THE NON-EQQ FORCES AND THE STRUCTURE OF SOLID HD AT HIGH-PRESSURE

Tile fact that HD molecules in the solid vibrate and gyrate around the ir centers of mass, substantially shifted relative to their geometrica l centers, entails a specific anisotropic interaction, which is absent in the homonuclear hydrogens. The physical reason behind the pressure -driven orientational phase transitions in the homonuclear nominally J = 0 hydrogens is recognized to be the electrical quadrupolar (EQQ) in teraction, which becomes progressively stronger with increasing pressu re. Our estimates show that in HD at relatively moderate pressure of a bout 10 kbar, the specific non-EQQ forces become predominant. The pres sure-induced orientational order in HD might have an entirely differen t nature as compared to the homonuclear hydrogens.