The equation of state and the temperature variation of the second order elastic constants of sodium and potassium

The equations are calculated using the model pseudopotential of Wallace. In calculating the vibrational contribution to the total pressure, the volume variation of the Gruneisen parameter is taken into account. The calculated isotherms at 300K and 100K are compared with the experimental measurements of other researchers. The agreement between theory and experiment at 300K is extremely good up to a pressure of 45 kilobars. The agreement at 100K is fairly good up to a pressure of 20 kbar. Using the same potential the temperature variation of the second order elastic constants are calculated making the approximation of replacing the normal mode frequency by the square root of the second moment mu 2. The calculated results indicate that in the case of C11 and C12 the temperature dependence has the right behaviour, whereas C44 almost comes out independent of temperature as against the experiment which shows a rapid decrease.