Nanocrystallinity as a Route to Metastable Phases: Rock Salt ZnO

A synthesis route to rock salt zinc oxide (rs-ZnO), high-pressure phase metastable at ambient conditions, has been developed. High-purity bulk nanocrystalline rs-ZnO has been synthesized from wurtzite (w) ZnO nanopowders at 7.7 GPa and 770–820 K and, for the first time, recovered at normal conditions. Structure, phase composition, and thermal phase stability of recovered rs-ZnO have been studied by synchrotron X-ray powder diffraction and X-ray absorption spectroscopy (XANES and EXAFS) at ambient pressure. Phase purity of rs-ZnO was achieved by usage of w-ZnO nanoparticles with a narrow size distribution as a pristine material synthesized by various chemical methods. At ambient pressure, rs-ZnO could be stable up to 360 K. The optical properties of rs-ZnO have been studied by conventional cathodoluminescence in high vacuum at room and liquid-nitrogen temperatures. The nanocrystalline rs-ZnO at 300 and 77 K has shown bright blue luminescence at 2.42 and 2.56 eV, respectively.