The configurational entropy theory and the heat capacity decrease of orientationally disordered crystals on cooling to 0K

Abstract

The validity of the Kauzmann extrapolation in the entropy theory for kinetic freezing of orientational disordered crystals is examined. It is found that the theory itself does not support this extrapolation nor does it lead to the inference of an Ehrenfest-type thermodynamic transition in the eauilibrium state at a temperature above 0K. It is argued that the heat capacity C_p and entropy S of the eauilibrium disordered state of a crystal would decrease along a stretched sigmoid shape curve to their zero values at 0 K, while the enthalpy H and volume would remain higher than in the ordered crystal state. This is elaborated by interpolating C_p between 0 K and the availabie high-temperature C_p values of the eauilibrium state of five substances, four orientationally disordered crystals and one nematic liauid crystal. The calculated H of the eauilibrium state at 0K is 20-41% of the enthalpy of transformation. The role of vibrational and configurational entropies has been discussed, and it is shown that the configurational part of C_p and S would not diverge at any temperature, nor would the configurational entropy be proportional to the excess entropy of the eauilibrium disordered state over the ordered state. The manner of decreases in C_p, S and H of the kinetically frozen state towards the eauilibrium state in our interpolation is characteristically different from the corresponding decrease in the Kauzmann eauilibrium state.