Pressure-induced phase transition in α- and β-BiNbO 4 .

BiNbO 4 has attracted a great deal of interest due to its excellent photocatalytic activities. Besides, it possesses rich polymorphism. Here, the structural stability and structural evolution of orthorhombic α- and triclinic β-BiNbO 4 were investigated via in situ X-ray diffraction patterns and Raman spectra up to 46.7 GPa. Upon compression, both BiNbO 4 samples become unstable. α-BiNbO 4 transformed into the monoclinic C 2/ c structure at 10.3 GPa, while β-BiNbO 4 possessed one P 1̄-to- P 1 isostructural phase transition around 12.7 GPa, and for the first time the crystal structure of each high pressure phase was identified. Both high pressure structures remained stable without obvious symmetry changes during compression to 46.7 GPa. In addition, both phase transitions were reversible upon decompression. These results provide insights to understand pressure-induced reversible phase transition in ABO 4 compounds with polymorphism.