  We calculated the phonon dispersion relations of ZnX (X=Se, Te) employing ab
initio techniques. These relations have been used to evaluate the temperature
dependence of the respective specific heats of crystals with varied isotopic
compositions. These results have been compared with mea- surements performed on
crystals down to 2 K. The calculated and measured data are generally in
excellent agreement with each other. Trends in the phonon dispersion relations
and the correspond- ing densities of states for the zinc chalcogenide series of
zincblende-type materials are discussed.
