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The emerging CdTe-BeTe semiconductor alloy that exhibits a dramatic mismatch in bond covalency / stiffness clarifying its vibrational-mechanical properties is used as a benchmark to test the limits of the percolation model (PM) worked out to explain the complex Raman spectra of the related but less contrasted Zn1‑xBex‑chalcogenides. The test is done by way of experiment (x< 0.11) – combining Raman scattering with X-ray diffraction at high pressure – and ab initio calculations (x~0 – 0.5, x~1). The (macroscopic) bulk modulus B0 drops below the CdTe value on minor Be incorporation, at variance with a linear B0 vs. x increase predicted ab initio, thus hinting at large anharmonic effects in the real crystal. Yet, no anomaly occurs at the microscopic (bond) scale as the regular bimodal PM-type Raman signal predicted ab initio for Be-Te in minority (x~0, 0.5) is (barely) detected experimentally. At large Be content (x~1) the same bimodal signal “relaxes” down to inversion – an unprecedented case. However, specific pressure dependencies of the regular (x~0, 0.5) and inverted (x~1) Be-Te Raman doublets are in line with PM-predictions. Hence, the PM applies as such to Cd1‑xBexTe – albeit in a “relaxed” form, without further refinement. This enhances the model’s validity as a generic descriptor of phonons in alloys.

The mechanical properties and stability of metal nanoparticle colloids under high-pressure conditions are investigated by means of optical extinction spectroscopy and small-angle X-ray scattering (SAXS), for colloidal dispersions of gold nanorods and gold nanospheres. SAXS allows us to follow in situ the structural evolution of the nanoparticles induced by pressure, regarding both nanoparticle size and shape (form factor) and their aggregation through the interparticle correlation function S(q) (structure factor). The observed behavior changes under hydrostatic and nonhydrostatic conditions are discussed in terms of liquid solidification processes yielding nanoparticle aggregation. We show that pressure-induced diffusion and aggregation of gold nanorods take place after solidification of the solvent. The effect of nanoparticle shape on the aggregation process is additionally discussed. We thank Professor Jan Dhont for helpful comments about nanoparticle diffusion in solid ethanol. F.R. acknowledges financial support from Projects PID2021-127656NB-I00 and MALTA-Consolider Team (RED2018-102612-T), and L.M.L.-M. from PID2020-117779RB-I00 and MDM-2017-0720, from the State Research Agency of Spain, Ministry of Science and Innovation. C.M.-S. acknowledges funding from the Spanish Ministry of Universities and the European Union-NextGeneration EU through the Margarita Salas research grant (C21.I4.P1). We acknowledge SOLEIL for the provision of synchrotron radiation facilities, and we would like to thank Dr. Javier Pérez, beamline supervisor, for assistance in using beamline SWING (proposals 20191731 and 20210678). This work benefited from the use of the SasView application, originally developed under NSF award DMR-0520547. SasView contains code developed with funding from the European Union’s Horizon 2020 research and innovation program under the SINE2020 project, grant agreement no. 654000.

We have studied the amorphization process of SnI

BaCoX 2O7 (X = As, P) are built on magnetic 1D units in which strong aperiodic undulations originate from incommensurate structural modulations with large atomic displacive amplitudes perpendicular to the chain directions, resulting in very unique multiferroic properties. High-pressure structural and vibrational properties of both compounds have been investigated by synchrotron X-ray powder diffraction and Raman spectroscopy at room temperature and combined with density functional calculations. A structural phase transition is observed at 1.8 GPa and 6.8 GPa in BaCoAs2O7 and BaCoP2O7, respectively. Sharp jumps are observed in their unit-cell volumes and in Raman mode frequencies, thus confirming the first-order nature of their phase transition. These transitions involve the disappearance of the modulation from the ambient-pressure polymorph with clear spectroscopic fingerprints, such as reduction of the number of Raman modes and change of shape on some peaks. The relation between the evolution of the Raman modes along with the structure are presented and supported by density functional theory structural relaxations.

International audience; The density and compressibility of nanoscale gold (both nanospheres and nanorods) and microscale gold (bulk) were simultaneously studied by X-ray diffraction with synchrotron radiation up to 30 GPa. Colloidal stability (aggregation state and nanoparticle shape and size) in both hydrostatic and nonhydrostatic regions was monitored by small-angle X-ray scattering. We demonstrate that nonhydrostatic effects due to solvent solidification had a negligible influence on the stability of the nanoparticles. Conversely, nonhydrostatic effects produced axial stresses on the nanoparticle up to a factor 10× higher than those on the bulk metal. Working under hydrostatic conditions (liquid solution), we determined the equation of state of individual nanoparticles. From the values of the lattice parameter and bulk modulus, we found that gold nanoparticles are slightly denser (0.3%) and stiffer (2%) than bulk gold: V 0 = 67.65(3) Å 3 , K 0 = 170(3)GPa, at zero pressure.

