Abnormal phase transition in BiNbO4 powders prepared by a citrate method

<font face="times new roman,times" size="3">Triclinic beta-BiNbO4 prepared below 750 degrees C and above 1040 degrees C (denoted as Low-beta and High-beta, respectively) and pure orthorhombic alpha-BiNbO4 at 900 degrees C were successfully derived from a citrate method and the phase transition from beta-BiNbO4 to alpha-BiNbO4 was first observed in BiNbO4 powders. This phenomenon proves that the abnormal phase transition from beta-BiNbO4 to alpha-BiNbO4 exists in BiNbO4 powder system. The synthesis of Low-beta powders can be attributed to the formation of the intermediate phase of Bi5Nb3O15 by the citrate method. With increasing temperature, the Low-beta phase gradually turns into alpha-BiNbO4 due to the thermodynamically metastable state of Low-beta. We also identified that the stress in pellet format can accelerate the phase transition from Low-beta to alpha 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.