High-pressure phase transition and properties of spinelZnMn2O4

Abstract

X-ray photoelectron spectroscopy, magnetic measurements, and a single-crystal x-ray structure determination at normal pressure have shown that Jahn-Teller active manganese ions in ${\mathrm{ZnMn}}_2{\mathrm{O}}_4$ are present in one valence state (III) on the octahedral sites of the spinel structure. The high-pressure behavior of ${\mathrm{ZnMn}}_2{\mathrm{O}}_4$ was investigated up to 52 GPa using the energy-dispersive x-ray diffraction technique and synchrotron radiation. The structural first-order phase transition from the body-centered to primitive-tetragonal cell takes place at $P_c=23\mathrm{}\mathrm{GPa}.$ The high-pressure phase is metastable down to normal pressure. The $c/a$ ratio reduces from 1.62 to 1.10 above $P_c$ and remains nearly pressure independent in the high-pressure phase. The transition is attributed to the changes in electron configuration of the ${\mathrm{Mn}}^{3+}$ ions. According to the crystal field theory, the $e_g$ electron of octahedrally coordinated ${\mathrm{Mn}}^{3+}$ is either in the $d_z^2$ orbital or in the $d_{x^2-y^2}.$ In the first configuration the ${\mathrm{MnO}}_6$ octahedron will be elongated and this is the case at normal pressure, while the second configuration gives the flattened octahedron. In the high-pressure phase some proportion of the $e_g$ electrons of the ${\mathrm{Mn}}^{3+}$ ions is moved to the $d_{x^2-y^2}$ level, which is revealed as an abrupt fall of observed magnitude of the distortion of the bulk crystal above $P_c.$