RAMAN AND BRILLOUIN SPECTROSCOPIC STUDIES OF CH4 SINGLE-CRYSTALS IN PHASE-I AND PHASE-II

A technique has been developed for obtaining single crystals of solid methane in phase II which can be reached only from within phase I at z ero pressure. Large (similar to 50 mm(3)) single crystals of CH4-I wer e grown in a cylindrical quartz cell and successfully cooled below the phase-transition temperature. Brillouin spectra were collected at dif ferent crystal orientations corresponding to rotation about the labora tory z axis. X-ray Laue diffraction techniques were applied to determi ne the orientation of the crystals in the laboratory frame. The elasti c constants of CH4-I were determined as a function of temperature from 90 to 20.5 K and the elastic constants of CH4-II were determined as a function of temperature from 19.5 to 15.5 K. The data in both phases were also used to calculate the Poisson ratio, bulk and shear moduli, the distribution of the acoustic velocities in high-symmetry planes, a nd the Debye temperature. Raman spectra of the fundamental modes nu(1) and nu(3) were recorded and their shifts and full widths at half heig ht were determined as a function of temperature through the phase tran sition. [S0163-1829(98)05429-0].