BaCoX

Using X-ray emission spectroscopy, we find appreciable local magnetic moments until 30 GPa to 40 GPa in the high-pressure phase of iron; however, no magnetic order is detected with neutron powder diffraction down to 1.8 K, contrary to previous predictions. Our first-principles calculations reveal a “spin-smectic” state lower in energy than previous results. This state forms antiferromagnetic bilayers separated by null spin bilayers, which allows a complete relaxation of the inherent frustration of antiferromagnetism on a hexagonal close-packed lattice. The magnetic bilayers are likely orientationally disordered, owing to the soft interlayer excitations and the near-degeneracy with other smectic phases. This possible lack of long-range correlation agrees with the null results from neutron powder diffraction. An orientationally disordered, spin-smectic state resolves previously perceived contradictions in high-pressure iron and could be integral to explaining its puzzling superconductivity.

International audience; Metallization and dissociation are key transformations in diatomic molecules at high densities particularly significant for modeling giant planets. Using X-ray absorption spectroscopy and atomistic modeling, we demonstrate that in halogens, the formation of a connected molecular structure takes place at pressures well below metallization. Here we show that the iodine diatomic molecule first elongates by ∼0.007 Å up to a critical pressure of Pc ∼ 7 GPa, developing bonds between molecules. Then its length continuously decreases with pressure up to 15–20 GPa. Universal trends in halogens are shown and allow us to predict for chlorine a pressure of 42 ± 8 GPa for molecular bond-length reversal. Our findings contribute to tackling the molecule invariability paradigm in diatomic molecular phases at high pressures and may be generalized to other abundant diatomic molecules in the universe, including hydrogen.

International audience; Pressure- and temperature-dependent Raman scattering in GeSe, SnSe, and GeTe for pressures beyond 50 GPa and for temperatures ranging from 78 to 800 K allow us to identify structural and electronic phase transitions, similarities between GeSe and SnSe, and differences with GeTe. Calculations help to deduce the propensity of GeTe for defect formation and the doping that results from it, which gives rise to strong Raman damping beyond anomalous anharmonicity. These properties are related to the underlying chemical bonding and consistent with a recent classification of bonding in several chalcogenide materials that puts GeTe in a separate class of “incipient” metals.

International audience; We have studied the amorphization process of SnI 4 up to 26.8GPa with unprecedented experimental details by combining Sn and I K edge X-ray absorption spectroscopy and powder X-ray diffraction. Standards and reverse Monte Carlo extended X-ray absorption fine structure (EXAFS) refinements confirm that the SnI 4 tetrahedron is a fundamental structural unit that is preserved through the crystalline phase-I to crystalline phase-II transition about 7 to 10GPa and then in the amorphous phase that appears above 20GPa. Up to now unexploited Iodine EXAFS reveals to be extremely informative and confirms the formation of iodine iodine short bonds close to 2.85Å in the amorphous phase at 26.8 GPa. A coordination number increase of Sn in the crystalline phase-II appears to be excluded, while the deformation of the tetrahedral units proceeds through a flattening that keeps the average I-Sn-I angle close to 109.5°. Moreover, we put in evidence the impact of pressure on the Sn near edge structure under competing geometrical and electronic effects.

The influence of pressure on the optical properties of nanostructured tetragonal Cu2Sb has been investigated jointly by experimental Raman spectroscopy (RS) and theoretical first-principles calculation based on density functional theory (DFT) varying pressure from zero (ambient condition) to 45 GPa. The DFT determined pressure dependence of the Raman active modes and the mode-Gruneisen parameters for each Raman mode are found to be in excellent agreement with those experimentally determined. Comparison of the pressure evolution of the experimental Raman data with those of DFT has allowed us to make the proper assignment of optical phonon modes.

DL-glutamic acid monohydrate crystal was synthesized from an aqueous solution by slow evaporation technique. The crystal was submitted to high-pressure (1 atm–14.3 GPa) to investigate its vibrational behavior and the occurrence of phase transitions. We performed Raman spectroscopy as probe and through the analysis of the spectra we discovered three structural phase transitions. The first one occurs around 0.9 GPa. In this phase transition, glutamic acid molecules suffer modifications in their conformations while water molecules are less affected. The second phase transition at 4.8 GPa involves conformational changes related to CO2−, NH3+ units and the water molecules, while the third one, between 10.9 and 12.4 GPa, involves motions of several parts of the glutamic acid as well as the water molecules. Considering the dynamic of high pressure, the second phase of DL-glutamic acid monohydrate crystal presented a better stability compared with the second phase of its polymorphs α and β L-glutamic acid. In addition, water molecules seem to play important role on this structural stability. All changes are reversible.

