MARTENSITIC TRANSITION IN SOLID H-2 UNDER PRESSURE STUDIED VIA ULTRASOUND PROPAGATION

We have studied the change in the transmitted signal amplitude of longitudinal sound in solid H-2 during the martensitic hcp-fcc transition. Samples were grown under pressures up to 1.2 kbar, producing molar volumes V equal to or smaller than that of solid D2 grown at zero pressure. The purpose was to see whether the nature of the phase transformation in the isotopic solids, after repeated thermal cycling, would become similar at equal density. Our experiments show, however, that the phase-transformation dynamics remain very different for both solids. Furthermore, we determine the volume dependence of the order-disorder transition temperature for an ortho concentration X = 0.75 and for 0.79 less-than-or-equal-to V/V0 < 1.0. Here V0 is the volume at zero pressure. Our results are compared with predictions and with previous work.