Thermal properties of pure and varistor ZnO at low temperatures

Specific heat data are reported on pure and varistor ZnO, 1.7–25 K, and thermal conductivity data are reported on varistor ZnO, 1.7–15 K. In pure ZnO interstitials give rise to an Einstein contribution (ωE=84 cm−1) to the specific heat above 10 K and to a Schottky contribution (δ≂10−5 eV) below 4 K due possibly to ordering. The calorimetric Debye temperature is 399.5 K. In varistor ZnO the specific heat is dominated below 20 K by compensating charge densities (∼1018 cm−3) on the interfacial barriers which give rise to two Schottky terms (δ=7×10−4 and 3×10−3 eV). All of these non-Debye excitations are localized (do not carry heat). No evidence is seen in the thermal conductivity for Kapitza-resistance effects at the heavily doped grain boundaries.