Anharmonic effects in thermodynamics and vacancy characteristics in alkali metals

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.