations, this fourth polymorph corresponds to an orthorhombic structure described by space group Cmca. This phase involves an increase of the atomic coordination number of vanadium and gadolinium. The results are compared to those reported on isomorphic compounds.

International audience; Following the emergence of two-dimensional (2D) materials, a large amount of work has been dedicated to this class of materials. Numerical simulations have proven to be powerful to analyze and predict structure-properties relationships. However, a recurrent issue that arises in some 2D compounds is the appearance of a dynamical instability as an unstable phonon branch in a small pocket close to the Brillouin zone center. The origin (numerical and/or physical) of this instability is hardly discussed. Here, using a rising 2D material, GaSe, as a case study, this issue is tackled by discussing the numerical techniques that may be used to lift the instability but also by understanding the fundamental origin of it. The interlayer distance is the crucial parameter and the ionicity of the compounds is the key physical property governing the propensity for instability. In many works, this distance is arbitrarily fixed to a value for which the absence of interactions between the periodic images of the layers is assumed. A careful control of the effect of this distance on the stability is required prior to subsequent calculations of physical properties.

International audience; In the present paper, graphite fluoride compounds CxF with various degrees of fluorination (x = 4, 2 and 1), i.e., with different C-F bonding characteristics, were systematically investigated using X-ray Raman Spectroscopy (XRS) and X-ray and neutron diffraction by means of Pair Distribution Function (PDF) analysis. For both techniques, ab-initio calculations allow the experimental results and structural/bonding assignments to be explained and/or confirmed. Presented here for the three stoichiometries are results for the carbon and fluorine first-neighbour distances, the evolution of the electronic band structure due to the change of the carbon hybridization upon fluorination, and also the nature of the final states for 1s core-electron excitations. Since fluorination is limited by the diffusion of fluorine in the carbon lattice, the process is less effective in the bulk, where a graphitic core remains. The cross-checked XRS, PDF data and Density Functional Theory (DFT) simulations appear also to be a powerful method to highlight the presence of a residual graphitic core, even at the highest fluorine content, in such semi-disordered and structurally complex compounds.

International audience; In this paper the structural and vibrational properties of intercalated fluorine graphite (C4F)n under compression up to 40 GPa are investigated by means of Raman spectroscopy, X-ray diffraction and ab initio calculations. The theoretical study was performed within the framework of the density functional theory, taking into account van der Waals corrections to the total energy. We studied the effect of anisotropic and isotropic deformation on the crystal properties, in relation to the layered structure of the sample and in particular to the CF bonding. The evolution with pressure of the phonon modes at the center of the Brillouin zone was calculated and compared with experimental data. Both our experimental data and theoretical models indicate that a structural transition occurs around 10 GPa. Above this pressure the (C4F)n structure becomes dynamically unstable, characterized by a “soft mode” and a change in symmetry due to anisotropic strain components.

In this work we study the pressure behaviour of the orpiment (monoclinic) and anorpiment (triclinic) layered structures of As2S3 by means of ab initio calculations performed within the density functional theory, as part of an ongoing theoretical and experimental joint effort to provide a comprehensive picture of the bonding of this interesting material and the evolution of its structural, electronic, and vibrational properties under pressure. © Published under licence by IOP Publishing Ltd., The authors acknowledge the financial support from the Ministerio de Economía y Competitividad (MINECO) of Spain through Projects No. MAT2013-46649-C04-02-P and MAT2013-46649-C04-03-P. Computer time in the MALTA computer cluster at the University of Oviedo, Spain, is also gratefully acknowledged (MINECO Project No. SD2007-00045).

XPS+EQUIPE-VIDE+LCS; International audience; We studied the high pressure behavior of poly(dicarbon monofluoride) (C2F)n up to 37 GPa both theoretically, within the first-principle framework of the density functional theory, taking into account van der Waals corrections to the total energy, and experimentally, combining in situ Raman spectroscopy, XRD and X-ray photoelectron spectroscopy (XPS) measurements. The (C2F)n structure has a complex mixed sp2 sp3 character where graphene layers are intercalated among sp3 carbon hybridized structural domains. We observed three phase transitions in the high pressure range explored. The first transition is observed around 12 GPa and is dominated by strain effects. Above 16 GPa the intercalated graphene layers corrugate and change from the sp2 to sp3 hybridization state. Above 25–27 GPa the new pressure-induced phase becomes unstable. We find that, by varying applied pressure or by changing the hybridization due to the fluorination of graphite, the band structure of (C2F)n can be modified, resulting in valence and conduction bands touching at the K point, as in graphene, thus turning (C2F)n into a semimetal. This could provide a new pathway to reversibly engineer the band structure and conductivity for applications in high energy density batteries.

