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We report the oxygen K-edge spectra of ices Ih, VI, VII, and VIII measured with X-ray Raman scattering. The pre-edge and main-edge contributions increase strongly with density, even though the hydrogen bond arrangements are very similar in these phases. While the near-edge spectral features in water and ice have often been linked to hydrogen bonding, we show that the spectral changes in the phases studied here can be quantitatively related to structural changes in the second coordination shell. Density-functional theory calculations reproduce the experimental results and support the conclusion. Our results suggest that non-hydrogen-bonded neighbors can have a significant effect also in the liquid water spectrum. We discuss the implications of the results for the actively debated interpretation of the liquid water spectrum in terms of local structure.

The phase diagram of KNbO3 was investigated by Raman scattering up to 30 GPa and from 10 to 500 K (750 K at 2 GPa). Macroscopic ferroelectricity vanishes on compression and the various ferroelectric solids, rhombohedral, orthorhombic and tetragonal (R–O–T), were found to exist in close domains of the pressure–temperature plane. Specifically the T solid vanishes below 200 K and the R and O solids exist, very likely, down to 0 K. Quantum suppression of the R solid likely occurs below 100 K. The transition to the cubic paraelectric phase is of the displacive type at low temperature whereas it is of the order–disorder type at high temperature. In the cubic solid, first-order Raman scattering, assigned to structural distortion due to dynamical disorder, was observed. The distortion is orthorhombic below 200 K and tetragonal above, which may stem from the rapid decrease with temperature of the Nb ion dwelling time in its allowed sites.

Pressure allows one to explore the energy landscape and the various conformations of macromolecules. This might be one of the most interesting prospects of combining macromolecular crystallography (MX) with pressure perturbation. This paper presents innovations in high-pressure macromolecular crystallography (HPMX). A new-generation diamond anvil cell extends the technical feasibility of HPMX experiments up to about 2.5 GPa (depending on the sample). Thanks to the large useful aperture (82°) provided by this cell and special loading techniques (use of splinter and/or multiple samples), HPMX can now be more readily applied to crystals with very anisotropic habits and/or low-symmetry space groups, as demonstrated by analysis of diffraction data collected at 140 MPa from orthorhombic urate oxidase crystals. The behaviour of hexagonal crystals of an octanucleotide was investigated up to 2 GPa.

The density functional perturbation theory is employed to study the vibrational properties of CuAlO 2 under pressure. The calculations are preformed using the pseudopotential wave method and the local density approximation for the exchange-correlation (XC) potential. The d electrons of Cu are treated as valence states. We present the phonon dispersion curves. Our results are in good agreement with the available experimental Raman scattering experiments. Ab initio calculations show the presence of a dynamical instability, possibly related with the experimentally observed phase transition.

II–VI and II1−x Mn x VI nanocrystals were prepared by the pressure cycle method using the Paris–Edinburgh cell. The recovered samples are nanocrystals in the cubic phase zinc-blend (ZB) structure and were characterized using transmission electron microscopy, electron diffraction, X-ray diffraction and Raman scattering. Transmission electron micrographs show that these nanocrystals are nearly spherical with diameters ranging from 20 to 50 nm depending on the sample under investigation. The Raman scattering measurements confirm the existence of II–VI nanocrystals in the cubic phase (ZB). The magnetic properties of Cd0.5Mn0.5Te nanoparticles were found to vary with the particle size and were different from those observed for the Cd0.5Mn0.5Te bulk initial samples. The χ vs. T data show temperature hysteresis due to spin-glass form, which occurs at T g=21 K, for both the bulk as well as for the recovered nanoparticle samples. The zero-field cooled and field-cooled χ vs. T curves for the nanoparticles showed a ...

Many deep-sea organisms need hydrostatic pressure for their survival. Thus, when retrieved from their natural environment for biological studies that cannot be conducted at the bottom of ocean, they must be returned to high pressure using appropriate pressure vessels. Our studies of organisms living at deep-sea hydrothermal vents lead us to develop such pressure vessels. Here, we present a system of small pressure reactors that were specifically conceived for the study of embryonic development, but may be also used for the study of small organisms, larvae, cells or bacteria. The system consists of two reactors fed by a common pressure line which can be manipulated separately using a set of valves. It is possible to carry out different operations (renewal of water, sampling) without depressurizing the reactors, a feature which proves crucial when carrying out long-term development experiments (over several days). The system has special chambers equipped with sapphire windows that allow the observation of e...

