High-pressure infrared spectra of solid methane are reported up to 30 GPa between 50 and 300 K. The symmetric stretching mode ( 1 ) was successfully used as a probe of the phase transitions. Seven different phases have been identified. Pressure and temperature-dependent studies allowed us to outline all the phase boundaries in this portion of the diagram. A high-pressure phase ͑HP͒, stable in all the temperature range analyzed, has been identified. The transition to this phase occurs at about 8 GPa at 50 K, and 25 GPa at 300 K. The wide range of stability of this phase suggests a single site-ordered structure. Group-theoretical and qualitative arguments point to hcp ͑D 6h factor group͒ as the favored crystal structure of the HP phase. The knowledge of the phase diagram allows us to outline the evolution of the crystal structure and of the site symmetries as the pressure increases. The low-pressure fcc crystalline modifications transform to the fully ordered hcp structure through intermediate tetragonal phases. Competition between molecular and crystalline fields determines a complex site-symmetry evolution. Similarities with analogous fcc-hcp evolution observed in rare gases and atomic systems support our conclusions.
An experimental setup for Fourier transform infrared (FTIR) studies in condensed matter at high pressure and low temperatures is described. We have adapted a close-cycle cryostat (T=20–300 K) to the sample compartment, which is used as a cryo chamber, of a FTIR spectrometer (frequency range 10–15 000 cm−1). A Cassegrain-type beam condenser is assembled to measure infrared absorptions of samples contained in a membrane diamond anvil cell (P up to 100 GPa). The tuning of the pressure and the cell alignment is performed from outside the evacuated instrument. An additional light path allows visual observation and in situ pressure calibration. The advantages of this system, demonstrated by its application to CH4 and Ar–(H2)2 crystals, are high radiation throughput, long time stability, visual observation of the sample, remote measurement and variation of the local pressure, and remote alignment of the cell with the IR beam.
The high frequency behavior of the dynamic structure factor, S(Q,ω), of liquid and supercritical neon is investigated by inelastic x-ray scattering at different temperatures and pressure. The spectral evolution is described in terms of a single-relaxation-time viscoelastic model. The occurrence of a positive dispersion in the sound velocity is clearly visible in both investigated thermodynamic phases. The anomalies in the dispersive behavior deeply reduce at the higher temperatures, probably, as a consequence of important changes in the first shell interactions. More generally, the atomic dynamics is dominated by a relaxation process whose time scale is in the range of fast microscopic degrees of freedom (≈10−13 s), and whose strength and typical time scale stay constant over all the explored liquid and supercritical regions.
Quantum effects in the teraherz dynamics of supercritical 4He have been studied as a function of both density rho and temperature T; they have been characterized through their effects on the second and third spectral moments of the dynamic structure factor S(Q, omega), measured by the inelastic x-ray scattering (IXS) technique. The IXS spectra were collected in the low-Q region below and around the position of the first diffraction peak Q(m), i.e., in a range relatively unusual in this kind of investigation. The measured spectral moments clearly show a departure from their high-T classical expected values. We observe, moreover, that the amplitude of quantum deviations increases slightly with increasing density. This experimental method allows us to extract, even in a region where the dynamics still maintains a collective character, such typical single particle properties as the mean atomic kinetic energy.
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