NTC thermistors are semiconducting ceramics with defined temperature characteristics of resistivity used for temperature measurement. The charge transport is based on a hopping-type motion of charge carriers between transition metal cations in octahedral sites of the spinel lattice. The majority of commercial NTC thermistor ceramics is fabricated using substituted spinel-type Ni-Co-Fe-Mn oxides. The operating temperature of most thermistors is limited to about 120°C; moreover, long-term stability of the resistivity is a critical issue. NiMn₂O₄, as one of the archetype spinel thermistors, is stable in air between 730 °C and 920 °C only. This spinel decomposes into NiO and a Mn-rich spinel at higher temperature, whereas at lower temperatures NiMnO₃ and Mn₂O₃ are formed. However, NiMn₂O₄­­ bulk thermistor ceramics are obtained as metastable spinels using proper synthesis conditions. Fe-substituted spinels, e.g.            NiFe_zMn_2-zO₄, exhibit thermodynamic stability over a wide temperature range and show improved ageing behavior.
To gain better understanding of the electrical properties and aging characteristics of thermistor spinel oxides, we investigated the cation distribution of NiMn₂O₄ and Fe-substituted spinels using in-situ neutron powder diffraction (NPD) in the temperature range from 25 °C up to 900 °C. Rietveld refinements of NPD data allow to extract a temperature dependent inversion parameter ν, describing the distribution of Ni- and Mn-cations between tetrahedral and octahedral sites of the spinel structure. Evaluation of temperature-dependent Seebeck coefficients using a modified Heikes’ formula allow to characterize the disproportionation p of Mn cations. Combining both results allows to derive the cation distribution Ni²⁺_1-vMn²⁺_pMn³⁺_v-p[Ni²⁺_vMn³⁺_2-v-pMn⁴⁺_p]O₄. The same methodology was used for Fe-substituted spinel oxides. Finally, the temperature dependent cation distribution is discussed in terms of its effect on thermodynamic stability, electrical properties and ageing behavior of NTC thermistor spinel ceramics.