A simple pseudopotential model is proposed, which allows the phonon spectra and temperature dependence of the lattice heat capacity of Ir and Rh be described with a high enough 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 determination of the characteristics (momenta) of the phonon density of states from the experimental values of the total heat capacity of 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 (more than by a factor of 1.5) decrease in the effective Debye temperature with increasing termperature. The temperature dependence of the mean square amplitude of atomic displacements in Ir and Rh has been calculated. Basing on the band calculations the manifestation of the Kohn singularities in the phonon spectra of Ir are discussed. §1. IntroductionFrom the physical point of view iridium and its analogue rhodium stand out by their unusual mechanical properties among other FCC metals (see discussion in Gornostyrev et al 1994). The peculiarities of mechanical properties (at least those inherent to high pure single crystals) should be eventually determined by the specific features of interatomic interactions in metal. Greenberg et al 1990 and Ivanov et al 1994 succeded in constructing of a pseudopotential model of pair forces for these metals wich describes surprisingly well (for transition metals) their elastic properties. In terms of this model the peculiarities of the Ir and Rh defect structure (Ivanov et al 1994, Gornostyrev et al 1994 could be analyzed, and the qualitative explanation of some of their peculiar mechanical properties, primarily, the possibility of brittle failure of single crystals after a long-term stage of plastic deformation could be given. At the same time it was found that in the description of the dispersion curves of phonons in Ir measured by Ivanov et al 1994 the model does not prove to be accurate enough for oscillation modes near the Brillouin zone boundaries. According to the calculations of Ivanov et al 1994 the maximum frequency of the phonon spectrum appears to be overestimated by 25 % comparing with the experimental value found from the results of tunnel experiments , and flattenings in the 1

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The calculations are performed for a broad range of the properties of Ca and Sr in the fcc and bcc phases. A detailed information on the magnitude and character of temperature dependence of anharmonic effects in the lattice dynamics over the entire Brillouin zone (frequency shifts and phonon damping, Gruneisen parameters) is given. A detailed comparison of the computational results for the heat capacity and thermal expansion with the experimental data is carried out; the theoretical results are in good agreement with the experiment.

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