Strongly reduced band gap in NiMn2O4 due to cation exchange

• Density functional based calculations were used to study collinear phase of NiMn2O4.
• The ground-state structure is a ferrimagnetic inverse spinel phase.
• The tetrahedral and octahedral Mn cations have ferromagnetic interactions.
• A 12.5% cation exchange introduces an impurity-like band in the original 1 eV gap.
• The 0.35 eV gap suggests a source of negative temperature coefficient of resistance.

NiMn2O4 is extensively used as a basis material for temperature sensors due to its negative temperature coefficient of resistance (NTCR), which is commonly attributed to the hopping mechanism involving coexisting octahedral-site Mn4+ and Mn3+. Using density-functional theory + Hubbard U calculations, we identify a ferrimagnetic inverse spinel phase as the collinear ground state of NiMn2O4. By a 12.5% cation exchange, a mixed phase with slightly higher energy can be constructed, accompanied by the formation of an impurity-like band in the original 1 eV band gap. This impurity-like band reduces the gap to 0.35 eV, suggesting a possible source of NTCR.