High-pressure phase of the cubic spinel ${\mathrm{NiMn}}_2{\mathrm{O}}_4$

It has been observed that the fcc spinel ${\mathrm{NiMn}}_2{\mathrm{O}}_4$ transforms to a tetragonal structure at about 12 GPa. The tetragonal phase does not revert to the cubic phase upon decompression and its unit-cell constants at ambient pressure are $a_0=8.65\mathrm{}\left(8\right)\mathrm{}$ and $c_0=7.88\mathrm{}\left(15\right)\mathrm{}\AA$ (distorted fcc). Within the experimental uncertainty, there is no volume change at the transition. The $c/a$ ratio of the tetragonal spinel is almost independent of pressure and equal to 0.91. The phase transition is attributed to the Jahn-Teller-type distortion and the ionic configuration can be assumed as ${(\mathrm{M}\mathrm{n}}^{3+}{)}_{\mathrm{tetr}}{[\mathrm{Ni}}^{2+}{\mathrm{Mn}}^{3+}{]}_{\mathrm{oct}}.$ The bulk modulus of the cubic phase is 206(4) GPa.