Recent experimental measurements and ab initio calculations in scheelite CaWO 4 , SrWO 4 , BaWO 4 , PbWO 4 , EuWO 4 and YLiF4 crystals reveal the existence of complex high-pressure phase diagrams, which present striking differences but also relevant similarities. In this work we show that the high-pressure structural sequence in the studied scheelites can be understood on the basis of the positions of the different ABX4 compounds in Fukunaga and Yamaoka's diagram and in Bastide's diagram. Our study can help to understand the phase diagrams and high-pressure phase transitions occurring in ABX4 compounds with scheelite, wolframite, fergusonite, zircon, or pseudoscheelite structures.

The structural properties of CaWO{sub 4}, SrWO{sub 4}, BaWO{sub 4}, PbWO{sub 4}, and EuWO{sub 4} scintillating crystals under pressure have been studied by X-ray powder diffraction, X-ray absorption near-edge structure measurements, Raman spectroscopy, and ab initio density functional theory calculations. The results obtained from these studies will be reviewed here and their differences and similitudes discussed.

In this paper, we combine a theoretical study of the structural phases of CaWO 4 and SrWO 4 under high pressure along with the results of angle-dispersive X-ray diffraction (ADXRD) and X-ray absorption near-edge structure (XANES) measurements of both tungstates up to approximately 20 GPa. The theoretical study was performed within the ab initio framework of the density functional theory (DFT) using a plane-wave basis set and the pseudopotential scheme, with the generalized gradient approximation (GGA) for the exchange and correlation contribution to the energy. Under normal conditions, CaWO 4 and SrWO 4 crystallize in the scheelite structure. Our results show that in a hydrostatic environment, both compounds undergo a scheelite-to-fergusonite phase transition with increasing pressure. We present a comparison of the evolution of the structural parameters, equation of state, and of the features of the transition, finding an overall excellent agreement between the experimental and theoretical results.

In this work we summarize the results of first-principles theoretical studies of the sequence of pressure-driven phase-transitions followed by the group-IVa elements Si and Ge. The agreement with the available experimental data is very good.

The equations of state and phase stability of calcium chalcogenides, CaS, CaSe and CaTe are investigated via first principles theoretical calculations. We report results of the structural properties of these compounds in the B1(NaCl) and the B2(CsCl) phases, the theoretical transition pressure and the volumes compression ratio. For CaTe we investigate the possible existence of an intermediate phase between the B1 and the B2 structure.

We acknowledge the financial support of the Spanish Ministerio de Economia y Competitividad under Grants MAT2010-21270-C04-02/03/04, CTQ2009-14596-C02-01, CSD2007-00045 and the Comunidad de Madrid and European Social Fund S2009/PPQ-1551-4161893. Access to the MALTA Cluster Computer (Universidad de Oviedo), the Atlante Super-computer (Instituto Tecnologico de Canarias, Red Espanola de Supercomputacion), and the MALTA Xcalibur Diffractometer (Universidad Complutense de Madrid) is gratefully acknowledged. C. G. A. wishes to thank the Agencia Canaria de Investigacion, Innovacion y Sociedad de la Informacion, and the European Social Fund of the Gobierno de Canarias for a fellowship. J.A.S. acknowledges financial support through the Juan de la Cierva fellowship program.

We report the results of a first-principles theoretical study of the high-pressure phase diagram of Si and Ge, with a particular emphasis on the Cmca and r8 phases which have been recently discovered in Si. Our results for these phases are in excellent agreement with the available experimental observations in the case of Si and we predict the formation of similar phases in Ge.

