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, K0, of 132.6 ± 3.1 GPa if the first pressure derivative of the bulk modulus is fixed to 4, and K0, of 102 ± 10 GPa for K0′ refined to 18.1 ± 5.6. Nonlinear compression behavior is observed for both crystallographic axes, with the c axis being more compressible than the a 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.