A Neutron Scattering Study of the Anharmonicity of Lattice Vibrations and Its Relation to the Melting of NaF

Abstract

Starting from a viewpoint that melting phenomena are caused by an instability of the lattice vibrations, neutron scattering studies were made on NaF to see how the anharmonic interaction of phonons develops with increasing temperature. The phonon frequency shift and the damping of the [110] TA₁ mode were measured as functions of wave number and frequency over a wide range of temperature from 300 to 1250 K. At temperatures very close to the melting point T_m=1266 K, high ω(q) phonons are heavily damped, but low ω(q) phonons are still surprisingly stable. A marked frequency softening was observed only for this mode; no evidence for premelting phenomena could be seen, and T_m appears quite suddenly. It is argued that T_m will be determined not by the bulk but by the surface potential.