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

Thermal behavior of the orthorhombic (α) and triclinic (β) 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 °C. The study revealed the sequence of thermal phase transformations and the new high-temperature modification, γ-BiNbO₄, which was formed above 1001 °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 (α, β, and γ). The average volumetric TECs of these three modifications were in the range 19-36 × 10^-6 °C^-1. The triclinic phase β-BiNbO₄ demonstrated the highest anisotropy of thermal expansion. α-BiNbO₄ was characterized by the minimal TEC and anisotropy, which indicated its greatest stability. The crystal structure of γ-BiNbO₄ was determined at 1100 °C using powder data and was refined using the Rietveld method (the α-LaTaO₄ structural type_, the space group Cmc2₁, a = 3.95440(3) Å, b = 15.0899(1) Å, c = 5.65524(5) Å, V = 337.458(5) ų, 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: α-BiNbO₄ → γ-BiNbO₄ → β-BiNbO₄ and β-BiNbO₄ → γ-BiNbO₄ → β-BiNbO₄ or, at slow heating, β-BiNbO₄ → (α-BiNbO₄) → γ-BiNbO₄ → β-BiNbO₄, where γ-BiNbO₄ is the high-temperature phase of bismuth orthoniobate.