RAMAN EXCITATIONS AND ORIENTATIONAL ORDERING IN DEUTERIUM AT HIGH-PRESSURE

High-resolution Raman spectra of ortho/para mixed crystals of deuteriu m to 4 K and pressures to 40 GPa reveal a number of phenomena. We find for para-containing samples a large enhancement of high-frequency vib ron sidebands, a total of 11 vibron peaks, and significant changes in the low-frequency rotational excitations upon passage into the low-tem perature phase. Analysis of the spectra reveals that the phase is orie ntational ordered and may form a superstructure derived from the high- temperature hcp lattice. The results also suggest that orientational f rustration accompanies the transition in ortho/para mixed crystals.