High-resolution Raman spectra of ortho/para mixed crystals of deuterium 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 vibron sidebands, a total of 11 vibron peaks, and significant changes in the low-frequency rotational excitations upon passage into the low-temperature phase. Analysis of the spectra reveals that the phase is orientational ordered and may form a superstructure derived from the high-temperature hcp lattice. The results also suggest that orientational frustration accompanies the transition in ortho/para mixed crystals.

• Received 22 January 1996

DOI:https://doi.org/10.1103/PhysRevB.54.R15590