Debye-Einstein model and anomalies of heat capacity temperature dependences of solid solutions at low temperatures

An approach is proposed for analysing the deviations of the heat capacity C-p(T) of solid solutions from the Kopp-Neumann rule (KNR) Delta DC(T) = C-p(T) - C-KNR(T). Temperature dependences of the heat capacity C-p(T) of selected compositions of systems (InP)(x) (InAs)(1-x) and (GaAs)(x) (InAs)(1-x) at temperatures of 5-300 K are analysed in the Debye-Einstein approximation. It was established that in the case of substitution of atoms in the cation subsystem (Ga3+ <-> In3+) with the same subsystem of anions (As3-), the positive values of Delta DC(T) at T < 100 K are due to the appearance of the lowfrequency Einstein mode, whereas the negative values of Delta DC(T) at T > 100 K are the result of a decrease in the fraction of the Debye contribution without changing the upper limit of the oscillation frequency. In the case of substitution in the cation subsystem (P3- <-> As3-) with the invariant cation subsystem (In3+) to the low-temperature positive contribution of the additional low-frequency Einstein mode, a positive part is added from the modified Debye mode having the characteristic temperature theta(D) below the additive value theta(DKNR). The adequacy of this model is confirmed by Raman scattering data.