Insights into Phase Control and Stabilisation of Metastable β-phase AMoO4 (A = Ni, Co, Ni/Co) Solid Solutions

Nickel- and cobalt-based molybdates, AMoO4, where A = Ni, Co, or Ni/Co, are of significant interest owing to their temperature-dependent phase behaviour and promising performance in electrochemical applications, particularly glucose sensing. This work explores the thermal stability and phase transitions of a series of Co and Ni molybdate solid solutions using in-situ variable-temperature X-ray diffraction (VT-XRD), with a focus on stabilising the high-temperature β-NiMoO4 phase at ambient conditions through cobalt substitution. VT-XRD measurements reveal that pristine NiMoO4 undergoes an α to β phase transition at approximately 300 °C. Progressive addition of Co systematically lowers the transition temperature to 200 °C for 5% Co and as low as 100 °C for 50% Co-substituted samples. Notably, pure CoMoO4 exhibits no β to α reverse transformation upon cooling, indicating kinetic stabilisation of the β-phase. These results demonstrate a strong compositional control over the α↔β phase transition in AMoO4 materials and highlight cobalt substitution as an effective strategy for stabilising the high-temperature β-NiMoO4 phase at or near room temperature.