Anharmonic lattice vibrations in noble metals

Both macroscopic and microscopic effects of anharmonicity in noble metals have been calculated in low-order perturbation theory. A strain-dependent Helmholtz free energy is used to evaluate the thermoelastic properties. The interactions are modeled according to a force-constant scheme which includes angular forces in the harmonic part of the potential. The cubic and quartic anharmonic force constants are related to third- and fourth-order elastic constants. It is shown that the cubic pairwise interactions have to be extended up to the third-neighbor shell, in order to fit experiments in Ag and Au. A slightly negative anharmonic contribution to the constant-volume specific heat is found, in good agreement with recent experimental inquiries. Phonon line shifts are computed along the high-symmetry directions of the Brillouin zone. Our results reproduce well neutron-scattering measurements which are available for copper at high temperatures. An enhanced line shift in longitudinal phonons of noble metals, along the [ 0 0] direction, is pointed out. © 1990 The American Physical Society.