Lattice dynamics of hcpHe4at high pressure

Abstract

The neutron-inelastic-scattering technique was used to measure the phonon dispersion relations in two high-density crystals of hcp ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ with molar volumes of 11.61 and 9.41 ${\mathrm{cm}}^3$/mol. These densities are on the order of twice that of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ at 30 bars. The crystals were grown from the melt in the fcc phase, and cooled across the fcc-hcp phase transition. The observed phonon spectra show anharmonic effects much less prominent than those observed in earlier measurements at 21 ${\mathrm{cm}}^3$/mol. In particular, multiphonon interference effects were not found to be very pronounced, while multiphonon scattering appeared to influence phonon line shapes strongly for large wave-vector and energy transfers. Elastic constants dėtermined from the initial slopes of the dispersion curves were found to be in good agreement with those calculated by Goldman using the first-order self-consistent-phonon theory. The dependence on volume of the phonon energies is discussed with reference to the earlier studies at 21.1 and 16.0 ${\mathrm{cm}}^3$/mol. No significant dispersion of mode Grüneisen parameters was found unlike the case of fcc He. Grüneisen parameters were found to depend on volume approximately in a manner given by Ahlers for the thermodynamic Grüneisen parameter.