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The exposure of RMn₂ (C15 or C14 cubic Laves phase, where R = Y; Dy; Ho or Er) to high deuterium pressure leads to formation of novel, unique YMn₂D₆, DyMn₂D₆, HoMn₂D₆ and ErMn₂D₆ deuterides with cubic Fm-3m symmetry. In spite of different structures and molecular volumes of parent RMn₂ compounds, the molar volumes of RMn₂D₆ deuterides are almost identical. In this paper, we present results of studies on RMn₂Dx (where R = Y, Dy, Ho and Er) submitted to compression up to 30 GPa in diamond anvil cell (DAC) combined with energy dispersive X-ray diffraction. The EOS (equation of state) parameters of the above four RMn₂D₆ samples and YMn₂D_x, with x ≤ 4 are compared. The EOS parameters of YMn₂D₆ are very similar to those of other RMn₂D₆ but very different than those of interstitial deuterides YMn₂D_x (x ≤ 4). The phase transition or segregation was not detected in RMn₂D₆ up to 30 GPa.

A study of a natural system concerning the growth of minerals (garnet) into metapelite host rocks from Calabria (southern Italy), which underwent pressure up to 1.2 GPa, is presented. Through the application of a thermodynamic model it was possible to determine P and T conditions achieved during metamorphic evolution of these rocks useful to reconstruct the related tectonic settings of poly-orogenic metamorphic complexes.

Gallium orthophosphate of low-cristobalite structure has been studied by Raman spectroscopy up to 28.5 GPa. Three phase transitions have been identified: 1.6, 3.1 and ∼7 GPa. Raman spectra (200 to 700 cm^-1) acquired above 3 GPa show signs of six-coordinated gallium ions, in agreement with earlier studies that the orthorhombic C222₁ structure transforms into the orthorhombic Cmcm structure. As pressure increases above 8 GPa, the Raman peaks associated with the low-cristobalite framework structure diminish gradually and replaced by peaks that are possibly related to isolated phosphate units. The spectra also show that the structure of the quenched sample retains that of the high-pressure phase prior to decompression.

We report the results of investigation of the thermopower (Seebeck effect) of a ThCr₂Si₂-structured heavy fermion single-crystalline YbPd₂Si₂ compound (itterbium-palladium-silicon, 1-2-2) under ultra high pressure P up to 22 GPa. At ambient conditions the thermopower S was found to be negative. Under pressurization S inverted its sign and exhibited a bend near 6 GPa. The compressibility of YbPd₂Si₂ also showed a bend near 6 GPa corroborating a supposition about phase transformation.

The agreement between the conception of minimal metal conductivity and experiment has been shown. It is found, that the smoothness of metal-insulator transition observed in crystal semiconductors is caused either by disorder and then we need the percolation theory to be used for data interpreting at T=0 K, or by small value of critical concentration of carriers. The critical concentration depends on the compensation level, classical and 'natural' widening of defect level or correlated distribution of impurities. According to experimental data, the characteristic parameters of transition in n-CdSnAs₂, p-CdSnAs₂<Cu>and p-Ge has been obtained by the extrapolation to T=0 K.

It has been revealed from pressure, temperature, magnetic and electric fields dependences of kinetic coefficients in doped compensated Ge<Au, Sb>, p-type InSb, InAs, CdSnAs₂<Cu>and HgTe, that the observed anomalies of electron transport in the listed above materials are linked to the presence of large-scale fluctuations of ionized impurity concentrations. It is shown, that the influence of chaotic potential begotten by these fluctuations becomes more noticeable when the temperature decreases or the pressure rises.

X-ray diffraction has been measured up to 19 GPa for liquid GeTe and up to 15 GPa for liquid GeSe using synchrotron radiations. Static structure factor S(Q) and pair distribution function g(r) was obtained. Both S(Q) of liquid GeTe and liquid GeSe change their shape with increasing pressure, indicating anisotropic contraction of the local structure. A sharp change in S(Q) and g(r) was observed in liquid GeTe between 1.8 GPa and 4.0 GPa. Pressure dependences of S(Q) and g(r) of liquid GeSe change around 6 GPa. In liquid GeSe a small pre-peak was observed around 1.2 Å^-1 up to 1.7 GPa which suggests the presence of the medium range order of covalent bond network. With increasing pressure the Peierls-type distortion becomes small and the bond angle decreases; the local structure becomes a rhombohedral structure. These pressure dependences of the structure of liquid GeTe and liquid GeSe are different from those of the crystals. These results are compared with those of other liquids and discussed in relation to the bonds between atoms in the liquids.

