  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.
