Lattice dynamics and phonon line shapes in Ar36 at high temperatures

The inelastic neutron scattering technique has been used to measure phonon frequencies and phonon line shapes in Ar36 at temperatures of 81 K [lattice constant a=5.465(2) A] and 55 K [a=5.440(2) A] primarily along the [100] and [110] directions. The 81-K data combined with earlier results for small wave vector by Fujii et al. were fitted to a force-constant model. Detailed comparisons are made of the observed phonon line shapes with those computed from the self-consistent harmonic approximation with added cubic anharmonicity (Glyde and co-workers) and molecular-dynamics simulations (Hansen and Klein). The agreement at high temperatures though still quite satisfactory, was found to be not as good as at low temperatures.