Zhuk Nadezhda A, Krzhizhanovskaya Maria G, Belyy Vladimir A, Makeev Boris A
Syktyvkar State University , Oktjabrskij prospect, 55 , Syktyvkar , Komi Republic 167001 , Russia.
Department of Crystallography, Institute of Earth Sciences , Saint-Petersburg State University , University Emb. 7/9 , St. Petersburg 199034 , Russia.
Inorg Chem. 2019 Jan 22;58(2):1518-1526. doi: 10.1021/acs.inorgchem.8b03024. Epub 2019 Jan 9.
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.
采用高温粉末X射线衍射（HTPXRD）和差示扫描量热法（DCS）在25 - 1200℃温度范围内研究了BiNbO的正交晶型（α）和三斜晶型（β）多晶型物的热行为。该研究揭示了热相变序列以及新的高温变体γ - BiNbO，其在1001℃以上形成并存在至BiNbO的熔化温度。BiNbO的不一致熔化表现为形成近似组成为BiNbO的立方相。本研究中使用HTPXRD方法评估热变形并计算BiNbO三种变体（α、β和γ）的热膨胀系数（TEC）。这三种变体的平均体积TEC在19 - 36×10℃范围内。三斜相β - BiNbO表现出最高的热膨胀各向异性。α - BiNbO的特征在于最小的TEC和各向异性，这表明其具有最大的稳定性。γ - BiNbO的晶体结构在1100℃下使用粉末数据确定，并采用Rietveld方法进行精修（α - LaTaO结构类型，空间群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%）。热分析和高温粉末X射线衍射方法表明，在加热过程中，原铌酸铋经历了以下相变序列：α - BiNbO → γ - BiNbO → β - BiNbO和β - BiNbO → γ - BiNbO → β - BiNbO或者在缓慢加热时β - BiNbO → (α - BiNbO) → γ - BiNbO → β - BiNbO，其中γ - BiNbO是原铌酸铋的高温相。
