Abnormal phase transition in BiNbO4 powders prepared by a citrate method

Triclinic β-BiNbO4 prepared below 750 °C and above 1040 °C (denoted as Low-β and High-β, respectively) and pure orthorhombic α-BiNbO4 at 900 °C were successfully derived from a citrate method and the phase transition from β-BiNbO4 to α-BiNbO4 was first observed in BiNbO4 powders. This phenomenon proves that the abnormal phase transition from β-BiNbO4 to α-BiNbO4 exists in BiNbO4 powder system. The synthesis of Low-β powders can be attributed to the formation of the intermediate phase of Bi5Nb3O15 by the citrate method. With increasing temperature, the Low-β phase gradually turns into α-BiNbO4 due to the thermodynamically metastable state of Low-β. We also identified that the stress in pellet format can accelerate the phase transition from Low-β to α phase of BiNbO4 in comparison with powder samples. It brings us new understanding of the BiNbO4 system and also provides a simple way to obtain BiNbO4 for microwave and photocatalytic applications.

► Pure BiNbO4 powders were successfully prepared by a citrate method using home-made Nb-citrate (Nb-CA) aqueous solution.
► Abnormal phase transition from β-BiNbO4 to α-BiNbO4 was first observed in BiNbO4 system.
► The synthesis of Low-β can be attributed to the formation of the intermediated phase Bi5Nb3O15 and the mechanism of the phase transition from β-BiNbO4 to α-BiNbO4 was proposed.
► We identified that the stress in pellet format can accelerate the phase transition compared with powder samples.