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Abstract

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.