The ultrasonic study of the elastic moduli of polycrystalline fullerite C₆₀ was carried out in the temperature range from liquid nitrogen to room temperature at pressures up to 1.4 GPa, including the regions of the orientational fcc-to-sc and glass transitions. The temperature dependencies of the elastic moduli at ambient pressure are in good agreement with previous studies. For the first time the pressure dependencies of the shear modulus and Poisson's ratio were determined. Particularly, the Poisson's ratio was found to increase from ≈0.3 at ambient pressures to ≈0.38 at p = 1 GPa. The observed high values of the Poisson's ratio, as well as the high values of pressure derivatives of the bulk modulus ≈23-25 are discussed in terms of the central m-n power potential and contribution of the non-central intermolecular forces due to intramolecular deformations.

The synthesis and the thermal stability of the high pressure and high temperature perovskite phase BiCrO₃ have been studied. BiCrO₃ can be synthesized in a wide range of pressure (1-6GPa) and temperature (690-865°C). The thermal behavior of BiCrO₃ depends on the atmosphere: it tends to be stable under inert gas whereas it decomposes at 650°C under O₂. The melting point of BiCrO₃ is determined by DTA under argon and a value of 1040°C is proposed. First SEM and TEM observations are also presented.

Pressure dependences of resistivity ρ(P) and Hall coefficient R_H have been measured for the novel high-temperature ferromagnetic semiconductor p-Cd_1-xMn_xGeAs₂ (x=0÷0.36) at the room temperature. Structural phase transitions with positions shifting towards low pressures, when percentage of manganese increases from 5.9 GPa for CdGeAs₂ to 4.8 GPa for p-Cd_0.64Mn_0.36GeAs₂, are found out. Some anomalies of dependence R_H(P), which we attribute to magnetic properties and the presence of impurities, are found out for the crystals with the greater percentage of manganese (x≥0.18).

X-ray absorption fine structure (XAFS) observations on iodine K edge of AgI have been performed under high pressure and high temperature up to 6.0 GPa and 1000K to investigate the local structure and effective potentials in high pressure phases. In EXAFS analysis, we have directly carried out the numerical integration of EXAFS function. The anharmonic effective pair potentials V(u)=au²/2+bu³/3! for I-Ag bond have been determined under pressures. α-AgI and disordered rock-salt type phase have super-ionic conduction behaviors. Ag ions occupy both octahedral and tetrahedral sites in the disordered phase and twenty percent of Ag ions occupy the tetrahedral site as a maximum value at 2GPa. The transition between the rock-salt type and disordered rock-salt type phases is a broad disorder type within the same structure. From the viewpoint of the local structure analyses on XANES and EXAFS spectra, some sudden changes are recognized near the phase transition point. Pressure influences greatly the effective pair potential of anti-bonding side and anharmonicity decreases with increasing pressure. Phonon dispersion relation of the rock-salt type AgI under pressure has been derived from EXAFS experiments. Analysis of EXAFS Debye-Waller factor is useful because the force constant can be decided directly even at high pressure and high temperature.

Phase equilibrium curves of H₂ + tetrahydrofuran and H₂ + tetra-n-butyl ammonium bromide mixed gas hydrates were measured. Each three-phase equilibrium curve converges at three-phase equilibrium point of pure tetrahydrofuran and tetra-n-butyl ammonium bromide hydrates, respectively. It is directly confirmed by use of Raman spectroscopy that H₂ is enclathrated in the hydrate cages by adding a small amount of tetrahydrofuran or tetra-n-butyl ammonium bromide. In both mixed gas hydrates, H₂ is selectively enclathrated in the small cage despite the concentrations of aqueous solution.

