Invariant Points and Phase Transitions in Deuterium at Megabar Pressures

Abstract

An extensive and detailed study of deuterium to 200 GPa and 20–300 K reveals a number of new features of its phase diagram. A well-defined Raman vibron discontinuity is observed across the I-III phase line above the I-II-III triple point at 167 GPa and 130 K. Evidence for additional invariant points is found, including one at 182( $\pm$3) GPa and 235( $\pm$15) K where the discontinuity disappears. Measurements of the phase III vibron reveal a new functional dependence of the order parameter with temperature and a previously undisclosed parallel to classical orientational phase transitions.