Anharmonic contribution to the entropy of solids. Analysis of KF

A method is described for analysing the explicit anharmonic contribution to the entropy which is obtained from experimental Cp data. It is known that there exists a temperature interval in which the experimental entropy at fixed volume, Sexp(V0, T), fulfils Salter's expansion; it is called the apparent quasiharmonic region. According to Barron's frequency shift, the anharmonic contribution to the entropy is found to be Delta Sanh(V0, T)=(A(V0)/3NK)EvCv where A(V0) is a constant characteristic of the crystal, EvCv is fitted in the apparent quasiharmonic region to an expansion, Sigma n=0infinity anT-2n, up to a temperature T. For T>T, the difference between the experimental values and the extrapolated values, obtained with the Salter expansion, is called Delta Sexpanh(V0, T) and therefore is equal to the difference between the anharmonic contribution to the experimental entropy and the anharmonic contribution to the extrapolated entropy. The method is applied to KF, and A=(-2.1+or-0.5)10-5K-1 is found. In the potassium halides the absolute value of A theta (2) is found to decrease when the radius of halide ion increases.