We report a first-principles study of high-pressure phases of Ge finding that a Cmca phase is stable in the pressure range 91-155 GPa, while at higher pressures the hcp phase has lower enthalpy. The structural parameters of the Cmca phase are similar to those of other Cmca phases found in Si (Si-VI) and Cs (Cs-V). Our results challenge the experimental report of a transition from the simple hexagonal phase to a double hexagonal close-packed phase in this material.

The Rhodes-Wohlfarth parameter extended to ferroelectrics by Tokunaga [J. Phys. Soc. Jap. 57, 4275 (1988)] is here analyzed within the Ф4 model. It is shown that it can be directly related with the displacive degree of the transition as described by the ratio C / E 0 , between the non-local coupling, C, driving the transition and the depth of the energy well, E0, associated with the distorted structure. However, the Rhodes-Wohlfarth parameter becomes asymptotically constant as C / E 0 decreases, i.e. for systems closer to the order disorder limit. Under this viewpoint, the very limited range of values observed for this experimental parameter is explained and is shown that, in general, it can only assess quantitatively the character of the transition in rather displacive cases. The argument can be generalized to more complex systems, and when applied to well known materials, a rough estimation of the displacive degree and the relevant microscopic energetic parameters in rather displacive ferroelectrics is possible.

We summarize the results of a pseudopotential study of the high-pressure stability and electronic structure of the “cinnabar” structure (space group P3121) in several III–V compounds, carried out within the framework of the ab initio density-functional theory with the local density approximation. Here we particularly focus on the dependence of the total energy of this structure on the internal parameters, and relate the stability of the form of the cinnabar structure observed experimentally in GaAs — and theoretically predicted for other III–V compounds on the basis of our results — to an increase of symmetry (to space group P6222) at a certain special value of the internal parameters. The agreement with the available experimental results for “cinnabar”-GaAs is very good.

We present a summary of the results of an ab initio total energy study of the high-pressure phase diagrams of AlP, AlAs, and AlSb, calculated within the local-density-functional theory implemented within a pseudopotential scheme. Besides including structures considered in previous theoretical studies (zinc-blende, rock-salt, CsCl, NiAs, and β-tin) we have also considered other structures such as Cmcm, cinnabar, and sc16, which have recently been found to be stable or metastable at high pressures in several III–V and II–VI compounds. Our results support the stability of a Cmcm phase in a certain range of pressure for each of the compounds studied.

The range of validity of Landau free-energy potentials with the usual approximation of constant coefficients for terms higher than quadratic has recently been questioned (J. Phys. R Condens. Matter 1 (1989) 8327). The frequent observation in real systems, within large temperature intervals, clearly outside any possible critical region, of power laws of the type mod Tc-T mod b for the order parameter has also been pointed out as an indication that certain simple general features in phase transitions, not related at all to critical phenomena, are beyond the usual approximations included in a Landau free-energy expansion. In particular, the value of the exponent h has been proposed to be related to the displacive and order-disorder degree of the system. In order to elucidate these questions, the temperature dependence of the Landau free energy corresponding to the three-dimensional Phi 4 model has been investigated using a straightforward Monte Carlo method. Different model parameters have been considered, ranging from typical displacive parameters to those approaching a pure order-disorder system. Following its formal definition, the Landau free energy at each temperature has been directly derived from the order parameter distribution in a Metropolis statistical sample. In contrast with other numerical methods used in previous literature, no approximation is introduced in the calculation, except inherent to the numerical method employed. It is shown that the temperature dependence of the Landau potential coefficients follows smooth simple laws that are outside the usual assumptions in Landau theory and can be related to the order-disorder degree of the system. The quadratic coefficient in the Landau potential exhibits a linear temperature dependence in large temperature intervals but shows a marked change in slope about the transition temperature. The quartic coefficient is shown to depend on temperature as strongly as the quadratic coefficient having a minimum around the transition point. The strong temperature dependence of this quartic coefficient is responsible for the 'non-classical' behaviour of the order parameter, which can be described by a power law.

