Nanocrystallinity as a Route to Metastable Phases: Rock Salt ZnO

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