The effect of the hydrostatic pressure on the CO ligand stretch vibration in cytochrome P450cam-CO bound with various substrates is studied by FTIR. The vibration frequency is linearily shifted to lower values with increasing pressure. The slope of the shift gives the isothermal compressibility of the heme pocket and is found to be related to the high-spin state content in an opposite direction to that previously observed from the pressure-induced shift of the Soret band. This opposite behaviour is explained by the dual effect of heme pocket water molecules both on the CO ligand and on electrostatic potentials produced by the protein at the distal side. The latter effect disturbs ligand-distal side contacts which are needed for a specific proton transfer in oxygen activation when dioxygen is the ligand. Their loss results in uncoupled H(2)O(2) formation.

We describe a high-pressure system built to load rare gases (He, Ar, Ne) in various types of diamond anvil cells, at room temperature. These gases are used as pressure transmitting media to obtain the best hydrostatic compression conditions in high-pressure experiments. Optical windows allow control of the loading process. The loading success rate is close to 100% and the initial pressures in the diamond anvil cell are in between 0.2 and 1 GPa. This system can easily be adapted for loading of various gaseous samples, including gas mixtures, which generally cannot be loaded by cryogenic methods.

Recently, it has been shown that CdTe has two sucessive phase transitions over a narrow pressure range at ≅3.5 GPa. In this work, the pressure cycle method using a Paris-Edinburgh cell up to 6 GPa has been applied to CdTe samples in order to obtain recovered CdTe nanocrystals which were characterized by high resolution transmission electron microscopy (HRTEM), electron diffraction and Raman scattering. Such retrieved nanocrystals are nearly spherical, with diameters ranging from 10 to 30 nm, and their structure is zinc-blende (ZB). Their Raman spectra is consistent with the CdTe phonon dispersion curves but reveal a phonon confinement effect.

The first part of this paper presents a short survey on the phase diagram of ice in the pressure range from 2 to 100 GPa from 10 to 300 K. The description is mainly devoted to the problem of the symmetrization of the O—H· · ·O bond, that is, the transformation of ice VII or VIII to ice X. In connection with this introduction, in the second part of the paper new vibrational and structural data obtained from Raman scattering and x-ray diffraction in ice VIII are given and these properties are compared with those of ice VII. Specifically, below 100 K, solid VIII vanishes at 62 GPa to give birth to a state which is the first step to ice X. This transition is marked by a softening of the lowest Raman-active lattice mode and the disappearance of the Raman signal. During the compression of ice VIII the O–H stretching mode wavenumber shows soft mode behaviour. It is found that the expansivity of ice VIII is negative above 10 GPa. Copyright © 2003 John Wiley & Sons, Ltd.

Investigations under pressure and at room temperature of BaTiO 3 and KNbO 3 using angle-dispersive powder X-ray diffraction are reported. Measurements were performed up to 35 and 30 GPa, respectively. The locations of the various transitions are consistent with other investigations. The variation of the cell volume was fitted to a Murnaghan type equation, the resulting bulk modulus B 0 is compared to the one computed from atmospheric Brillouin scattering data. A good agreement is obtained.

We report on the observation of precursor effects of the rhombohedral-to-cubic phase transition in Indium Selenide (InSe) with several experimental techniques. The pressure at which these precursor defects are first observed depends on the sensitivity of the experimental technique. In transport measurements, which are very sensitive to low defect concentrations, precursor effects are observed 5 to 6 GPa below the phase transition pressure whereas in X-ray diffraction measurements precursor effects are only observed 2 GPa below the phase transition pressure. We report optical absorption measurements, in which the precursor effects are shown by the growth and propagation of dark linear defects appearing 3 GPa below the phase transition pressure. On the base of a simple model of the stress field around edge dislocations, we attribute the darkening of the InSe samples to local phase transitions to a high-pressure modification along linear dislocations. These results agree with room-pressure and high-pressure ...

