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High-Temperature Crystal Chemistry of α-, β-, and γ-BiNbO Polymorphs. | LitMetric

Thermal behavior of the orthorhombic (α) and triclinic (β) polymorphs of BiNbO 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-BiNbO, which was formed above 1001 °C and existed up to the melting temperature of BiNbO. The incongruent melting of BiNbO was characterized by the formation of the cubic phase with the approximate composition BiNbO. The HTPXRD method was used in this study to evaluate thermal deformations and to calculate thermal-expansion coefficients (TEC) of the three modifications of BiNbO (α, β, and γ). The average volumetric TECs of these three modifications were in the range 19-36 × 10 °C. The triclinic phase β-BiNbO demonstrated the highest anisotropy of thermal expansion. α-BiNbO was characterized by the minimal TEC and anisotropy, which indicated its greatest stability. The crystal structure of γ-BiNbO was determined at 1100 °C using powder data and was refined using the Rietveld method (the α-LaTaO structural type the space group Cmc2, a = 3.95440(3) Å, b = 15.0899(1) Å, c = 5.65524(5) Å, V = 337.458(5) Å, R = 4.82, R = 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-BiNbO → γ-BiNbO → β-BiNbO and β-BiNbO → γ-BiNbO → β-BiNbO or, at slow heating, β-BiNbO → (α-BiNbO) → γ-BiNbO → β-BiNbO, where γ-BiNbO is the high-temperature phase of bismuth orthoniobate.

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http://dx.doi.org/10.1021/acs.inorgchem.8b03024DOI Listing

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