The compression behavior of zircon-type samarium orthovanadate, SmVO4, has been investigated using synchrotron-based powder x-ray diffraction and ab-initio calculations up to 21 GPa. The results indicate the instability of ambient zircon phase at around 6 GPa, which transforms to a high-density scheelite-type phase. The high-pressure phase remains stable up to 21 GPa, the highest pressure reached in the present investigations. On pressure release, the scheelite phase is recovered. Crystal structure of high-pressure phase and equations of state (EOS) for the zircon- and scheelite-type phases have been determined. Various compressibilities such as bulk, axial and bond, estimated from the experimental data are found to be in good agreement with the results obtained from theoretical calculations. Calculated elastic constants show that the zircon structure becomes mechanically unstable beyond the transition pressure. Overall there is good agreement between experimental and theoretical findings., 13 pages, 12 figures, 3 tables

The electronic band-structure and band-gap dependence on the $d$ character of ${A}^{2+}$ cation in $A$WO${}_{4}$ wolframite-type oxides is investigated for different compounds ($A$ $=$ Mg, Zn, Cd, and Mn) by means of optical-absorption spectroscopy and first-principles density-functional calculations. High pressure is used to tune their properties up to 10 GPa by changing the bonding distances establishing electronic to structural correlations. The effect of unfilled $d$ levels is found to produce changes in the nature of the band gap as well as its pressure dependence without structural changes. Thus, whereas Mg, Zn, and Cd, with empty or filled $d$ electron shells, give rise to direct and wide band gaps, Mn, with a half-filled $d$ shell, is found to have an indirect band gap that is more than 1.6 eV smaller than for the other wolframites. In addition, the band gaps of MgWO${}_{4}$, ZnWO${}_{4}$, and CdWO${}_{4}$ blue-shift linearly with pressure, with a pressure coefficient of approximately 13 eV/GPa. However, the band gap of multiferroic MnWO${}_{4}$ red-shifts at \ensuremath{-}22 meV/GPa. Finally, in MnWO${}_{4}$, absorption bands are observed at lower energy than the band gap and followed with pressure based on the Tanabe-Sugano diagram. This study allows us to estimate the crystal-field variation with pressure for the MnO${}_{6}$ complexes and how it affects their band-gap closure.

The mercury chalcogenides HgSe and HgTe show peculiar behavior under moderate compression such as the unusual observation of a ``hidden'' transition from their ambient-pressure zincblende phase to an orthorhombic metastable phase with space group $C{222}_{1}$. We present here the results of an ab initio pseudopotential study within the framework of the density-functional theory of the stability of the low-pressure zincblende phases of HgSe and HgTe and the transition to the $\text{C}{222}_{1}$ structure. We relate the observation of the $\text{C}{222}_{1}$ phase to an instability in the transverse-acoustic phonon branches of the zincblende phases of these materials. We have studied in detail the mechanism of the $\text{zincblende}\ensuremath{\rightarrow}\text{C}{222}_{1}$ transition as well as the structural evolution of the $\text{C}{222}_{1}$ phase under pressure.

Room-temperature Raman scattering was measured in ${\text{CdWO}}_{4}$ up to 43 GPa. We report the pressure dependence of all the Raman-active phonons of the low-pressure wolframite phase. As pressure increases changes in the Raman spectra are detected at 20 and 35 GPa due to the onset of reversible structural phase transitions. We also report ab initio total-energy and lattice-dynamics calculations for the different phases of ${\text{CdWO}}_{4}$. They helped us determine the crystalline structure of the high-pressure phases. Experimental and theoretical results suggest the coexistence of two structures from 20 to 35 GPa: one with tetragonal symmetry and another with triclinic symmetry. Beyond 35 GPa the monoclinic $\ensuremath{\beta}$ fergusonite is proposed as the structure of ${\text{CdWO}}_{4}$. The pressure evolution of the lattice parameters and the atomic positions of wolframite ${\text{CdWO}}_{4}$ are also theoretically calculated and an equation of state is reported.

