FREEZING OF MOLECULAR-HYDROGEN AND ITS ISOTOPES IN POROUS VYCOR GLASS

We have made a detailed ultrasonic study of freezing and melting of mo lecular H-2, HD and D-2 in the pores of Vycor glass. The behavior was similar to that seen in previous measurements with argon and helium. T he hydrogen liquids undercooled about 2.5 K below their bulk triple po ints before freezing began and there was substantial hysteresis betwee n freezing and melting. The velocity and attenuation began to increase suddenly at the onset of freezing. The velocity continued to increase to the lowest temperatures (2 K) and the attenuation had a broad peak at about two thirds of the freezing temperature. We attribute these e ffects to stress relaxation via thermally activated vacancy motion in the solid hydrogen, an interpretation confirmed by looking at the freq uency dependence of the velocity and attenuation. The magnitude of the velocity and attenuation changes increased in going from H-2 to HD to D-2, as expected based on their increasing densities and elastic cons tants. However, there were no qualitative differences between the boso n (H-2 and D-2) and fermion (HD) cases nor, for that matter, between h ydrogen and argon. We believe that essentially all the hydrogen was fr ozen a few tenths of a kelvin below T-F, at the point where the meltin g/freezing hysteresis began. If even a few per cent of the hydrogen ha d remained liquid and become superfluid at some lower temperature, it would have been seen as a further increase in the velocity and a criti cal attenuation peak. The sensitivity of our ultrasonic measurements a llowed us to make accurate measurements of the freezing and melting te mperatures of the different liquids in Vycor. We found that the fracti onal undercooling, (T-B-T-F)/T-B, increased as the molecular mass decr eased which may indicate the importance of quantum effects on the liqu id-solid interfacial energy sigma(1s).