Ultrasound propagation near the martensitic order-disorder transition in solid H2 and D2

Abstract

The behavior of the longitudinal and transverse sound velocities and the qualitative change of acoustic attenuation are reported for solid H 2 and D 2 at constant volume upon repeated thermal cycling through the martensitic transition between the orientationally disordered and the ordered (cubic) phases. The observed relative velocity changes across the transition in D 2 are only about 10−3 –10−2 , whereas the acoustic attenuation is strongly dependent upon the history of thermal cycling. In general the attenuation increases dramatically as the crystal is cooled for the first time through the transition and is thermally cycled. The behavior in solid H 2 is quite different: In the ordered phase the sound attenuation is so great that the velocity cannot be measured, but upon warming the solid back into the disordered phase, the sound signal returns practically to its original intensity, even after several cyclings. It is shown that the observations are consistent with a soft-mode model for the martensitic transformation and that the effect of the order-disorder transition on the sound velocity is likely to be small.