LOW-TEMPERATURE SPECIFIC HEATS AND RELATED THERMODYNAMIC PROPERTIES OF SODIUM FLUORIDE AND CAESIUM BROMIDE

Measurements of the specific heats of NaF and CsBr between 20 and 300°K are reported. The results are believed to be accurate to within 0.5% down to 60°K, falling to 2% at 25°K. The Grüneisen parameters γ = βV/CVχT = βV/CPχS of NaF and CsBr have been calculated from the present heat capacity measurements in conjunction with earlier thermal expansion and elasticity data, and in the case of NaF from the pressure derivatives of the elastic constants. In common with NaCl and NaI the Grüneisen parameter of NaF falls with diminishing temperature, but this parameter is independent of temperature for CsBr, in common with CsCl and CsI. The quasi-harmonic approximation has been applied to calculate the maximum frequencies vD(n) of the Debye distributions having the same nth moments as the actual crystals for -3 <or= n <or= 6, interpolation being employed in evaluating those parts of the integrals occurring between 0 and 25°K for -2 <or= n <or= 1. The variations display characteristic minima, and it has been observed that the dependence of vD(n) upon n diminishes for the positive moments as the ratio of the masses of the constituent ions approaches unity for both face-centred and body-centred alkali halides.