The room-temperature Raman scattering was measured in ${\text{ZnWO}}_{4}$ up to 45 GPa. We report the pressure dependence of all the Raman-active phonons of the low-pressure wolframite phase. As pressure increases additional Raman peaks appear at 30.6 GPa due to the onset of a reversible structural phase transition to a distorted monoclinic $\ensuremath{\beta}$-fergusonite-type phase. The low-pressure and high-pressure phases coexist from 30.6 to 36.5 GPa. In addition to the Raman measurements we also report ab initio total-energy and lattice-dynamics calculations for the two phases. These calculations helped us to determine the crystalline structure of the high-pressure phase and to assign the observed Raman modes in both the wolframite and $\ensuremath{\beta}$-fergusonite phases. Based upon the ab initio calculations we propose the occurrence of a second phase transition at 57.6 GPa from the $\ensuremath{\beta}$-fergusonite phase to an orthorhombic $Cmca$ phase. The pressure evolution of the lattice parameters and the atomic positions of wolframite ${\text{ZnWO}}_{4}$ are also theoretically calculated, and an equation of state reported.

The compression process in the α-phase of europium trimolybdate was revised employing several experimental techniques. X-ray diffraction (using synchrotron and laboratory radiation sources), Raman scattering and photoluminescence experiments were performed up to a maximum pressure of 21 GPa. In addition, the crystal structure and Raman mode frequencies have been studied by means of first-principles density functional based methods. Results suggest that the compression process of α-Eu2(MoO4)3 can be described by three stages. Below 8 GPa, the α-phase suffers an isotropic contraction of the crystal structure. Between 8 and 12 GPa, the compound undergoes an anisotropic compression due to distortion and rotation of the MoO4 tetrahedra. At pressures above 12 GPa, the amorphization process starts without any previous occurrence of a crystalline-crystalline phase transition in the whole range of pressure. This behavior clearly differs from the process of compression and amorphization in trimolybdates with β′-phase and tritungstates with α-phase., We thank Diamond Light Source for access to beamline I15 (EE1746) that contributed to the results presented here. Part of the diffraction measurements were performed at the 'Servicio Integrado de Difraccion de Rayos X (SIDIX)' of University of La Laguna. This work has been supported by Ministerio de Economia y Competitividad of Spain (MINECO) for the research projects through the National Program of Materials (MAT2010-21270-C04-01/02/03/04, MAT2013-46649-C41/2/3/4-P and MAT2013-43319-P), the Consolider-Ingenio 2010 MALTA (CSD2007-00045), the project of Generalitat Valenciana (GVA-ACOMP/2014/243) and by the European Union FEDER funds. C Guzman-Afonso wishes to thank ACIISI and FSE for a fellowship. J A Sans thanks the FPI and 'Juan de la Cierva' programs for fellowships.

Room-temperature Raman scattering has been measured in lead tungstate up to 17 GPa. We report the pressure dependence of all the Raman modes of the tetragonal scheelite phase PbWO4-I or stolzite, space group I41 /a, which is stable at ambient conditions. Upon compression the Raman spectrum undergoes significant changes around 6.2 GPa due to the onset of a partial structural phase transition to the monoclinic PbWO4-III phase space group P21 /n. Further changes in the spectrum occur at 7.9 GPa, related to a scheelite-to-fergusonite transition. This transition is observed due to the sluggishness and kinetic hindrance of the I → III transition. Consequently, we found the coexistence of the scheelite, PbWO4-III, and fergusonite phases from 7.9 to 9 GPa, and of the last two phases up to 14.6 GPa. We have performed ab initio latticedynamics calculations, which have greatly helped us in assigning the Raman modes of the three phases and discussing their pressure dependence. The Raman modes of the free WO4 molecule are discussed.

We report the results of both angle-dispersive x-ray diffraction and x-ray absorption near-edge structure studies in $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4}$ and $\mathrm{Pb}\mathrm{W}{\mathrm{O}}_{4}$ at pressures of up to $56\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $24\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, respectively. $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4}$ is found to undergo a pressure-driven phase transition at $7.1\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ from the tetragonal scheelite structure (which is stable under normal conditions) to the monoclinic fergusonite structure whereas the same transition takes place in $\mathrm{Pb}\mathrm{W}{\mathrm{O}}_{4}$ at $9\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. We observe a second transition to another monoclinic structure which we identify as that of the isostructural phases $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4}\text{\ensuremath{-}}II$ and $\mathrm{Pb}\mathrm{W}{\mathrm{O}}_{4}\text{\ensuremath{-}}III$ (space group $P{2}_{1}∕n$). We have also performed ab initio total-energy calculations which support the stability of this structure at high pressures in both compounds. The theoretical calculations further find that upon increase of pressure the scheelite phases become locally unstable and transform displacively into the fergusonite structure. The fergusonite structure is, however, metastable and can only occur if the transition to the $P{2}_{1}∕n$ phases were kinetically inhibited. Our experiments in $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4}$ indicate that it becomes amorphous beyond $47\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$.