New results on the phase diagram of KNbO 3 determined, from 10 to 650 K up to 30 GPa, with Raman scattering are given. On increasing pressure the Curie temperature T c was found to shift down regularly from 700 K, at atmospheric pressure, to 20 K at ∼25 GPa. Up to 20 GPa, the pressure dependence of T c follows a classical regime described by the Ising model where the dipolar interaction follow a i 2 /v law, where i is the off-center displacement and v the unit cell volume. This behaviour found also for the ice VII-VIII is very likely encountered in a number of (anti)ferroelectric-paraelectric transformations involving positional disorder.

In this paper we describe two experiments, in the near- and mid-infrared, designed to investigate the evolution under pressure of the GaTe refractive index for polarization parallel and perpendicular to the crystallographic c-axis (in the layer plane). The refractive index dispersion for both light polarizations has been determined up to 5.5 GPa. It is found that the refractive index increases faster in the direction perpendicular to the c-axis than along the c-axis. To find out the origin of such a difference we used a Phillips-Van Vechten model and arrived at the conclusion that it is due to the different pressure behaviour of the Penn gap for each polarization.

Raman spectra of MgSiO3-perovskite (Mg-pv) were recorded at simultaneous high-pressure and low-temperature conditions. This allowed to estimate characteristic frequencies (nu(i)) and other mode parameters as a function of both pressure and temperature. The cross derivatives partial derivative(2)nu(i)/partial derivative T partial derivative P were measured for the first time, thus providing new insights into the lattice dynamics of Mg-pv. These parameters are negative for the two lowest frequency modes at 250 and 255 cm(-1) (approximate to-6 X 10(-4) cm(-1) GPa(-1) K-1) and positive for the other modes (+3 X 10(-4) to +5 X 10(-4) cm(-1) GPa(-1) K-1). These measurements were combined with previously published vibrational density of states (VDoS) for deriving entropy, specific heat, thermal pressure, equation of state (EoS), and various thermoelastic parameters of Mg-pv at mantle P-T conditions. The calculations were performed using a general anharmonic formulation including the spectroscopically measured parameters, It is shown that anharmonic effects are relatively small in this compound under geophysically relevant conditions especially for the EoS. The model is used to discuss the discrepancies in the pressure and temperature calibrations in diamond-anvil cells and multianvil presses. Finally, a complete thermodynamic data set for (Mg0.9Fe0.1)SiO3-perovskite is proposed along lower mantle geotherms, It is shown that a pure perovskite lower mantle is unlikely to exist. (C) 2000 Elsevier Science B.V, All rights reserved.

Energy dispersive X-ray diffraction was used at room temperature to investigate the phase diagram of the KNbO 3 perovskite up to 19 GPa. The results confirm the location of the ferroelectric–paraelectric transition at ∽9 GPa and support the transformation of the orthorhombic solid to the tetragonal solid in the 4–5 GPa pressure range. The variation of the cell volume was fitted to a Murnaghan equation of state whose parameters are discussed.

The Raman active modes of CuAlS2 have been investigated at ambient temperature under high pressure using nitrogen gas as pressure transmitting medium up to 16.3 GPa. An assignment of the symmetry of the zone-centered modes is proposed on the basis of their pressure dependence as compared to that of the corresponding modes in the zincblende structure (ZnS). In this sense, a schematic representation of the cubic Brillouin zone showing its high symmetry points and the correlation with the CuAlS2 chalcopyrite lattice modes is presented. The high pressure effect on the phonon frequencies is discussed in terms of the Grineisen parameters. A structural first-order phase transition was observed at about 16.3 GPa. After a pressure cycle was completed, the Raman scattering measurements performed on the retrieved samples gave some information about the size of the crystallites created by the phase transition.

