Thermal analysis and high-temperature X-ray diffraction study of BiNbO₄

Phase transformations of bismuth orthoniobate BiNbO₄ were studied by the methods of differential scanning calorimetry and high-temperature X-ray diffraction. The DSC curve in the heating–cooling mode of the orthorhombic samples (α-BiNbO₄) showed endothermic and exothermic effects associated with the phase transition from the orthorhombic modification to the triclinic one at the temperatures of ~ 1328 K and ~ 1270 K. This phase transition passed through a high-temperature phase of γ-BiNbO₄, in which reflections were indicated in the temperature range of 1323 K → 1353 K → 1263 K. The X-ray diffraction pattern of the sample quenched in air (77 K) and calcined at the temperature from this range, 1323 K, corresponded to the triclinic modification. During the heating, the triclinic modification (β-BiNbO₄) was transformed into the high-temperature phase of γ-BiNbO₄, which was stable up to ~ 1270 K. Then, it was transformed into the triclinic modification again. The DSC curve of the sample of triclinic BiNbO₄ showed endo- and exothermic effects at temperatures of ~ 1274 K and ~ 1270 K associated with the transition into the high-temperature phase of γ-BiNbO₄. The cooling of the sample leads to the phase transition from the triclinic to the orthorhombic modification of BiNbO₄. Thermostating at 1223 K for 1 h and quenching in air the compact sample or dispersed powder of bismuth orthoniobate of the triclinic modification led to the formation of the orthorhombic modification.