Low temperature heat capacity of bulk and nanophase ZnO and Zn _1-xCo_xO wurtzite phases

The low temperature heat capacity of the ZnO-CoO solid solution system was measured from 2 to 300 K using the heat capacity option of a Quantum Design Physical Property Measurement System (PPMS). The thermodynamic functions in this temperature range were derived by curve fitting. The standard entropies of bulk ZnO and bulk ZnO-CoO (wurtzite, 18 mol% CoO) at T = 298.15 K were calculated to be (43.1 ± 0.4) J · mol^-1 · K^-1 and (45.2 ± 0.5) J · mol^-1 · K^-1, respectively. The surface entropy of ZnO was evaluated to be (0.02 ± 0.01) mJ · K^-1 · m^-2, which is essentially zero. No sharp magnetic transitions were observed in the solid solution samples. The nanophase solid solution, 12 mol% CoO, appears to bind H₂O on its surface more strongly than ZnO.