Negative thermal expansion induced by coordination number change in high-entropy oxides

Coordination number changes in chemical bonds sometimes produce a gigantic volume change, as exemplified by the α–β transition in tin. We have achieved a giant negative thermal expansion (NTE) using four-to-six coordination number change in high-entropy oxides. Thermal expansion anomalies appear in the equimolar oxide (MgCoNiCuZn)O in the crossover temperature range from a multiple-phase state including four-coordination at low temperatures to a single-phase state with six-coordination. By changing the cation ratio, a giant NTE occurs with a volume change exceeding 2% at temperatures higher than 800 °C. Furthermore, doping Ca or Na promotes atomic movement accompanying the crossover and also improves volume change reversibility. Comparison of powder x-ray diffraction analysis and dilatometry results demonstrates that the coordination number change induces NTE.