Ultrasonic propagation in solid H₂: The effects of orientational ordering

Abstract

A report is given of the observation below 1 K of the sound attenuation δα expected from progressive orientational ordering in hcp H₂ with ortho concentrationsX<0.53. The experiments were carried out at 10 and 30 MHz. The amplitude of this effect depends on the coupling between the lattice vibrations and the molecular orientation, and should be maximum when the acoustic angular frequency ω is comparable with the orientational relaxation rate τ⁻¹. The average rate can be roughly estimated from NMR longitudinal relaxation timeT ₁ measurements. Such a maximum for δα was indeed observed in the expected temperature range. At high enough temperatures, δα was found to be proportional toT ⁻¹₁ . which is consistent with predictions in the high temperature limit. Furthermore, the transition between the hcp and the fcc phases forX>0.53 is studied by means of the large changes in the sound propagation at the transition, and the phase diagram thus obtained is compared with results from x-ray and pressure measurements. The new observations explain some previous discrepancies in results using different methods. The difference between solid H₂ and D₂ regarding the stabilization of the cubic structure above the orientational ordering transition is also discussed. Calculations of the respective energy barriers ΔE to be overcome during the martensitic transition are suggested.