I.J. and A.K. are grateful to the Latvian Council of Science project no. lzp-2018/2-0353 for financial support. The research leading to these results has been partially supported by the project CALIPSOplus under the Grant Agreement No. 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020., High pressure energy-dispersive X-ray absorption spectroscopy is a valuable structural technique, especially, when combined with a nano-polycrystalline diamond anvil cell. Here we present recent results obtained using the dispersive setup of the ODE beamline at SOLEIL synchrotron. The effect of pressure and temperature on the X-ray induced photoreduction is discussed on the example of nanocrystalline CuO. The possibility to follow local environment changes during pressure-induced phase transitions is demonstrated for α-MoO (Formula presented.) based on the reverse Monte Carlo simulations., Horizon 2020 project CALIPSOplus under the Grant Agreement No. 73087; Latvian Council of Science project no. lzp-2018/2-0353; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

Manganese oxide is a prototype of an antiferromagnetic Mott insulator. Here, we investigate the interplay of magnetic ordering and lattice distortion across the N\'eel temperature ${T}_{N}$ under pressure using neutron and x-ray diffraction. We find an increase in ${T}_{N}$ with a rate of $d{T}_{N}/dP=+4.5(5)\phantom{\rule{0.28em}{0ex}}\mathrm{K}/\mathrm{GPa}$, an increase in the rhombohedral distortion $\ensuremath{\alpha}$ by $d\ensuremath{\alpha}/dP=+0.{018}^{\ensuremath{\circ}}/\mathrm{GPa}$, as well as a volume striction which is insensitive to pressure. These results allow retrieving the dependence of the coupling constants ${J}_{1}$ and ${J}_{2}$ on interatomic distances and compare it to first-principles predictions. Antiferromagnetic diffuse scattering was observed up to $\ensuremath{\approx}1.2{T}_{N}$, and long-range magnetic order appears at room temperature at 42 GPa.

International audience; We present a polarization-dependent pure transverse-optic (TO) Raman study of high-quality Cd x Zn 1−x Se single crystals with zincblende (cubic) structures (x 0:3) covering both the phonon and phonon-polariton variants of the TO modes, using suitable backward and near-forward scattering geometries, respectively. Insight into the native phonon regime of the phonon-polaritons is obtained at intermediate composition of the random crystal (x ¼ 0:5, 0:3) and at the (Cd,Zn)-dilute limits (x 0:1; using prototype impurity motifs) by applying ab initio codes to large supercells (64-216 atoms), with special attention to both the Raman intensities and the phonon frequencies. The experimental (Raman) and theoretical (ab initio) results converge onto a percolation-type three-phonon [1 Â (Cd-Se), 2 Â (Zn-Se)] pattern for Cd x Zn 1−x Se. On the practical side, the interplay between the oscillator strengths of the two Zn-Se Raman modes is used to diagnose a pronounced trend toward local clustering in the studied crystals, presumably an early sign of the composition-induced zincblende ⇿ wurtzite structural transition (x 0:3). The deviation from the ideal Zn ⇿ Cd random substitution is estimated by working out a zincblende-version of the percolation model equipped with a relevant order parameter κ. The model is based on a sensitivity of the Zn-Se vibration to its local environment at the second-neighbor scale, independently supported by ab initio calculation of the Raman spectra in their dependence on κ (adjusted by simulated annealing)

Structural and electronic transformation taking place in α-FeOOH goethite have been studied by Fe K-edge x-ray absorption spectroscopy at pressures up to 50 GPa. These studies have shown the symmetrization of FeO6 octahedra coinciding with the Fe3+ high to low spin transition at pressure above ~45 GPa. Our data are in excellent agreement with the results of recent single crystal XRD and Mossbauer spectroscopy studies (Xu et al 2013 Phys. Rev. Lett. 111 175501), supporting the H-bonds symmetrization in iron oxyhydroxide, resulting from the Fe3+ high-to-low spin crossover at above 45 GPa. Our study shows an applicability of the x-ray absorption spectroscopy in a further study of the H-bonds symmetrization phenomenon.

The development of luminescent mechanochromic materials depends mainly on the possibility to rationally design them with the desired properties. Molecular copper iodide clusters constitute an unprecedented family of compounds exhibiting great changes of their luminescence properties upon mechanical stress. From previous studies, the mechanochromic properties of cubane [Cu4I4L4] (L = organic ligand) clusters have been attributed to modifications of cuprophilic interactions induced by mechanical solicitation. In this study, we ascertain our hypothesis by choosing to study the luminescence mechanochromism of a [Cu4I4(PPh3)4] cluster which presents two crystalline polymorphs exhibiting strikingly different Cu–Cu bond lengths. As forecasted, only one of these two polymorphs exhibits mechanochromic properties. Structural and optical characterization methods are reported along with structural characterization under controlled pressure allowing a precise analysis of the structural changes occurring under mechanical stress. In addition to confirming our mechanism based on enhancement of cuprophilic interactions under pressure, this study demonstrates the possibility of prediction of mechanochromic properties in the family of copper iodide compounds that constitutes a step further toward the rational design of stimuli-responsive materials.

