  Low-temperature heat capacities (Cp) of nanostructured rock salt (rs-ZnO) and
wurtzite (w-ZnO) polymorphs of zinc oxide were measured in the 2-315 K
temperature range. No significant influence of nanostructuring on Cp of w-ZnO
has been observed. The measured Cp of rock salt ZnO is lower than that of
wurtzite ZnO below 100 K and is higher above this temperature. Using available
thermodynamic data, we established that the equilibrium pressure between
nanocrystalline w-ZnO and rs-ZnO is close to 4.6 GPa at 300 K (half as much as
the onset pressure of direct phase transformation) and slightly changes with
temperature up to 1000 K.