Using angle-dispersive x-ray powder diffraction and image plate detector, the equations of state of H{sub 2}O and D{sub 2}O ice VII, corrected for deviatoric strain, were determined up to 106 GPa, and 60 GPa, respectively. Our data were analyzed on the basis of the universal equation of state of Vinet {ital et al.} This treatment indicates the occurrence of two second- or higher-order phase transitions at about 15 and 66 GPa, in agreement with the pressure ranges where the nature of the proton disorder is expected to change. {copyright} {ital 1997} {ital The American Physical Society}

The Raman spectrum of icosahedral \ensuremath{\alpha}-boron presents a very sharp peak at $525{\mathrm{cm}}^{\ensuremath{-}1}$ that was consistently rejected from the lattice modes but is still unexplained. New Raman scattering experiments under pressure are compared with ab initio lattice dynamics calculations. The very good agreement of the mode frequencies and their pressure coefficients yields unambiguous assignment of all observed features, including the $525{\mathrm{cm}}^{\ensuremath{-}1}$ line which is a highly harmonic librational mode of the icosahedron and mainly involves bond bending. This mode is also identified in the Raman spectrum of other icosahedral boron-rich solids.

A study based on photoluminescence and absorption measurements as a function of temperature and pressure for PbSe nanocrystals with sizes in the range 3-13 nm reveals the influence of size quantum confinement on the observed variation. In the case of the temperature variation, the effective bandgap changes from showing a positive rate of change to showing a negative one (for a quantum dot 3 nm in diameter), which can be accounted for by incorporating a linear variation of the carrier effective masses into a simple calculation of the exciton ground state in the quantum dot. In the case of the pressure variation, we observe a clear inverse correlation between the absolute value of the pressure coefficient and the nanocrystal size, a signature of quantum size confinement, with values changing from -76 to -41 meV GPa⁻¹ for quantum dots ranging from 13 to 3 nm in diameter, respectively, clearly smaller in absolute value than the rate for bulk PbSe (-84 meV GPa⁻¹). We used again the hypothesis of a linear variation of the carrier effective masses with pressure in order to fit this experimental variation quantitatively.

Phase transformations of styrene (C8H8) at room temperature and up to 32 GPa have been studied by visual observations, Fourier transform infrared spectroscopy, and Raman scattering. At room temperature, styrene freezes at 0.3 GPa and undergoes a solid‐to‐solid transformation around 3.5 GPa. Around 15 GPa the extent of the reaction monostyrene→polystyrene becomes significant. At 32 GPa the reaction is nearly completed and, contrary to the general trend, the phenyl group is still stable at this pressure. This uncommon behavior is very likely correlated with the amorphous character of the polystyrene formed under pressure.

The ternary compound CuAlSe2 is a direct energy gap semiconductor (Eg = 2.6 eV at 300 K) crystallizing in the tetragonal chalcopyrite structure. In this work the optical absorption edge and the Raman active modes of CuAlSe2 were measured as a function of pressure up to 30 GPa. The measurements were performed in a membrane diamond-anvil cell at ambient temperature using neon gas as pressure transmitting medium. The direct energy gap (Γ Γ) increases linearly with pressure at the rate of 4.7 × 10−2 eV GPa−1. At 6.7 GPa the character of the fundamental gap changes to pseudodirect (Γ Γ). This gap decreases with pressure at a rate of −2.9 × 10−2 eV GPa−1. The effect of pressure on the phonon frequencies is discussed in terms of the Gruneisen parameters. A first-order structural phase transition was observed at 12 GPa in the upstroke and at 2 GPa in the downstroke.

International audience; The high-pressure behavior of tetragonal SrMoO4 was analyzed by Raman and optical-absorption measurements. Pressures up to 46.1 GPa were generated using diamond-anvil cells and Ne or N2 as quasi-hydrostatic pressure-transmitting media. A reversible phase transition is observed at 17.7 GPa. A second transition is found at 28.8 GPa and the onset of a third one at 44.2 GPa. The pressure dependence of Raman-active modes is reported for the different phases and the pressure evolution of the fundamental band-gap reported for the low-pressure phase. The observed changes in the Raman spectra contradict the structural sequence determined from previous experiments performed under higher non-hydrostaticity. This fact suggests that deviatoric stresses can influence pressure-driven transitions in scheelite-type oxides. We also report total-energy, lattice-dynamics, and band-structure calculations. They reproduce accurately the behavior of the physical properties of the low-pressure phase and predict the occurrence of phase transitions at pressures similar to experimental transition pressures. According to theory, the high-pressure phases have monoclinic and orthorhombic structures, which are much more compact than tetragonal scheelite. Theoretical results and experiments are compared with previous studies.

