CuMn2O4: properties and the high-pressure induced Jahn-Teller phase transition

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Abstract
Single crystal x-ray diffraction, x-ray photoelectron spectroscopy and magnetic susceptibility measurements at normal pressure have shown that, in spite of two Jahn-Teller active ions in CuMn2O4, the crystal is cubic with partly inverse spinel structure, the inversion parameter being \mbox{$x = 0.8$}. The cation configuration at normal pressure was determined as Cu0.2+Mn2+0.8[Cu2+0.8Mn3+0.2Mn4+1.0]O4. The high-pressure behaviour of the crystal was investigated up to 30 GPa using the energy dispersive x-ray diffraction technique and synchrotron radiation. A first-order phase transition connected with a tetragonal distortion takes place at Pc = 12.5 GPa, the c/a ratio being 0.94 at P = 30 GPa. The high-pressure phase has been described in terms of ligand field theory and explained by the changes to the valence and electronic configuration of the metal ions, leading to the formula Cu2+0.2Mn3+0.8[Cu2+0.8Mn3+1.2]O4. The electron configuration of the tetrahedrally coordinated Cu2+ and Mn3+ is (e4)t5 and e2t2, respectively. On the other hand, the electron configuration of Cu2+ located at octahedral sites is (t62g)e3g. While six electrons with antiparallely aligned spins occupy the triplet (t62g), three electrons on the orbital eg can be distributed in two ways (double degeneracy): (dx2-y2)1(d z2)2 and (dx2-y2)2(dz2)1. The first alternative leads to an axially elongated octahedron; the second one causes flattening of the octahedron. The contraction of the c axis indicates, that in the high-pressure phase the second configuration with unpaired electron on the dz2 orbital occurs. A similar effect of the octahedral contraction brings the orbital degeneracy of Mn3+ with the t32ge1g distribution. It follows that at high pressure the ligand field forces the two metals to take the valences that they show in the parent oxides CuO and Mn2O3.