Techniques for calculating properties of materials at high pressures

It is shown that, according to the equation of classical thermodynamics (pa rtial derivative U/partial derivative p)(T) = V(beta p - alpha T), where be ta is compressibility and alpha the volume expansion coefficient, the inter nal energy of materials decreases with increasing pressure, in conflict wit h the energy conservation law. The assumptions leading to this conflict are analyzed. It is assumed that the only inherent characteristic of a materia l is its internal energy U and not enthalpy H or heat q. In connection with this, materials should be characterized by energy capacity C-U, rather tha n by heal capacity C-p. These characteristics are related by C-U = C-p - al pha pV. The relative difference between C-U and C-p increases with pressure and ranges, depending on material, from 0.5 to 5% at 10(8) Pa and from 7 t o 9% at 10(9) Pa. An equation is derived for the pressure derivative of int ernal energy, according to which this derivative is positive, in accordance with the energy conservation law. In addition, equations are obtained for calculating the internal-energy and free-energy changes caused by an increa se in pressure at constant temperature.