Mn₃O₄ under High Pressure and Temperature: Thermal Stability, Polymorphism, and Elastic Properties

We report results of in situ synchrotron X-ray diffraction studies of hausmannite up to 7.2 GPa and 1273 K. The Mn₃O₄ tetragonal spinel is found to transform to a 9.6% denser polymorph of the CaMn₂O₄-type structure at 7.2 GPa and 673 K, under milder conditions than those of any transformations to postspinel phase described so far. Upon heating at high pressure, the Mn₃O₄ phase undergoes decomposition and finally disappears in favor of MnO at temperatures above 1073 K. A fit of the pressure dependence of the volume of tetragonal phase to the Birch–Murnaghan equation of state yields the bulk modulus, <i>K</i>₀, of 132.6 ± 3.1 GPa if the first pressure derivative of the bulk modulus is fixed to 4, and <i>K</i>₀, of 102 ± 10 GPa for <i>K</i>₀′ refined to 18.1 ± 5.6. Nonlinear compression behavior is observed for both crystallographic axes, with the <i>c</i> axis being more compressible than the <i>a</i> axis. Axial ratios in CaMn₂O₄-type Mn₃O₄ with temperature are also estimated. Surprisingly, the lattice distortion of the marokite-like phase is not preserved upon releasing pressure and temperature to ambient conditions

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