Nanostructuring and stabilization of metastable rock-salt ZnO: Impact of high-pressure media and compression geometry

For complete and reproducible recovery of a high-pressure polymorph of zinc oxide, rock-salt ZnO (rs-ZnO), nanostructured wurtzite ZnO (w-ZnO) is typically used as a precursor for high-pressure synthesis. In the case of polycrystalline w-ZnO, only small amounts (less than 30 vol.%) of disordered/nanosized rs ZnO were occasionally observed in the recovered products. Here we report the conditions for the synthesis of single-phase rs ZnO from microcrystalline (40-50 µm) w ZnO powder at 7.7 GPa and 2000 K. The complete recovery of metastable rs-ZnO is possible only by using a multianvil apparatus for quasi-hydrostatic (triaxial) compression/decompression and pyrolytic boron nitride as a pressure medium. Single-phase rs ZnO was produced as colorless nanocrystalline well-sintered bulks with Vickers hardness up to 7 GPa - the record value for ZnO due to a fortunate combination of Hall-Petch nanostructuring effect and high intergranular purity. This unexpected phenomenon can be related to the mechanism of the direct and reverse phase transitions in ZnO, which requires "uniaxial" tensile deformation. Texture analysis of the recovered samples, as well as previous kinetic studies and ab initio simulation of strain-structure relationships, strongly support this model. Thus, 3-axial decompression is a more efficient tool – neglected until now – for reproducible recovery of high-pressure ZnO-based materials than the nature of the w-ZnO precursor and the presence of isostructural rock-salt phases.