THE SEARCH FOR A UNIVERSAL EQUATION OF STATE CORRECT UP TO VERY HIGH-PRESSURES

The universal equations of state of solids recently proposed by severa l authors have been examined by comparing them with the theoretical re sults calculated by the augmented-plane-wave method and the quantum-st atistical model proposed by Kalitkin and Kuz'mina from low to ultra-hi gh pressures. It has been shown that the Vinet equation is in good agr eement with the theoretical results both for the P-V relation and for the pressure dependence of the isothermal bulk modulus up to 10 TPa (V /V-0 = 0.20) for monatomic solids and up to 1 TPa (V/V-0 = 0.35) for d iatomic solids. The Kumari-Dass and the Dodson equations become less s uccessful below V/V-0 = 0.7 if the zero-pressure values for B-0, B'(0) and B ''(0) are used. For monatomic solids the Holzapfel equation has a very similar structure to that of the Vinet equation at low and med ium compressions and it is in good agreement with the theoretical valu es up to ultra-high pressures. For the application to polyatomic solid s a remedy for the shortcomings of the Vinet equation at very high pre ssures is given on the basis of the quantum-statistical model. The res ulting equation is in good agreement with the theoretical values from low to ultra-high pressures both for monatomic and for diatomic solids .