Non-Debye behaviours of heat capacities of cubic II–VI materials

On the basis of an appropriate four-oscillator version of a representative dispersion-related hybrid model we perform detailed analyses of isobaric heat capacity data available for cubic ZnS, ZnSe, ZnTe, CdTe, HgSe, and HgTe. Characteristic non-Debye behaviours of the C_p(T) data sets under study, which are manifested above all in form of non-monotonic dependences (maxima) of the respective C_p(T)/T³ curves in the cryogenic region, are described in terms of two Einstein oscillators for short-wave transversal acoustic (TA) phonons in combination with relatively weak components of Debye and non-Debye type due to long-wave acoustic phonons. This prominent non-Debye feature is represented alternatively in the form of non-monotonic dependences (minima) of conventional Debye temperature curves, Θ_D(T). The close correlation between the low-temperature asymptotic (decreasing vs. increasing) sections of Θ_D(T) vs. C_p(T)/T³ curves is described by simple algebraic formulae. The maxima positions of the latter are shown to be nearly proportional to the centre of gravity positions of the respective TA phonon spectra sections. The inherent non-Debye nature of the whole phonon density of states (PDOS) spectra is shown to find its global expression in characteristic snakelike shapes of the equivalent, moment-related phonon energy curves.