Synthesis and Thermal Stability of Cubic ZnO in the Salt Nanocomposites
  Cubic zinc oxide (rs-ZnO), metastable under normal conditions, was synthesized from the wurtzite modification (w-ZnO) at 7.7 GPa and ~800 K in the form of nanoparticles isolated in the NaCl matrix. The phase transition rs-ZnO \rightarrow w-ZnO in nanocrystalline zinc oxide under ambient pressure was experimentally studied for the first time by differential scanning calorimetry and high-temperature X-ray diffraction. It was shown that the transition occurs in the 370-430 K temperature range and its enthalpy at 400 K is -10.2 \pm 0.5 kJ mol-1. 
P.S. Sokolov, A.N. Baranov, Zh.V. Dobrokhotova, V.L. Solozhenko
cond-mat.mtrl-sci