INVARIANT POINTS AND PHASE-TRANSITIONS IN DEUTERIUM AT MEGABAR PRESSURES

An extensive and detailed study of deuterium to 200 GPa and 20-300 K r eveals a number of new features of its phase diagram. A well-defined R aman vibron discontinuity is observed across the I-III phase Line abov e the I-II-III triple point at 167 GPa and 130 K. Evidence for additio nal invariant points is found, including one at 182(+/-3) GPa and 235( +/-15)K where the discontinuity disappears. Measurements of the phase III vibron reveal a new functional dependence of the order parameter w ith temperature and a previously undisclosed parallel to classical ori entational phase transitions.