The stability range at room temperature of the orthorhombic phase of ${\mathrm{KNbO}}_{3}$ was investigated under pressure using optical observation and Raman scattering techniques. It was observed that the ferroelectric orthorhombic solid is stable up to the 9--10 GPa pressure range, where it transforms to the paraelectric cubic solid through a weakly first-order transformation.

We analyze optical absorption, transmission, luminescence, and Raman scattering in $n$-type GaN at hydrostatic pressures up to 30 GPa. The results show freeze-out of free electrons above $\ensuremath{\sim}20\mathrm{GPa}$ due to trapping at levels that are resonant at ambient pressure but become gap states at high pressures. Ab initio calculations show that both the N vacancy and the Ga interstitial undergo this transition at $\ensuremath{\sim}20\mathrm{GPa}$, but the vacancy should be more abundant. The pressure dependence of the yellow luminescence indicates that a transition between a shallow donor and a deep acceptor is involved.

A large optical‐aperture membrane diamond anvil cell designed for infrared spectroscopy is described. The cell offers definite advantages compared to existing systems. Other possibilities concerning x‐ray diffraction analyses with the cells are mentioned.

International audience; High-resolution Brillouin scattering experiments are performed under hydrostatic pressures ranging from 0 to 10 GPa in vitreous silica. The hypersound-attenuation coefficient exhibits a sharp maximum located at 2 GPa, which appears to coincide with the well-known minimum in sound velocity. Two main processes contribute to sound damping: the thermal relaxation of local defects and the anharmonic interactions with thermal vibrations. By analyzing the temperature dependence of the velocity and subtracting the changes produced by the above mechanisms, the unrelaxed velocity is found. It is constant at low temperatures and increases anomalously above a temperature onset.We show that the onset strongly depends on pressure, which is in agreement with a dynamical origin of the structural changes that produce the hardening.

The transition line between ice VII and ice VIII has been determined by Raman scattering for H2O and D2O. Above 15 GPa the transition temperature decreases linearly with pressure, and then drops to 0 K around 62 and 72 GPa for H2O and D2O, respectively. An analysis, using the pseudospin formalism, shows that the linear regime is due to the decrease with pressure of the dipolar interaction, while the proton tunnelling remains negligible. The proton disorder becomes efficient only from 60 GPa for H2O (or 72 for D2O) where it limits the stability range of the ordered phase VIII. Our analysis suggests that ice X, the so‐called ice with hydrogen at the midpoint of the O‐‐‐O bonds, would be obtained above 80 GPa (H2O) or 100 GPa (D2O).

Phase transformations of thiophene (C4H4S) and deuterated thiophene (C4D4S) were studied by visual observations and Fourier‐transform infrared spectroscopy up to 30 GPa. At room temperature, thiophene freezes at 0.4 GPa and undergoes a solid‐to‐solid transition around 6 GPa and probably another one at 20 GPa. It is shown that an irreversible incomplete transition occurs at 16 GPa. The product of the reaction, a white solid, can be recovered at atmospheric pressure. The infrared analysis of the recovered product gives evidence of a polymer formed by coupling between rings derived from thiophene, but with saturated bonds. Physicochemical arguments suggest the formation of a fibrous compound.

In this work we report high-pressure (HP) and high-temperature (HT) ex situ and in situ experiments in ${\text{BaWO}}_{4}$. Starting from powder samples of ${\text{BaWO}}_{4}$, scheelite structure $(I{4}_{1}/a)$, we reached conditions of 2.5--5.5 GPa and 400--1100 K using a Paris-Edinburgh press. The quenched samples were characterized by x-ray diffraction and Raman measurements at ambient conditions. Depending upon the final $P\text{\ensuremath{-}}T$ conditions we found either the scheelite or the monoclinic ${\text{BaWO}}_{4}$-II $(P{2}_{1}/n)$ structure. We also performed HP-HT in situ Raman measurements in a single crystal of ${\text{BaWO}}_{4}$ using a resistive-heated diamond-anvil cell. The transition from scheelite to the ${\text{BaWO}}_{4}$-II phase was observed at 5 GPa for $T=621\text{ }\text{K}$. Ab initio lattice-dynamics calculations have been performed in order to characterize the vibrations of the ${\text{BaWO}}_{4}$-II phase. Finally we carried out in situ powder angle-dispersive x-ray diffraction synchrotron measurements on ${\text{BaWO}}_{4}$ compound following different $P\text{\ensuremath{-}}T$ paths, extending its measured phase diagram in the 2--6 GPa and 300--2000 K range.

