Heat capacity of ZnO: Isotope effects

We have measured the heat capacity of zinc oxide for several single crystals with different isotopic compositions in the 5-350 K temperature range. We have performed first-principles calculations in order to analyze the dependence of the heat capacity on the isotope mass of the oxygen and zinc atoms. This dependence provides us relevant information to understand thermodynamic properties from a microscopic point of view, and to validate theoretical models of phonon dispersion relations and atomic displacements. The Zn mass affects mainly the acoustic phonons, thus leading to isotopic effects on the heat capacity mostly at low temperatures, whereas the O mass affects mainly the optic phonons, which become thermally active at higher temperatures. This behavior is reproduced quantitatively by the calculations, and is also in agreement with theoretical predictions reported for wurtzite GaN, an isostructural semiconductor.