Relationship between electronic and crystal structure in Cu-Ni-Co-Mn-O spinels

Local atomic and crystal structures around Cu and Mn atoms in Mn1.68Cu0.6Ni0.48Co0.24O4 spinel samples fabricated by metal-organic decomposition synthesis at different annealing temperatures were investigated by X-ray absorption fine structure analysis. There are two distinct copper cations, Cu1+ and Cu2+, both of which maintain tetrahedral coordination. The bond-length distances are Cu1+-O = 2.00 Å and Cu2+-O = 1.80 Å. The manganese cations are for the most part octahedral. The spinels prepared at low temperature (600 °C) contain smaller (Mn4+-O = 1.88 Å) undistorted MnO6 octahedrons corresponding to Mn4+ valence, whereas the manganese octahedrons in high-temperature materials (800 °C and higher) were larger and had a pronounced tetragonal distortion pertaining to Mn3+ oxidation state (Mn3+-O = 1.93 Å and 2.11 Å). By rising the fabrication temperatures, relative concentration of the species of Mn4+ and Mn3+ varies as a result of the reaction represented by Cu1+ + Mn4+ ⇒ Cu2+ + Mn3+, implying irreversible temperature-induced structural transformation. Atomic coordinates in the low-temperature phase are similar to those found in the ideal cubic spinel with oxygen parameter u = 0.27, whereas local environments of the Cu and Mn atom correspond to the tetragonal CuMn2O4 phase (space group I41/amd). Unlike in CuMn2O4, orientation of the lattice distortions is random, however, the long-range cubic spinel structure is retained at all time.