We performed high-pressure ADXRD studies on Fe₅Si₃ and Ni₂Si up to 75 GPa. No evidence of the occurrence of a phase transition was observed in them. Fe₅Si₃ was found to compress isotropically, but an anisotropic compression was observed in Ni₂Si. These results are supported by ab initio total-energy calculations, which for Fe₅Si₃ also predicted a transition at 283 GPa from the hexagonal P6₃/mcm phase to a cubic phase. High-pressure melting studies were conducted on FeSi up to 70 GPa. We found a change in the melting slope at 12 GPa, which is attributed to the intersection of the melting curve with the phase boundary between -FeSi and CsCl-type FeSi. Finally, an equation of state for Fe₅Si₃ and Ni₂Si is reported.

We report on a full potential linear muffin-tin orbital calculation of the electronic structure of zircon at ambient conditions as well as under high pressure. The calculations reproduce the experimentally observed pressure induced phase transition from the zircon to scheelite structure. The calculated transition pressure of 8 GPa compares very well with the experimental value of about 10 GPa. Our calculated equation of state and the volume collapse associated with the crystallographic change are also in very good agreement with experiment.

Effect of hydrostatic pressure on the birefringence and dielectric properties of CuInP₂S₆ crystals is studied. The pressure behaviour of the phase transition temperature confirms that the phase transition in these crystals belongs to the order/disorder type. An additional line of birefringence anomalies in the ferrielectric phase is revealed. Based on the studies of the optical and dielectric properties of CuInP₂S₆ crystals their (p, T) diagram was constructed.

The phase decomposition phenomenon is found in the hexagonal ω-phase of the Ti—Zr system under high pressure. The ω → ω₁ + ω₂ decomposition of the equiatomic TiZr alloy occurred due to long thermobaric treatment at P = 5.5±0.6 GPa and T = 710±30 K. The chemical compositions of the ω₁- and ω₂-phases recovered to ambient conditions were estimated from the X-ray data to be around Ti₂₀Zr₈₀ and Ti₈₃Zr₁₇. The experimental data were used to calculate the mixing energy and the top of the decomposition curve in the isobaric T-C diagram of this system. We find that the equilibrium T-C phase diagram of the Ti-Zr system at pressures above ∼8 GPa is of the eutectoid type with the high-temperature β-phase and the low-temperature ω₁- and ω₂-phases.

We studied the electrical conductivity of ACrO3 (A=Sr, Ca) perovskites performed at various pressures up to 40 GPa using diamond anvil cell techniques. The samples were synthesized under high pressure high temperature. Pressure induced metallizations were observed in both samples. However the xray diffraction experiments with synchrotron radiation source indicated no discernable crystal structural transition up to 60 GPa. Therefore the pressure induced metallizations were ascribed to electronic type phase transitions. It possibly came from the change of electronic structure due to an orbital ordering evolution induced by pressure.

Melting temperatures of iron up to 130 GPa and compressibility to 150 GPa were measured by X-ray diffraction in the double-sided laser-heated diamond cell. New experimental features are the very tightly focused X-ray beam, in situ alignment of X-ray beam and hot spot, and rapid data acquisition. The melting temperatures are in good agreement with previous optical measurements supporting 'low' melting temperatures in the core. Above 70 GPa only the (hcp)-Fe phase is found. The measured densities of the annealed iron samples yield an accurate equation of state. The c/a ratio of iron decreases with pressure but shows an increase with temperature and at core conditions is nearly the same as at low pressure.

Synchrotron x-ray diffraction measurements of liquid Sb under pressure have revealed that the structure in the low pressure range is not simple-cubic-like nor slightly Peierls-type distorted one, although the coordination number and the pair correlation function show some similarities to that of simple-cubic-like liquid. Structural changes were observed at around 3GPa and 6.5-9GPa. The changes of the peak positions and the coordination number are reported.

A novel experimental apparatus for measuring gas absorption has been adopted to study sorption of carbon dioxide CO₂ and vinylidenefluoride (VDF) in a semi crystalline polymer, namely poly(vinylidenefluoride) (PVDF). The experimental technique allows to calculate simultaneous absorption of different components in the polymer phase, through a gas chromatographic determination of the composition of the fluid phase. To take into account indirectly the polymer swelling, a suitable not-absorbable probe, namely argon, has been added to the fluid phase. Concentrations of molecules absorbed in the polymer phase were simply calculated through mass balance equations written for each component in the two phases. Several different operating conditions were investigated. Temperature was kept constant at 50°C, while pressure varied between 8 and 35 MPa. Sorption was measured for pure CO₂ and VDF and for mixtures containing around 0.20 w/w and 0.40 w/w of VDF in CO₂, to analyse how the presence of a different component could affect the sorption of the other. Results show that pure CO₂ presents a solubility more than three fold higher than pure VDF at the same pressure conditions. On the other side, when both gases were present in the mixtures, the sorption of CO₂ seems to be strongly inhibited by VDF, whereas this latter presents a solubility quite similar to the pure gas if operating at the same partial pressure.

