Peculiarities of phonon spectra and lattice heat capacity in Ir aned Rh

Abstract

A simple pseudopotential model is proposed, which allows the phonon spectra and temperature dependences of the lattice heat capacity of Ir and Rh to be described with a sufficiently high accuracy. A careful comparison of the calculated and experimental values of the lattice heat capacity is carried out, with the procedure of the identification of the phonon contribution to the heat capacity and the determination of the characteristics (momenta) of the phonon density of states from the experimental values of the total heat capacity of a metal at a constant pressure being described in detail. The results of the theoretical calculations explain, in particular, such peculiar feature of Ir and Rh, unusual for cubic metals, as a sharp (by a factor of more than 1.5) decrease in the effective Debye temperature with increasing temperature. The temperature dependence of the mean square amplitude of atomic displacements in Ir and Rh has been calculated. On the basis of the band calculations the manifestation of the Kohn singularities in the phonon spectra of Ir is discussed.