Cluster approximation analysis of the phase transition in Cd(CN)2•CCl4 and N(CH3)4[CdCu(CN)4]•CCl4 clathrates

The phase transition found at 220 K in Cd(CN)(2). CCl4 and at 190 K in N(CH3)(4)[CdCu(CN)(4)]. CCl4 by Raman spectroscopy was analysed using the cluster approximation, where the interactions between two atoms of the guest molecule and neighboring atoms are taken into account exactly and the rest is replaced by a mean field potential. The second order phase transition is derived and the first order behavior also obtained. The observed spectral changes of both systems are explained well in terms of the cluster approximation, from which we can deduce the parameters on the interactions.