EVALUATION OF THE HIGH-TEMPERATURE PRESSURE DERIVATIVE OF THE GRUNEISEN CONSTANT FROM THE TEMPERATURE-VARIATION OF THE ELASTIC-MODULI

An average quasiharmonic formulation is used to estimate the second-or der effective Gruneisen parameter, and to express the high-temperature strain derivative of the first-order effective Gruneisen parameter (E GP) in terms of the variation with temperature of the elastic constant s Delta C-alpha beta(T) of several alkali halides. To a good approxima tion the magnitude of the strain derivatives is determined by the valu e of a C-alpha beta-EGP and are positive for LiF, NaF and NaCl at room temperature. From the EGP's strain derivatives we deduce the hydrosta tic pressure dependence of the average Gruneisen constant gamma from r oom to high temperatures for NaCl, KCl and KBr. Good agreement is obta ined between the calculated partial derivative gamma/partial derivativ e p of NaCl at 300 and 800 K and experimental data up to 1 GPa. We fin d for the alkali halides that, in general, the decrease of gamma with increasing pressure is due to predominant fourth-order effects on the temperature variation of their elastic constants. The present method o f evaluation of partial derivative gamma/partial derivative p does not require the use of pressure derivatives of the elastic moduli. (C) 19 98 Elsevier Science Ltd. All rights reserved.