PECULIARITIES OF PHONON-SPECTRA AND LATTICE HEAT-CAPACITY IN IR AND PH

A simple pseudopotential model is proposed, which allows the phonon sp ectra and temperature dependences of the lattice heat capacity of Ir a nd Rh to be described with a sufficiently high accuracy. A careful com parison of the calculated and experimental values of the lattice heat capacity is carried out, with the procedure of the identification of t he phonon contribution to the heat capacity and the determination of t he characteristics (momenta) of the phonon density of states from the experimental values of the total heat capacity of a metal at a constan t pressure being described in detail. The results of the theoretical c alculations 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 temperatur e. The temperature dependence of the mean square amplitude of atomic d isplacements in Ir and Rh has been calculated. On the basis of the ban d calculations the manifestation of the Kohn singularities in the phon on spectra of Ir is discussed.