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 under 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.