We report results of Raman spectroscopy studies on single crystals of ${\text{B}}_{12}{\text{P}}_{2}$ and ${\text{B}}_{12}{\text{As}}_{2}$ under pressure to 120 GPa at 300 K. In the 75--85 GPa range in both compounds several additional modes appear while several other split. These results evidence a transition to a distorted structure with a strong deformation of the icosahedra. The Raman spectra measured at ambient down from 120 GPa show that this transformation is reversible.

Boron seems to be the indispensable ingredient to obtain materials with high hardness. It exists under many different structures. The simplest member of the family, α-boron is based on an arrangement of slightly distorted B12 icosahedra in a rhombohedral unit cell. B12X2 compounds, with X = O, P, As, are based on the α-boron structure, with the X atoms in the inter-icosahedra space. The stability of α-boron has been studied by x-ray diffraction and Raman scattering up to 100 GPa, showing no sign of phase transformation, and by Raman scattering for B12P2 and B12As2 with the observation of phase transformation in the 80 GPa range.

The structural properties of magnetite ${\text{Fe}}_{3}{\text{O}}_{4}$ under pressure have been extensively studied and are well established. This is not the case for its electronic and magnetic properties. We report a high-pressure x-ray magnetic circular dichroism study of magnetite up to 41 GPa. We observe a continuous evolution of the magnetic moment in the whole pressure range, in agreement with a band-structure description of the electronic state of magnetite. We highlight distinct regimes of magnetism. The x-ray absorption spectra do not suggest any major modification in the local structure in this pressure range.

The phase boundary between ice VII and ice VIII of deuterated ice (D2O) was determined by Raman scattering up to 51 GPa. From 2 to 51 GPa, the temperature of the stability range of ice VIII was found to decrease from ∼278 to 158 K.

Visual observations and Fourier-transform infrared spectroscopy were used to investigate thiophene (C4H4S) up to 30 GPa. A second-order phase transition was found around 6 GPa. Above 16 GPa thiophene undergoes an irreversible transformation. The product recovered at ambient exhibits the characteristics of a polymer.

Phase transformations of thiophene (C4H4S) and deuterated thiophene (C4D4S) were studied by visual observations and Fourier‐transform infrared spectroscopy up to 30 GPa. At room temperature, thiophene freezes at 0.4 GPa and undergoes a solid‐to‐solid transition around 6 GPa and probably another one at 20 GPa. It is shown that an irreversible incomplete transition occurs at 16 GPa. The product of the reaction, a white solid, can be recovered at atmospheric pressure. The infrared analysis of the recovered product gives evidence of a polymer formed by coupling between rings derived from thiophene, but with saturated bonds. Physicochemical arguments suggest the formation of a fibrous compound.

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.

We present here a new diamond anvil cell designed to combine high hydrostatic pressure up to 10 GPa, non-destructive, pulsed magnetic fields up to 60 T and low temperature as low as 2 K. Both optical and transport measurements are now achievable. Magneto-photoluminescence set-up under pressure is described in great detail. We report photoluminescence experiments on oriented ruby single crystals exhibiting direct observation of the intrinsic Zeeman splitting of excited states under pressure.

International audience; In this work, we address the question of pressure-induced phase transitions in amorphous germanium.We discuss both the pressurization and depressurization cycles on an amorphous film, showing the appearance of various stable and metastable (LDA–HDA) phases by Raman scattering and X-ray absorption spectroscopy. In particular, we report evidence of recrystallization in the metastable ST12 phase driven by the density of voids of the film.