The effect of pressure on the Raman spectrum of PbWO4 has been investigated up to 48 GPa in a diamond-anvil cell using neon as pressure-transmitting medium. Changes are detected in the Raman spectrum at 6.8 GPa as a consequence of a structural phase transition from the tetragonal scheelite structure to the monoclinic PbWO4-III structure. Two additional phase transitions are detected at 15.5 and 21.2 GPa to the previously unknown crystalline phases IV and V. The last one remains stable up to 43.3 GPa. At 47.7 GPa all Raman modes disappear, which could be caused by a pressure-induced amorphization. All structural changes are reversible, being the scheelite phase recovered at ambient pressure. However, the two most intense modes of the PbWO4-III phase are still present after full decompression, indicating that this phase coexists as a minority metastable phase with the scheelite phase after pressure release. The wavenumber of the Raman modes and their pressure dependencies are reported for the four crystalline phases. The present reported results are compared with previous studies., 24 pages, 4 figures, 4 tables

DL‑isoleucine single crystals were grown by the slow evaporation method at ambient temperature. Their vibrational properties were studied at ambient temperature as a function of pressure by Raman scattering. At ambient conditions the mode assignment was done in terms of the Potential Energy Distribution (PED) through density functional theory calculations. Both nitrogen and neon were used as pressure transmitting media. The pressure-dependent investigation shows modifications in the Raman spectra recorded between 30 and 3200 cm−1 that were interpreted as phase transitions undergone by the crystal between 1.3 and 1.9 GPa and between 3.6 and 5.1 GPa. Finally, stress was simulated on the unit cell of the crystal from ambient up to 5.0 GPa.

Crystalline polymorphs possess different physical properties, and phase changes between those polymorphs may affect the properties of engineered materials such as drugs. This is very well illustrated by the large effort that is put into the capability to predict phase behaviour of pharmaceuticals to avoid the unexpected appearance of different crystal forms. Much progress has been made, but one of the remaining challenges is (the accuracy in) the prediction of phase behaviour as a function of temperature. Obviously, predictions should at a certain point be verified against experimental data; however, it may not always be easy to elucidate the phase behaviour of a given compound experimentally, because thermodynamically and kinetically controlled phenomena occur in a convoluted fashion in experimental data. The present paper discusses the trimorphism of l-tyrosine ethyl ester as an example case of how experimental data in combination with the thermodynamic tenets lead to a consistent phase diagram, which can be used as the basis for pharmaceutical formulations and for comparison with polymorph predictions by computer. The positions of the two-phase equilibria I-II, I-III, and I-L have been obtained experimentally. Using the Clapeyron equation and the alternation rule, it has been shown how the positions of the other equilibria II-L, III-L, and II-III can be deduced in combination with the stability rankings of the phases and the phase equilibria. The experimental data have been obtained by synchrotron X-ray diffraction, Raman spectroscopy, and thermal analysis as a function of pressure and temperature. Furthermore, laboratory X-ray diffraction as a function of temperature and differential scanning calorimetry have been used. At room temperature, form II is the most stable phase, which remains stable with increasing pressure, as it possesses the smallest specific volume. Form I becomes stable above 33 °C (306 K), but with increasing pressure it turns into form III. On thermodynamic grounds, form III is expected to have a stable domain at very low temperatures.

Phonon dispersion ω(q) and phonon density of states g(ω) of the high-pressure rocksalt phase of InN are calculated ab initio, at different pressures, using the density functional perturbation theory. Born effective charge e ∗ T , LDA values of dielectric con- stant ∞ and of the electronic gap E g are obtained as well. Building on the theoretical knowledge of phonon frequencies and their evolution with pressure, we attempt to interpret the set of six bands found previously in Raman spectra of epitaxial layers and of InN-nanowires.

International audience; Motivated by the prediction of surface Kondo breakdown in topological Kondo insulators, we investigated the valence and magnetism of SmB6 with x-ray absorption and magnetic circular dichroism at the Sm M 4,5 and L 2 edges with surface and bulk sensitivity, respectively. A higher Sm valence state at the surface and in the bulk under pressure indicates the surface-and pressure-induced bulk Kondo breakdown. We confirmed the different magnetization origin from the surface (f electron) to the bulk (d electron) and ascertained the direct relationship between the disappearance of the Sm L 2-edge x-ray magnetic circular dichroism signal and the closure of the d-f hybridization gap in SmB6 bulk under pressure, paving the way for further investigations of the Kondo insulators. Above P c = 8.5 GPa, a more localized 4f state with higher valence and the disappearing of the 5d magnetism are observed, suggesting the closure of the hybridization gap in the bulk.

