Thermal expansion of NaCl, KCl and CsBr by X-ray diffraction and the law of corresponding states

Abstract

The coefficients of thermal expansion of NaCl, KCl and CsBr are determined accurately at different temperatures using a diffractometer, Geiger counter, chart recorder and a specially designed furnace. Equations are given for the variation of the lattice constants with temperature. The temperature dependence of the thermal expansion at high temperatures is shown to be related to the concentration of thermally generated Schottky defects. The energies of formation of Schottky pairs in the three halides are estimated and are found to be consistent with those deduced from ionic conductivity studies. The reduced parameters α/(α)M/2 and T/TM give a common curve for all the halides, TM being the melting point and (α)M/2 the value of α at T = ½ TM. The curve is a straight line in the limits ~ 0.30 < T/TM < ~ 0.65 and is found to deviate considerably at higher temperatures. Assuming that the deviation is due to defects, the energies of formation of Schottky defects for the varies halides of Li, Na, K, Rb and Cs are estimated and found to agree excellently with the experimental and theoretical values.