A detailed study of the pressure-induced phase transitions in amorphous Ge $(a\text{-Ge})$ up to 17 GPa is reported combining Raman scattering, x-ray absorption spectroscopy (XAS) measurements, and density-functional theory calculations. $a\text{-Ge}$ samples were films obtained by evaporation and characterized by different density of voids. Specific and reproducible phase transitions (interpreted as disorder-disorder, disorder-order, and order-disorder) are observed on pressurization and depressurization, depending on the initial density of voids. Details of the structural and vibrational properties of the various phases have been obtained by XAS and Raman-scattering data analysis. Samples showing a low density of voids transform first into a metallic disordered phase (8 GPa) and to GeII at higher pressures while those with higher density of voids transform directly to the GeII phase (10.6 GPa). Upon depressurization, the first nucleates into the GeIII metastable phase in the 7.2--2.3 GPa pressure range, while the others return to the amorphous state below 5 GPa. The behavior upon depressurization shows that the initial morphology determines the transitions experienced by the pressurized sample down to ambient pressure.

International audience; We present a systematic and comparative study of the pressure-induced solidification of 11 frequently used pressure transmitting fluids using the ruby fluorescence technique in a diamond anvil cell. These fluids are 1 : 1 and 5 : 1 iso-n pentane, 4 : 1 deuterated methanol-ethanol, 16 : 3 : 1 deuterated methanol-ethanol-water, 1 : 1 FC84-FC87 Fluorinert, Daphne 7474, silicone oil, as well as nitrogen, neon, argon and helium. The data provide practical guidelines for the use of these fluids in high pressure experiments up to 50 GPa.

Boron is an atomic species at the origin of the hardest materials. It has one of the most complicated chemistry among the elements. The simplest member of the family, α-boron, is based on an arrangement of slightly distorted B12 icosahedron in a rhombohedral unit cell. It has been studied by x-ray diffraction and Raman scattering up to 100 GPa, showing no sign of phase transformation.

We have studied the crystal and electronic structure of monoclinic (MC) InSe under pressure finding a reversible phase transition to a ${\mathrm{Hg}}_{2}{\mathrm{Cl}}_{2}$-like tetragonal phase. The pressure evolution of the crystal structure was investigated by angle-dispersive x-ray diffraction and Raman spectroscopy in a diamond-anvil cell up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. From the diffraction experiments, we deduced that MC InSe becomes gradually more symmetric under pressure, transforming the crystal structure into a tetragonal one at $19.4\ifmmode\pm\else\textpm\fi{}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. This phase transition occurs without any volume change. Raman measurements under pressure confirmed the occurrence of a monoclinic-to-tetragonal transformation. The nondegenerate modes in the MC phase, especially the ${A}_{g}^{4}$ modes, exhibit a negative pressure coefficient, converging with the ${B}_{g}^{1}$ modes, and becoming an $Eg$ mode in the tetragonal phase. The experimental results are interpreted through density-functional theory (DFT) electronic-structure and total-energy calculations, which showed that beyond $18\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ the tetragonal phase is the most stable phase. It is also shown that along the continuous change from monoclinic to tetragonal InSe, there is a progressive decrease of the band gap and eventually, in the tetragonal phase, there occurs a small band overlap. However, the Raman-effect and optical-absorption measurements suggest that this overlap is probably due to the usual DFT band-gap underestimation. Tetragonal InSe is most likely a low-gap semiconductor. The bonding in the monoclinic phase and that in the tetragonal InSe phase are compared.

International audience; We present spectacular evidence of metallization of amorphous Ge films under high pressure using optical micrographs and Raman-scattering and x-ray-absorption spectroscopies. This transformation is identified as a low-density amorphous LDA to high-density amorphous HDA transition, relevant to a large class of systems including amorphous ice, semiconductors, and oxides. We have discovered that this transition initiates at the surface of the LDA sample, characterized intrinsically by a thickness dependent density of voids. Contrary to the case of transitions involving stable crystalline solid phases, our observations show that pressure-induced phase transitions in inhomogeneous amorphous samples are morphology driven and are favored for lower defect void densities. The metal disordered phase is observed first at 8 GPa for a lower density of voids, transforming to a metastable ordered phase upon depressurization. The local HDA structure is characterized by an increase in the first-neighbor coordination number, average distance, and variance.