Solid ${\mathrm{NH}}_{4}\mathrm{F}$ displays intriguing parallels with ice despite its apparently ionic character. Here we investigate its phase diagram in low-temperature and high-pressure regions using Raman spectroscopy, x-ray diffraction, and ab initio structure search calculations. We focus on the high-pressure cubic phase which resembles that found in ice under pressure and is also the ambient pressure phase of other ammonium halides. We detect a disorder-order transition above 10 GPa, recalling those found both in other ammonium halides and in ice. The transition reveals itself in the pressure dependence of several Raman modes as well as through the progressive splitting of lattice and bending modes of the cubic phase at low temperatures. An in-depth analysis of the Raman modes and their evolution is made.

International audience; Under a high pressure of helium, the volume change of amorphous silica is much smaller than expected from its elastic properties. This is due to helium insertion in the free volume of the glass network. Here, we report spectroscopic experiments using either helium or neon as penetrating pressurizing media and molecular simulation that indicate a relationship between the amount of gas adsorbed and the strain of the network. A generalized poromechanical approach, describing the elastic properties of microporous materials upon adsorption, is shown to successfully describe the physics of deformation of such silica glasses in which the free volume exists only at the sub-nanometer scale.

International audience; The high-pressure behavior of CaWO4 wasanalyzed at room temperature by Raman spectroscopy.Pressure was generated using a diamond-anvil cell and Ne aspressure-transmitting medium. The pressure range of previousstudies has been extended from 23.4 to 46.3 GPa. Theexperiments reveal the existence of two reversible phasetransitions. The first one occurs from the tetragonal scheelitestructure to the monoclinic fergusonite structure and isobserved at 10 GPa. The onset of a previously unknownsecond transition is found at 33.4 GPa. The two high-pressurephases coexist up to 39.4 GPa. The Raman spectra measuredfor the low-pressure phase and the first high-pressure phase areconsistent with previous studies in the pressure range wherecomparison is possible. The pressure dependence of all the Raman-active modes is reported for different phases. We also reporttotal-energy and lattice-dynamics calculations, which determine the occurrence of two phase transitions in the pressure rangecovered by the experiments. The first transition is in full agreement with experiments (scheelite-to-fergusonite). According tocalculations, the second-highest pressure phase has an orthorhombic structure (space group Cmca). Details of this structure, itsRaman modes, and its electronic band structure are given. The reliability of the reported results is supported by the consistencybetween the theoretical and experimental values obtained for transition pressures, phonon frequencies, and phonon pressurecoefficients.

The structural and vibrational properties of SrWO

International audience; Phase-pure samples of a metastable allotrope of silicon, Si-III or BC8, were synthesized by direct elemental transformation at 14 GPa and similar to 900 K and also at significantly reduced pressure in the Na-Si system at 9.5 GPa by quenching from high temperatures similar to 1000 K. Pure sintered polycrystalline ingots with dimensions ranging from 0.5 to 2 mm can be easily recovered at ambient conditions. The chemical route also allowed us to decrease the synthetic pressures to as low as 7 GPa, while pressures required for direct phase transition in elemental silicon are significantly higher. In situ control of the synthetic protocol, using synchrotron radiation, allowed us to observe the underlying mechanism of chemical interactions and phase transformations in the Na-Si system. Detailed characterization of Si-III using X-ray diffraction, Raman spectroscopy, Si-29 NMR spectroscopy, and transmission electron microscopy are discussed. These large-volume syntheses at significantly reduced pressures extend the range of possible future bulk characterization methods and applications.

X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) are two complementary structural techniques. Their combination improves the understanding of the effect of pressure on materials as illustrated by examples taken from studies on different types of materials (semiconductors, molecular solid, ferroelectric perovskite and gas mixture). The introduction of nanopolycrystalline diamonds anvils has extended XAS to high-energy edges with the possibility to use energy-scanning XAS beamlines where XRD can be performed in addition to XAS experiments.

International audience; High-pressure polarized Raman spectra of vitreous silica are measured up to 8 GPa in a diamond-anvil cell atroom temperature. The combined use of either a nonpenetrating pressurizing medium—argon—or a penetrating one—helium, allows one to separate density from stress effects on the Raman frequencies. In the framework of a simple central force model, the results emphasize the distinct role played by the shrinkage of the intertetrahedral angle Si-O-Si and the force-constant stiffening during the compression. The polarization analysis further reveals the existence of an additional isotropic component in the high-frequency wing of the boson peak. The pressure dependence of the genuine boson peak frequency is found to be much weaker than previously reported and even goes through a minimum around 2 GPa in remarkable coincidence with the anomalous compressibility maximum of silica.

