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The heat liberated by isothermal compression permits the determination of the thermodynamic properties of liquid water−heat capacity, compressibility, and expansivity. The results, as a function of pressure and temperature, have been extrapolated as far down as −40 °C. They confirm the existing data in the region of the supercooled liquid and also the existence of a divergence of the measured properties. This could be explained as a lambda transition occuring at −45 °C on isobars p = 0. The abnormal heat capacity which characterizes this transition is well described as a function of the temperature by a formula with critical coefficients as already suggested by others.

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. 0 1995 American Institute of Physics.

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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.

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.

Angle dispersion X-ray diffraction (AXDX) experiments on nitromethane single crystals and powder were performed at room temperature as a function of pressure up to 19.0 and 27.3 GPa, respectively, in a membrane diamond anvil cell (MDAC). The atomic positions were refined at 1.1, 3.2, 7.6, 11.0, and 15.0 GPa using the single-crystal data, while the equation of state (EOS) was extended up to 27.3 GPa, which is close to the nitromethane decomposition threshold pressure at room temperature in static conditions. The crystal structure was found to be orthorhombic, space group P2(1)2(1)2(1), with four molecules per unit cell, up to the highest pressure. In contrast, the molecular geometry undergoes an important change consisting of a gradual blocking of the methyl group libration about the C-N bond axis, starting just above the melting pressure and completed only between 7.6 and 11.0 GPa. Above this pressure, the orientation of the methyl group is quasi-eclipsed with respect to the NO bonds. This conformation allows the buildup of networks of strong intermolecular O...H-C interactions mainly in the bc and ac planes, stabilizing the crystal structure. This structural evolution determines important modifications in the IR and Raman spectra, occurring around 10 GPa. Present measurements of the Raman and IR vibrational spectra as a function of pressure at different temperatures evidence the existence of a kinetic barrier for this internal rearrangement.

The room-temperature Raman spectra of Sr 1−x Ca x TiO 3 (SCT) for x = 0.06 (SCT06), 0.12 (SCT12) and 0.30 (SCT30) are shown to contain first-order Raman lines corresponding to E g and B 1g modes, which appear below 105 K in pure SrTiO 3 . The E g line shows slight asymmetry, indicating the orthorhombic structure of SCT for x 0.06. The low-temperature Raman spectra of SCT06 and SCT12 contain Raman-active modes of symmetry B 2g and E g , predicted by group theory but not observed in SrTiO 3 or SCT0.7 (x = 0.007) by previous workers. The polar hard modes TO 2 (175 cm −1 ) and TO 4 (550 cm −1 ) are present even at room temperature for SCT06 and SCT12 due to Ca 2+ -centred ferroelectric microregions. The intensity of these modes starts increasing below 55 and 80 K for SCT06 and SCT12 respectively, which are nearly 2 T c , where T c is the temperature of the smeared dielectric peak. The presence of such an extended precursor region is akin to relaxor behaviour. For SCT30, two new lines at 79 and 128 cm −1 appear below the paraelectric to antiferroelectric phase transition temperature of 230 K. It is shown that Raman lines characteristic of both ferroelectric and antiferroelectric phases are present in SCT12 below 100 K, confirming the frustration model for the highly smeared dielectric response for this composition.