High-Temperature Crystal Chemistry of α-, β-, and γ-BiNbO4 Polymorphs

Thermal behavior of the orthorhombic (alpha) and triclinic (beta) polymorphs of BiNbO4 was studied by the methods of high-temperature powder X-ray diffraction (HTPXRD) and differential scanning calorimetry (DCS) in the temperature range 25-1200 degrees C. The study revealed the sequence of thermal phase transformations and the new high-temperature modification, gamma-BiNbO4, which was formed above 1001 degrees C and existed up to the melting temperature of BiNbO4. The incongruent melting of BiNbO4 was characterized by the formation of the cubic phase with the approximate composition Bi3NbO7. The HTPXRD method was used in this study to evaluate thermal deformations and to calculate thermal-expansion coefficients (TEC) of the three modifications of BiNbO4 (alpha, beta, and gamma). The average volumetric TECs of these three modifications were in the range 19-36 x 10(-6) degrees C-1. The triclinic phase beta-BiNbO4 demonstrated the highest anisotropy of thermal expansion. alpha-BiNbO4 was characterized by the minimal TEC and anisotropy, which indicated its greatest stability. The crystal structure of gamma-BiNbO4 was determined at 1100 degrees C using powder data and was refined using the Rietveld method (the alpha-LaTaO4 structural type, the space group Cmc2(1), a = 3.95440(3) angstrom, b = 15.0899(1) angstrom, c = 5.65524(5) angstrom, V = 337.458(5) angstrom(3), R-wp = 4.82, R-Bragg = 3.61%). The methods of thermal analysis and high-temperature powder X-ray diffraction revealed that, during the heating, bismuth orthoniobate underwent the following sequence of phase transitions: alpha-BiNbO4 -> gamma-BiNbO4 ? beta-BiNbO4 and beta-BiNbO4 -> beta-BiNbO4 -> beta-BiNbO4 or, at slow heating, beta-BiNbO4 -> (alpha-BiNbO4) -> gamma-BiNbO4 -> beta-BiNbO4, where gamma-BiNbO4 is the high-temperature phase of bismuth orthoniobate.