Angle-dispersive x-ray-diffraction and x-ray-absorption near-edge structure measurements have been performed on ${\mathrm{CaWO}}_{4}$ and ${\mathrm{SrWO}}_{4}$ up to pressures of approximately 20 GPa. Both materials display similar behavior in the range of pressures investigated in our experiments. As in the previously reported case of ${\mathrm{CaWO}}_{4}$, under hydrostatic conditions ${\mathrm{SrWO}}_{4}$ undergoes a pressure-induced scheelite-to-fergusonite transition around 10 GPa. Our experimental results are compared to those found in the literature and are further supported by ab initio total-energy calculations, from which we also predict the instability at larger pressures of the fergusonite phases against an orthorhombic structure with space group Cmca. Finally, a linear relationship between the charge density in the $A{\mathrm{O}}_{8}$ polyhedra of $AB{\mathrm{O}}_{4}$ scheelite-related structures and their bulk modulus is discussed and used to predict the bulk modulus of other materials, like hafnon.

A landau expansion of the free energy has been used to calculate the temperature behaviour of the specific heat of the incommensurate phase of (NH4)2BeF4 in the vicinity of the incommensurate → commensurate transition. The proposed expansion includes a secondary order parameter along with sixth order terms in the primary order parameter, and accurately predicts the anomalous jump of the observed specific heat at the transition temperature. The calculated curve for cp is in good agreement with the available experimental data.

The optical-absorption edge of single crystals of CaWO4, SrWO4, BaWO4, and PbWO4 has been measured under high pressure up to ~20 GPa at room temperature. From the measurements we have obtained the evolution of the band-gap energy with pressure. We found a low-pressure range (up to 7-10 GPa) where alkaline-earth tungstates present a very small Eg pressure dependence (-2.1 < dEg/dP < 8.9 meV/GPa). In contrast, in the same pressure range, PbWO4 has a pressure coefficient of -62 meV/GPa. The high-pressure range is characterized in the four compounds by an abrupt decrease of Eg followed by changes in dEg/dP. The band-gap collapse is larger than 1.2 eV in BaWO4. We also calculated the electronic-band structures and their pressure evolution. Calculations allow us to interpret experiments considering the different electronic configuration of divalent metals. Changes in the pressure evolution of Eg are correlated with the occurrence of pressure-induced phase transitions. The band structures for the low- and high-pressure phases are also reported. No metallization of any of the compounds is detected in experiments nor is predicted by calculations., 26 pages, 1 table, 6 figures

SbPO4 is a complex monoclinic layered material characterized by a strong activity of the non-bonding lone electron pair (LEP) of Sb. The strong cation LEP leads to the formation of layers piled up along the a-axis and linked by weak Sb-O electrostatic interactions. In fact, Sb is 4-fold coordination with O similar to what occurs with the P-O coordination, despite the large difference of ionic radii and electronegativity between both elements. Here we report a joint experimental and theoretical study of the structural and vibrational properties of SbPO4 at high pressure. We show that SbPO4 is not only one of the most compressible phosphates but also one of the most compressible compounds of the ABO4 family. Moreover, it has a considerable anisotropic compression behavior with the largest compression occurring along a direction close to a-axis and governed by the compression of the LEP and the weak inter-layer Sb-O bonds. The strong compression along the a-axis leads to a subtle modification of the monoclinic crystal structure above 3 GPa leading from a 2D to a 3D material. Moreover, the onset of a reversible pressure-induced phase transition is observed above 9 GPa, which is completed above 20 GPa. We propose that the high-pressure phase is a triclinic distortion of the original monoclinic phase. The understanding of the compression mechanism of SbPO4 can aid in understanding the importance of the ion intercalation and catalytic properties of this layered compound., Main text - 43 pages Supplementary Information - 13 pages