Crystal structure of a new low-valence niobium compound: K2-xNb4O3(F, O)3F

The structure of a new low-valence niobium phase K2-xNb4O3(F, O)3F is investigated by single-crystal X-ray diffraction analysis. The compound is obtained by electrocrystallization on a cathode from molten salts that contain both the Nb(IV) and Nb(V) fluoride and oxyfluoride complexes dissolved in fluorides of alkali metals. For two crystals with x=0.75 (I) and 0.32 (II), the unit-cell parameters in space group P4/mmm are a=4.137, c=21.860 A (I) and a=4.148, c=21.582 A (II). The refinement in the anisotropic approximation of atomic thermal vibrations converged at R=0.038 (I) and R=0.055 (II). In structures I and II, the sheets of the NbO and KF structures alternate along the four-fold axis. The general crystal chemical formula is [[Nb4O3(O, F)2]·[KF][(K,□)(F,O)]. Structures I and II differ in the number of vacancies in the K positions, the degree of deformation of the KF networks, and the strength of bonding between the NbO and KF fragments. It is shown that the new phase is a structural homologue of compounds M Nb4O6 (M=Ba or K). The m''NbO''·''MX'' and ''NbO''·n''MX'' homologous series are compared. The occurrence of the m''NbO''·n''MX'' series (M=Ba or K: X=O or F; and n and m are integers) is predicted