The low-temperature heat capacity of the Sb₂Te_3-x Se _x solid solution from experiment and theory

The lattice dynamics of Sb₂Te_3-x Se _x (x  =  0, 0.6, 1.2, 1.8, 3) mixed crystals have been studied by a combination of low-temperature heat-capacity measurements between 2-300 K and first-principles calculations. The results from the experimental and theoretical investigations are in excellent agreement. While Sb₂Se₃ can be considered as a harmonic lattice oscillator in this temperature range, for the isostructural compounds Sb₂Te₃, Sb₂Se_0.6Te_2.4, Sb₂Se_1.2Te_1.8 and Sb₂Se_1.8Te_1.2 (tetradymite structure type; R [Formula: see text] m) a small anharmonic contribution to the total heat capacity has to be taken into account at temperatures above 250 K. For the compounds which crystallize in the tetradymite structure type the experimental and theoretical data show unambiguously that the exchange of Te by Se leads to an increase of the bonding polarity and consequently to a hardening of the bonding which is reflected in an increase of the Debye temperatures with increasing Se contents. In addition, our studies clearly demonstrate that the mixed crystals in the stability field of the tetradymite structure type are characterized by a strong non-ideal mixing behavior.