High pressure and high temperature study the phase transitions of BiNbO4

Phase transitions of both α-BiNbO4 and β-BiNbO4 were investigated under high pressure and high temperature (HPHT) (0–5 GPa and 300–1800 °C, respectively) using X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectroscope (EDS). The stable regimes for the BiNbO4 phases were also determined. Experiments showed that both α-BiNbO4 and β-BiNbO4 could convert to a new high pressure phase HP-BiNbO4 above 3 GPa, and the transition temperatures decreased with increasing pressures. Below 3 GPa, β-BiNbO4 keeps stable up to 1800 °C, while α-BiNbO4 converts to β-BiNbO4 when temperature is higher than 1150 °C. By heating the metastable HP-BiNbO4 phase at room pressure, HP-BiNbO4 converts to α-BiNbO4 at 600 °C. Above 1150 °C, HP-BiNbO4 would transform to β-BiNbO4. Phase relationships of Bi2O3, Nb2O5, α-BiNbO4, β-BiNbO4 and HP-BiNbO4 were also summarized.

► A new high pressure phase (HP-BiNbO4) was discovered above 3 GPa.
► HP transformation boundaries of BiNbO4 were defined up to 5 GPa, 1800 °C.
► The quenched HP-BiNbO4 would convert to the starting phases when heating.
► Phase relations of Bi2O3, Nb2O5, α, β and HP-BiNbO4 are summarized.