International audience; DL-valine crystal was studied by Raman spectroscopy under hydrostatic pressure using a diamond anvilcell from ambient pressure up to 19.4 GPa in the spectral range from 40 to 3300 cm1. Modifications inthe spectra furnished evidence of the occurrence of two structural phase transitions undergone by thisracemic amino acid crystal. The classification of the vibrational modes, the behavior of their wavenumberas a function of the pressure and the reversibility of the phase transitions are discussed.

Near-forward Raman scattering combined with ab initio phonon and bond length calculations is used to study the phonon-polariton transverse optical modes (with mixed electrical and mechanical character) of the II-VI ZnSeS mixed crystal under pressure. The goal of the study is to determine the pressure dependence of the poorly resolved percolation-type Zn-S Raman doublet of the three oscillator [1(Zn-Se),2(Zn-S)] ZnSe68S32 mixed crystal, which exhibits a phase transition at approximately the same pressure as its two end compounds (~14 GPa, zincblende-to-rocksalt), as determined by high-pressure x-ray diffraction. We find that the intensity of the lower Zn-S sub-mode of ZnSe68S32, due to Zn-S bonds vibrating in their own (S-like) environment, decreases under pressure (Raman scattering), whereas its frequency progressively converges onto that of the upper Zn- S sub-mode, due to Zn-S vibrations in the foreign (Se-like) environment (ab initio calculations). Ultimately, only the latter sub-mode survives. A similar phonon freezing was earlier evidenced with the well-resolved percolation-type Be-Se doublet of ZnBeSe [Pradhan et al. Phys. Rev. B 81, 115207 (2010)], that exhibits a large contrast in the pressure-induced structural transitions of its end compounds. We deduce that the above collapse and convergence process is intrinsic to the percolation doublet of a short bond under pressure, at least in a ZnSe-based mixed crystal, and not due to any pressure-induced structural transition., 20 pages, 7 figures

The compressional and shear wave velocities of seven series of hydrous aluminosilicate glasses were measured by Brillouin scattering at room temperature and pressure as a function of water content. The glasses were quenched from high temperature and 0.2 or 0.3 GPa pressure. The dry end-members range from highly polymerized albitic and granitic compositions, to depolymerized synthetic analogues of mantle-derived melts. For each set of glasses, the adiabatic shear and bulk moduli have been calculated from the measured sound velocities and densities. These moduli are linear functions of water content up to 5 wt% H2O, the highest concentration investigated, indicating that both are independent of water speciation in all series. For anhydrous glasses, (∂ V /∂ P )S changes from about −0.3 to −0.7 × 10−15 m3/(Pa·mol) with increasing degree of polymerization. For the same compositions, the partial molar (∂ V /∂ P )S of the water component changes from near zero to −0.8 × 10−15 m3/(Pa·mol) with increasing degree of polymerization, such that dissolved water amplifies the differences in rigidity between the anhydrous glasses. This strong variation indicates that the solubility mechanisms of water depend strongly on silicate composition. A simple linear model reproduces (∂ V /∂ P )S to within measurement uncertainty, with glasses becoming more compressible as NBO/T decreases and as alkalinity increases. The dissolved water component also becomes more compressible as NBO/T decreases, but less compressible as alkalinity increases. In strongly depolymerized structures, water causes an increase in sound velocities even though it induces a decrease in density. The general rule according to which sound velocities correlate with density is, therefore, violated, for in these cases water dissolution results in a decrease of compressibility. Barring any strongly anomalous effect of water on configurational compressibility, the trends observed for glasses should be still more divergent for melts. Consequently, hydrous mafic and ultramafic liquids are predicted to have a very low compressibility. The shear modulus of hydrous glasses varies little with bulk composition or water content, supporting the use of a single composition-independent pre-exponential factor in models of melt viscosity.

Raman spectroscopy was used to investigate the vibrational properties of l -glutamic acid in the β-form in a diamond-anvil cell up to pressures of 21.5 GPa in the spectral region between 20 and 3270 cm −1 . From the analysis of the results it was observed that the crystal undergoes an impressive series of phase transitions in this pressure range. The phase transitions are accompanied by modifications in a number of modes and discontinuities in the wavenumber versus pressure plots, as well as by modifications in the bands associated to both rocking and stretching modes of NH 3 + . Upon decompression to ambient pressure the original spectra were recovered indicating that the phase transitions are reversible.

