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 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

Volume variation of Gruneisen parameters offcc transition metals

“…The present value of Ni is also high. However, γ values of Pd, Pt, Co, Ir and Rh are quite reasonable compared to experimental and other theoretical values (Barrera and Batana 1993a;Katsnelson et al 1997).…”

“…Our calculations show that the relation (γ/Ω) = constant holds good for fcc transition metals. 2⋅05 (Katsnelson et al 1997) However, the results of volume variation of γ for Ni, Ag, Au and Rh are not satisfactory. This point is worth discussing.…”

“…In other words, a simple model potential of Heine-Abarenkov (1964) type, as used by Antonov et al is not adequate atleast for Ag, Au, Ni, and Rh. In fact, recently Katsnelson et al (1997) have carried out an extensive study of phonon spectra and specific heat of Rh with Heine-Abarenkov type model potential. These authors found that the form factor obtained from Heine-Abarenkov model requires modification in certain region of q-space.…”

Thermal expansion and the equation of state of Ir and Rh

“…§1. IntroductionAs it is stated in the literature (Gornostyrev et al 1994, Ivanov et al 1994, Katsnelson et al 1996 referred to as I), Ir and its analogue Rh are set off from other FCC metals because of some distinctive features of their physical properties: unusual deformation-induced failure, peculiar defect structure, specific temperature dependence of the effective Debye temperature, etc. Moreover, the situation is not trivial because in the case of Ir, unlike the most, if not all of the other FCC metals a rather broad range of properties (elastic moduli, phonon spectra and lattice heat capacity) can be described in terms of a simple pseudopotential model (see I and references therein).…”

Thermal transport in DNA

“…The Debye temperatures obtained are lower than the values extracted from the low temperature specific heat measurement (420K),40 but close to the specific heat measurement made in the approximate range θR/2 to θR, i.e. high-temperature value (290 K) 55. The fitting results for two Ir films on the DNA fiber sample are: 21.96×10 −8 Ωm and 22.49×10 −8 Ωm for αρ, 259.4 K and 285.9 K for θR.…”