Anharmonic effects in thermodynamics and vacancy characteristics in alkali metals

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Abstract
Anharmonic contributions to free energy, pressure, specific heat, Cpan(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 Cvan amount to 0.3-0.5 of the difference Cp-Cv and most of them are in good agreement with experiment. The volume dependence of Cvan is very weak and anharmonic pressure Pan 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 Cd (T) of Na, K, Rb, and Cs have been determined from the experimental specific heat Cp and the calculated Cvan and Cp-Cv. The vacancy concentrations nv and formation energies E1vF have been estimated from the Cd(T). The values of nv obtained are larger, and those of E1vF smaller, than most previous estimates.