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Journal of Structural Chemistry

2012 year, number 5


A. D. Vasiliev, N. M. Laptash
Keywords: potassium-sodium oxopentafluoroniobate, crystal structure, O/F order, concomitant polymorphism
Pages: 922-926


X-ray crystallography is used to analyze the concomitant polymorphism of KNaNbOF5 crystals. The second β-modification of the compound, crystallizing in the tetragonal crystal system is found for the first time: space group P4/nmm, a = 5.9352(2) Å, с =8.5487(5) Å, V = 301.14(2) Å3, Z = 2, R1 = 0.0095. Parameters of the orthorhombic noncentrosymmetric structure of the α-phase, previously described by Poeppelmeier et al. (J. Am. Chem. Soc., 129, 13963-13969 (2007)), are refined and a comparative analysis of both structures is performed. The structures are characterized by the complete ordering of oxygen and fluorine atoms, the Nb-O distance in the α-phase (1.738(1) Å) being noticeably longer than that in the β-phase (1.709(2) Å). In β-KNaNbOF5, alternating NbOF5 and NaOF5 octahedra share vertices, while in α-KNaNbOF5, they share both vertices and edges. The existence of the non-polar centrosymmetric β-modification of KNaNbOF5 cancels the assumption of the substantial contribution of potassium cations to the polar structure of α-KNaNbOF5.