The acoustic velocities, adiabatic elastic constants, bulk modulus, elastic anisotropy, Cauchy violation, and density in an ideal solid argon (Ar) have been determined at high pressures up to 70 GPa in a diamond anvil cell by making new approaches of Brillouin spectroscopy. These results place the first complete study for elastic properties of dense Ar and provide an improved basis for making the theoretical calculations of rare-gas solids over a wide range of compression.

A methane hydrate (MH) single crystal was synthesized in a diamond anvil cell to investigate its intrinsic high-pressure properties. With increasing pressure, the cubic sI phase of MH changed to the MH-II phase at P ) 0.9 GPa and room temperature, and this phase remains stable up to P ) 1.9 GPa, which was visually observed by optical microscopy. In situ Raman spectra for CH 4 molecules encaged in different cages of MH-II show two vibrational bands; the higher frequency band shows a remarkable increase in its frequency versus pressure (17.0 cm -1 /GPa), and the lower band shows a progressive increase in frequency with pressure (6.3 cm -1 /GPa). These results are interpreted on the basis of two different structures recently reported for MH-II. Above P ) 1.9 GPa, MH-II crystals visually decomposed and the O-H stretching Raman band of host cages became unobservable, indicating no more existence of the cage structure. Raman spectra of CH 4 molecules in MH-III show almost the same behavior as those of pure solid methane up to at least 5.2 GPa, which may be consistent with the existence of a new type of MH.

Vibrational properties of Ba doped Si clathrate are investigated at high pressures up to 20 GPa by Raman spectroscopy. Vibrations related with Ba encaged in the Si cages are observed below 100 cm(-1) by low-frequency Raman measurements of Ba8Si46 and Ba6.6Si46 clathrates. The high-pressure Raman spectra obtained for both compression and decompression processes reveal a new phase transition at 7 GPa, and the reversibility of the phase transition at 15 GPa. We investigate the mechanisms of these phase transitions, the interaction between the guest and host frame, and the volume dependence of the vibrational modes.

A new phase of solid bromine was discovered at a pressure region above 80 GPa by Raman scattering experiments with a diamond anvil high-pressure cell. This phase was found to be the same as the iodine phase V with an incommensurate structure [Nature (London) 423, 971 (2003)] which appears between the molecular phase I and the monatomic phase II. In the incommensurate phases of both bromine and iodine, Raman active soft modes were clearly found in the low frequency region. The data suggest that the monoatomic phase II occurs above 30 and 115 GPa for iodine and bromine, respectively.