LATTICE DYNAMICS OF NEON AT 2 DENSITIES FROM COHERENT INELASTIC NEUTRON SCATTERING

Phonon dispersion relations for two single crystals of neon were measured by inelastic neutron scattering. Growth of the crystals by a high-pressure technique permitted a study of Ne lattice dynamics at densities differing by 4%. Measurements in the principal symmetry directions of the fcc lattice were made at 4.7 °K on a crystal with a lattice parameter of 4.462±0.002 Ã. These data could be well fitted with a spectrum calculated by self-consistent phonon theory with a Lennard-Jones pair potential. An all-neighbor quasi-harmonic calculation with the Lennard-Jones potential also provided a good match after calculated energies were adjusted for a difference in theoretical and experimental lattice parameters. The two models utilized different values for the Lennard-Jones parameters in the calculations. A Born-von Kármán analysis indicated that interatomic forces are short-range and predominantly central. Dispersion curves in the [001] direction were measured for a second sample at 4.7 and 25 °K at constant volume (lattice parameter 4.402±0.002 Ã). The temperature shift of the frequencies was small, averaging (-2.3±1.3)% for the transverse and (2.3±2.4)% for the longitudinal mode. A Born-von Kármán analysis of data for this higher-density sample indicated the appearance of significant noncentral forces. Comparison of phonon measurements at 4.7 °K for the two samples yielded average quasiharmonic-mode Grüneisen parameters γj of 2.9±0.5 and 4.2±0.6, respectively, for the [001] transverse and longitudinal branches. © 1969 The American Physical Society.