After pressurization to 30 GPA, in a diamond anvil cell, benzene transforms, at room temperature, to a white solid which is stable at ambient pressure. We report here the infrared spectroscopy analysis performed under pressure and at ambient conditions. These preliminary results show that the transformation involves an opening of the benzene rings leading to a highly cross‐linked polymer.

This paper reports on two new technical developments concerning sample environments for X-ray magnetic circular dichroism (XMCD). The first measurements under high pressures of up to 30 GPa are described. The difficulties of combining the techniques of high pressure and XMCD are commented on. The second development involves the use of a fast-switching magnetic field. A new superconducting device is used to perform XMCD measurements on paramagnetic compounds in magnetic fields of up to 6 T. The small amplitude of the XMCD signal imposes, for a given signal-to-noise ratio, a noise less than a few 10−5. The signal-to-noise ratio is improved by the use of a series of acquisitions, switching the magnetic field between each acquisition. A very fast switching mechanism has been built based on mechanical rotation of a superconducting coil, with the sample kept in place inside the coil. The XMCD signals at the L II,III-edges of paramagnetic rare-earth compounds have been measured at 4.5 K in fields of up to 6 T with a switching time of 11 s.

We have studied the vibrational properties of $\mathrm{Cu}\mathrm{Al}{\mathrm{O}}_{2}$ by means of Raman scattering in ambient conditions, at low temperature, and also at high pressure. Results are discussed in the framework of an ab initio calculation. Raman active modes have wave numbers ${\ensuremath{\sigma}}_{{E}_{g}}=418.1\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and ${\ensuremath{\sigma}}_{{A}_{1g}}=767.2\ifmmode\pm\else\textpm\fi{}0.3\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Polarized measurements with single crystals have confirmed their symmetry. We present and discuss the phonon-dispersion curves. Below $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the temperature dependence of the Raman active modes is almost linear, with coefficients $\ensuremath{\partial}{\ensuremath{\sigma}}_{{E}_{g}}∕\ensuremath{\partial}T=(\ensuremath{-}4\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}∕\mathrm{K}$ and $\ensuremath{\partial}{\ensuremath{\sigma}}_{{A}_{1g}}∕\ensuremath{\partial}T=(\ensuremath{-}1.0\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}∕\mathrm{K}$. Most of the temperature shift is associated with thermal expansion. The pressure dependence is given by $\ensuremath{\partial}{\ensuremath{\sigma}}_{{E}_{g}}∕\ensuremath{\partial}P=2.72\ifmmode\pm\else\textpm\fi{}0.07\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}∕\mathrm{GPa}$ and $\ensuremath{\partial}{\ensuremath{\sigma}}_{{A}_{1g}}∕\ensuremath{\partial}P=4.96\ifmmode\pm\else\textpm\fi{}0.12\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}∕\mathrm{GPa}$. We observe a reversible phase transition at $34\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, which, as already has been shown to happen in $\mathrm{Cu}\mathrm{Ga}{\mathrm{O}}_{2}$, we relate to the existence of a dynamical instability.

The behaviour of the d(GGTATACC) oligonucleotide has been investigated by X-ray crystallography at 295 K in the range from ambient pressure to 2 GPa (approximately 20,000 atm). Four 3D-structures of the A-DNA form (at ambient pressure, 0.55, 1.09 and 1.39 GPa) were refined at 1.60 or 1.65 A resolution. In addition to the diffraction pattern of the A-form, the broad meridional streaks previously explained by occluded B-DNA octamers within the channels of the crystalline A-form matrix were observed up to at least 2 GPa. This work highlights an important property of nucleic acids, their capability to withstand very high pressures, while keeping in such conditions a nearly invariant geometry of base pairs that store and carry genetic information. The double-helix base-paired architecture behaves as a molecular spring, which makes it especially adapted to very harsh conditions. These features may have contributed to the emergence of a RNA World at prebiotic stage.