Numerous investigations have been performed on Ce metal since the discovery of the γ → α phase transformation, where a face-centered cubic structure is believed to collapse isostructurally with a volume change of ∼17%. However, two questions have yet to be answered definitively. First, is the transformation truly isostructural or is the face-centered cubic structure lost in α-Ce due to symmetry breaking? Second, if the transformation is isostructural does the face-centered cubic structure stay in crystallographic orientation through the volume collapse? Here, we use high-pressure and high-temperature X-ray diffraction measurements to examine single and polycrystalline samples of Ce in the vicinity of the γ ↔ α transformation. This was achieved by successive continuous compression and decompression in a diamond anvil cell at temperatures under, at and above the critical point. Our results show that the crystal structure remains face-centered cubic for both the γ and α phases. The results also show that the face-centered cubic structure retains its crystallographic orientation, simply reducing in volume during the γ → α phase transformation. Upon transformation to α, polycrystalline samples show increased diffraction peak broadening, while single crystals show increased streaking. These changes in diffraction can be attributed to increased damage and lattice misorientation from the transformation. Using a simple atomic lattice model, we show that a periodic array of misfit edge dislocation is necessary to accommodate the large volume difference at the γ–α interface and this could act as a source of the edge dislocations needed to produced previously observed deformation bands.

In this work, we have investigated the structural behavior of cuproscheelite up to 33.9 GPa by means of high-pressure single-crystal X-ray diffraction (SXRD) and extended X-ray absorption fine structure (EXAFS). According to EXAFS, beyond 9 GPa a phase transition takes place. On the basis of SXRD, the transition is from the triclinic (P1) structure to a monoclinic (P2/c) structure isotypic to wolframite. The transition implies abrupt changes of CuO6 and WO6 octahedra, but no coordination change. Further, we report the role played by the Jahn–Teller distortion of the CuO6 octahedra on the mechanism of the phase transition as well as the changes in the behavior of the Cu–O bonds for the triclinic and monoclinic phases of CuWO4. A second phase transition was detected at 22.5 GPa. Finally, a low-temperature experiment has been carried out at 100 K. No structural changes were detected for CuWO4, confirming the already known triclinic structure stability at room pressure and low temperature.

Raman spectroscopy investigations of dl-alanine crystal under high pressures have been carried out up to 18.0 GPa. For instance, around 1.0 GPa and between 1.7 and 2.3 GPa changes in the Raman profile were observed and associated to conformational changes of the molecules in the unit cell or to a phase transition accompanied to slight conformational change of the molecule through CH and CH3 groups. Moreover, between 6.0 and 7.3 GPa, the appearance of a new low energy lattice modes and to the splitting of a band assigned to the stretching vibration of the CCH3 moiety were related to a second phase transition. Finally, changes in lattice modes, red shift of the band associated to CCH3 stretching and increasing of line-width of the band associated to the wagging of CO2, between 11.6 and 13.2 GPa, are ascribed to a third phase transition. On release of pressure the original phase was obtained again.

We present the results of x-ray absorption measurements carried out in Zn 1− x Mn x O thin films under high pressure. The Mn environment remains essentially the same for nominal Mn concentrations given by x = 0.05 , 0.1, 0.15 and 0.25. Both the XANES (X-ray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorption Fine Structure) indicate that Mn occupies the Zn site, being surrounded by four oxygen atoms at 2.02±0.01 A. The substitutional hypothesis is reinforced by comparing the differences between the ambient (wurtzite) and high pressure (rocksalt) spectra, which correspond to tetrahedral and octahedral Mn environments.

International audience; We report a combined study of the structural and electronic properties of the spinel-type semiconductor MnIn2S4 under high pressures by means of X-ray diffraction (ADXRD), X-ray absorption (XAS), and op- tical absorption measurements. The three techniques evidence a reversible structural phase transition near 7 GPa, that according to ADXRD measurements is to a double-NaCl structure. XAS measurements evi- dence predominant tetrahedral coordination for Mn in the spinel phase that does not noticeably change with increasing pressure up to the phase transition. XAS measurements indicate that the static disorder in- creases considerably when the sample reverts from the double-NaCl phase to the spinel phase. Optical ab- sorption measurements show that the direct gap of MnIn2S4 exhibits a nonlinear behaviour with a positive pressure coefficient at pressures below 2.5 GPa and a negative pressure coefficient between 2.5 and 7 GPa. The pressure behavior of the bandgap seems to be affected by the defect concentration. The dou- ble-NaCl phase also exhibits a bandgap with a negative pressure coefficient.

International audience; For the development of applications based on mechanochromic luminescent materials, acomprehensive study of the mechanism responsible for the emission changes is required. We report thestudy of a mechanochromic copper iodide cluster under hydrostatic pressure, which allows control ofcrystal packing via modification of the intermolecular interactions. In situ single-crystal powder X-raydiffraction analysis and emission measurements under pressure permit one to establish a directcorrelation between the molecular structure and luminescence properties and, in particular, todemonstrate that cuprophilic interactions are responsible for the stimuli-responsive luminescenceproperties of such multinuclear coordination compounds.

