Thermal expansion and the equation of state of Ir and Rh

Abstract

The simplest anharmonic characteristics of Ir and Rh are discussed in the framework of a previously developed simple pseudopotential model which describes the elastic moduli, phonon spectra and the lattice heat capacity in the harmonic approximation of these metals succesfully. The microscopic Gruneisen parameters, the dependences of the elastic moduli on pressure, the coefficient of thermal expansion and the equations of state at the finite temperatures have been calculated. The ab initio calculations of the energy-band structure and the equation of state for Ir at T = 0 have been done to test the model for adequacy at high pressures. The values of different contributions (zero-point oscillations, quasiharmonic, etc.) in the considered thermodynamic characteristics of Ir and Rh are discussed. §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). Even in sp-metals the situation is less favorable: to describe the elastic moduli of Ca allowance must be made for the singular contributions to energy which result from the proximity of the Fermi surface and the faces of the Brillouin zone (Katsnelson et al 1990), whereas for Al it is necessary to take into account the contributions of three body forces. Nevertheless, in describing the phonon spectra of Al it is impossible to attain an accuracy 1

Thermophysical properties of iridium at finite temperature

“…Our results are in excellent agreement with the results obtained by Katsnelson et al [38] using I for all compression range and in excellent agreement for smaller compressions up to 20% and in reasonably good agreement at higher compressions using MI. From above comparison, we find that our re-116401-4 sults are better than other theoretical results, [38,39] which confirm the applicability of local form of the pseudopotential used in the present calculation which has only one adjustable parameter r c (core radius). Further, EOS is a test of approximations used in the theory for the electron-electron interaction which seems to be properly accounted for.…”

“…To the best of our knowledge, for the first time, in this paper, we have carried out the study of thermal expansion of Ir up to melting temperature because, at higher temperatures, the experimental results of β P change appreciably and the theoretical studies are limited up to 900 K. By conducting a detailed comparison of our results with experimental results, we find that our results are almost identical to experimental results obtained by Touloukian et al (recommended values) up to 1500 K. [34] The maximum deviation of our computed results with the results obtained by Touloukian et al is 6.9% at 2500 K. The results obtained by Arblaster [35] are also in good agreement with 2.78% variation at 2500 K. The results obtained by Caldwell [36] are in excellent agreement with our computed results up to 1300 K. The results obtained by Halvorson [37] are in excellent agreement with our theoretical results at low temperature and high temperature with maximum deviation of 1.05% at 1800 K. Our computed results are also better than the theoretical results obtained by Ferah et al [5] and Katsnelson et al [38] Ferah et al [5] used molecular dynamics simulations by using modified Morse potential and employed embedded atom method (EAM) to study temperature variation of thermal expansion. Katsnelson et al [38] used pseudopotential obtained by Ivanov et al (I) and modified form of the pseudopotential of Ivanov et al (MI) to study the temperature variation of thermal expansion. From such detailed comparisons, we find that the simple method used in this paper properly accounts for anharmonic effects.…”

“…In order to compare our computed results of volume thermal expansion with the experimental and other theoretical results, we have converted the results in the form of relative volume thermal expansion. The temperature variation of relative volume thermal expansion along with the experimental findings [34][35][36][37] and other theoretical results [5,38] is shown in Fig. 3.…”

“…Our computed P-Ω isotherm at 300 K is compared with experimental findings [39] as well as theoretical results [38,39] in Fig. 4 up to 40% compression.…”