Abstract: A nickel-cobalt-manganese oxide monocrystal having a cubic spinel structure over a broad concentration range of manganese/cobalt/nickel, including the entire possible range of manganese to nickel ratios, provided that the resulting monocrystal has a cubic spinel structure. A flux method is described for producing the monocrystals. Also disclosed are sensors, based on the monocrystals, having desirable electrical properties In particular, the sensors of the present invention are highly accurate temperature sensors or thermistors having high sensitivity, good reproducibility and improved aging characteristics. Performance of a sensor based on the monocrystal permits the preparation of a thermistor standard which can be used to evaluate the performance of ceramic devices.