Lithium borohydride (LiBH₄) was compressed up to 10 GPa using a diamond-anvil-cell to investigate its high-pressure structure. In-situ x-ray diffraction profiles indicated a pressure-induced transformation at 1.1 GPa, which was consistent with the previous experimental observation such as Raman scattering spectroscopy. The high-pressure phase was indexed on a tetragonal symmetry of P4₂/mmc, which was not corresponding some structural models proposed by previous calculation studies. An unknown substance (presumably another Li-B-H compound), which was contained in the starting material, also transformed into its high-pressure phase at 0.6 GPa without any relation to the transformation of LiBH₄.

The supramolecular chemistry of peracetylated-β-CD (perAc-β-CD) as a host for triphenyphosphine derivatives has been studied in supercritical (scCO₂) using UV absorption spectroscopy. It was found that the association constant in scCO₂ at 40°C and 300 bar is 10 to 1000 times smaller compared to analogous systems in aqueous solvent. Studies of the thermodynamics of the inclusion process found an enthalpy of association of -30 kJ/mole and an entropy of -55 J/moleK. This difference with respect to water is attributed to the absence of the hydrophobic effect in scCO₂ due to the much smaller polarity of scCO₂ versus water. To further explore the effect of the solvent on the association constant, values of solvent polarity intermediate between the limits of scCO₂ and water could be useful. To this end, methanol is added to the scCO₂ as a cosolvent to modify the overall polarity. By taking advantage of the tunability of the supercritical phase, a wide range of solvent polarity can be accessed by varying the temperature (35 to 50°C), pressure (200 to 400 bar), and mole fraction of methanol. This study is pursued to obtain a better comprehension of the inclusion process in CO₂-based supercritical fluids to make the association constant of the phosphine-perAc-β-CD system comparable to those observed in aqueous systems.

The contribution summarizes the results of recent studies of phase transitions induced by high pressure in a number of molecular organic crystals, such as polymorphs of paracetamol, chlorpropamide, polymorphs of glycine, L- and DL-serine, β-alanine. The main attention is paid to the following topics: (1) Reversible / irreversible transformations; (2) Different behavior of single crystals / powders; (3) The role of pressure-transmitting liquid; (4) The role of the kinetic factors: phase transitions on decompression, or after a long storage at a selected pressure; (5) Isosymmetric phase transitions; (6) The role of the changes in the hydrogen bond networks / intramolecular conformational changes in the phase transitions; (7) Superstructures / nanostructures formed as a result of pressure-induced phase transitions.

Zr-rich, La-doped lead zirconate titanate ceramic samples with composition near Pb_0.99La_0.01(Zr_0.91Ti_0.09)O₃ (hereafter PLZT) were studied by time-of-flight neutron diffraction and dielectric measurements at 295 K and 1 bar and at 250 K versus increasing and decreasing pressure to 0.55 GPa. The diffraction data at 295 K show that the sample has the rhombohedral ferroelectric R3c structure (F_R(LT)) and remains in that phase upon cooling to 250 K. As pressure is increased at 250 K, a transition is observed above 0.3 GPa to essentially the antiferroelectric CaTiO₃ orthorhombic Pnma (A_O) phase. There appear two other peaks in the neutron spectra, suggesting a possible incommensurate cell for this A_O. After the initial drop of the F_R(LT) content, the transformation remains incomplete to 0.55 GPa with ∼10% of the sample retaining the F_R(LT) low pressure phase. On pressure release, 91% of the F_R(LT) phase is recovered with the remainder remaining in the A_O phase. Isothermal dielectric data at 250 K suggest a structural transition with an onset near 0.37 GPa for increasing pressure, consistent with the diffraction results.

A thermodynamic model of the free energy as a function of volume and temperature for tin with taking into account the polymorphic transformation and melting effects is proposed. Results of equation-of-state and phase-diagram parameters calculations for this metal are compared with available experimental data on static and shock compression at high pressures.