International audience; L-threonine crystal was studied by Raman spectroscopy under pressure in the spectral range from 50 to3300 cm1. The pressure range of a previous study has been extended from 4 to 27.0 GPa. Modificationsin the whole spectrum give us evidence of three structural phase transitions undergone by this aminoacid as well as two conformational change. The classification of the vibrational modes and the behaviorof their frequencies as a function of the pressure are presented.

International audience; Raman spectra of rhombohedral boron monosulfide (r-BS) were measured under pressures up to34 GPa at room temperature. No pressure-induced structural phase transition was observed, whilestrong pressure shift of Raman bands towards higher wavenumbers has been revealed. IR spectroscopyas a complementary technique has been used in order to completely describe the phononmodes of r-BS. All experimentally observed bands have been compared with theoretically calculatedones and modes assignment has been performed. r-BS enriched by 10B isotope was synthesized,and the effect of boron isotopic substitution on Raman spectra was observed and analyzed.

International audience; Amino acid crystals are important systems for investigating chemical bonds of biochemical interest.Raman spectroscopy was used to study the vibrational properties of monohydrated L-asparagine crystalsubmitted to high pressure. Under these conditions amino acids can undergo phase transition and,eventually, as is the case of L-alanine, transform to an amorphous phase. In this work monohydrated Lasparaginewas submitted to pressures up to 30 GPa, twice the pressure for which L-alanine was observedto present the solid–amorphous phase transition. Up to the maximum pressure reached in theexperiments, no evidence of amorphization was found in the present work. However, severalmodifications in the entire spectral range of the Raman spectrum (30–3600 cm1) suggest monohydratedL-asparagine undergoes a solid–solid phase transition at 10 GPa. The reversibility of the phase transitionas well as the behavior of bands associated with hydrogen bond is also given.

Extended X-ray Absorption Fine Structure and Raman studies were performed to follow the structural and vibrational behavior of mechanical alloyed Ni75Se25 mixture, containing trigonal Ni3Se2 nanocrystalline phase, when exposed to high-pressure conditions. An increase in the local structural order with pressure increasing was observed by means of Debye–Waller factor analysis. The relative Ni nearest-neighbours distances (d) were determined as a function of pressure (P). From the fitting of these d × P curves with Murnaghan's equation, the inverse linear compressibility of nanocrystalline phases (Ni3Se2 and Ni) and their derivates were determined. The high-pressure Raman experiments resulted in a poor determination of the phonons dispersion curve for Ni3Se2 phase, but they were very important to confirm the absence of nanocrystalline Se in the mechanical alloyed Ni75Se25 mixture, showing that all Se was consumed to form the Ni3Se2 phase.

Extended x-ray absorption fine structure studies were performed to follow the structural behaviour of FeSe2 and FeSe/Fe7Se8 nanocrystalline phases produced from a mechanically alloyed Fe25Se75 mixture when exposed to high pressure. A phase transition from marcasite FeSe2 to NiAs-type FeSe/Fe7Se8 was previously reported when the milling time was increased from 20 to 62 h; this study proposes to verify the possibility of inducing the same phase transition in high-pressure experiments using a diamond anvil cell.

We report energy dispersive x-ray diffraction experiments carried out in the Te rich side of ZnSexTe1-x alloys with the aim of characterizing the cinnabar phase. In the samples studied (x =0.55, 0.2, 0.1 and 0.0) the cinnabar phase is observed both in the upstroke and downstroke, between either the zincblende and rocksalt phases (x =0.55, 0.2), or between the zincblende and Cmcm phases (x =0.1, 0.0). In ZnSe0.2Te0.8 the rocksalt phase is transformed to the Cmcm phase if additional pressure is applied. The dependence of the cinnabar's lattice parameters with composition is given by a=4.062 — 0.262x and c =9.434 — 0.575x.

Here we show that the longitudinal (L) and transverse (T) phonon densities of states can be extracted from linearly and circularly polarized Raman spectra. A test of the method for a-Si yields results consistent with the L and T densities of states inferred by analogy with the crystalline state. For a-SiO2, for which the density of states is not known, both the L and T densities of states are obtained. The results show that the broad peak at around 70 cm−1, commonly called the boson peak, is almost purely T in nature.

X-ray absorption and diffraction measurements on InAs up to 80 GPa reveal the presence of a high-pressure short-range chemically disordered Pmma structure and shows no evidence of the CsCl phase. The combination of the two techniques proved to be essential to fully describe the new phase. Since Pmma symmetry has recently been shown to be a direct consequence of a phonon instability of CsCl, this result highlights recent theoretical calculations that are able not only to identify instabilities in statically stable phases, but also to predict the occurrence of alternative symmetries.