KNb4O5F and NbO2 crystal structures. Structural aspect of chemical decomposition of K2-xNb4O3(O,F)(3)F in the melt of sodium and potassium chlorides

The structures of the KNb4O5F (I) (synthesized for the first time) and the NbO2 (II) phases formed together with NbO during decomposition of reduced o xofluoroniobate K2-xNb4O3(O,F)(3)F (III) in the (NaCl + KCl) melt are studi ed by X-ray methods. The tetragonal structures of compounds I and II are re fined up to R = 0.06 and R = 0.0186, respectively. The unit-cell parameters are determined as a = 4.155(3) and c = 8.377(6) Angstrom, sp. gr. P4/mmm f or compound I, a = 13.698(3) and c = 5.976(1) Angstrom, sp. gr. 14(1)/a for compound II, and a = 4.151(3) and c = 21.64(2) Angstrom, sp. gr. P4/mmm fo r compound III. Decomposition of compound III is considered in terms of dif ferent stabilities of its structural modules (the two-dimensional periodic NbO and the KF fragments). The formation of NbO and compound I is associate d with the decomposition of KF fragments and the preservation of NbO fragme nts. In compound I, only one of two KF networks (similar to those separatin g the NbO fragments in compound III) is preserved. In NbO, two-dimensional condensed NbO fragments form a three-dimensional framework. Compound II has neither of the two modules observed in III, but the NbO4 squares forming a n NbO fragment are preserved. The defects of the real structure of compound I, which give rise to the formation of a one-dimensionally disordered KNb4 O5F structure, are analyzed. The discrepancies between the structural chara cteristics of compound II obtained in the present study and reported earlie r are discussed.