Anharmonic effects in thermodynamics and vacancy characteristics in alkali metals

Abstract

Anharmonic contributions to free energy, pressure, specific heat, C_p^an(T), and thermal expansion, ( Delta v/v)_an, have been calculated for five alkali metals. A pseudopotential model that gives a rather accurate description of the atomic properties of these metals is used. In the range of T where vacancy concentration is negligible the calculated contributions C_v^an amount to 0.3-0.5 of the difference C_p-C_v and most of them are in good agreement with experiment. The volume dependence of C_v^an is very weak and anharmonic pressure P_an is quite small. The contribution ( Delta v/v)_an is determined mainly by the bulk modulus quasiharmonic variation due to thermal expansion. The lattice defect heat capacities C^d (T) of Na, K, Rb, and Cs have been determined from the experimental specific heat C_p and the calculated C_v^an and C_p-C_v. The vacancy concentrations n_v and formation energies E_1v^F have been estimated from the C^d(T). The values of n_v obtained are larger, and those of E_1v^F smaller, than most previous estimates.