Home – Home – Jornals – Journal of Structural Chemistry 2012 number 5

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) ų, 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.