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

TL;DR

This paper introduces a pseudopotential model that accurately describes phonon spectra and heat capacity in Ir and Rh, explaining their unique temperature-dependent properties and phonon features.

Contribution

A new pseudopotential model is developed to accurately predict phonon spectra and heat capacity in Ir and Rh, including analysis of Kohn singularities.

Findings

The model reproduces experimental heat capacity data accurately.

Ir and Rh exhibit a sharp decrease in Debye temperature with temperature.

Kohn singularities are discussed in relation to phonon spectra.

Abstract

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.