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 Mn 3 O 4 tetragonal spinel is found to transform to a 9.6% denser polymorph of the CaMn 2 O 4 -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 Mn 3 O 4 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, K 0, of 132.6 ± 3.1 GPa if the first pressure derivative of the bulk modulus is fixed to 4, and K 0, of 102 ± 10 GPa for K 0 ′ 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 CaMn 2 O 4 -type Mn 3 O 4 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.