CRYSTAL-STRUCTURE OF A NEW LOW-VALENCE NIOBIUM COMPOUND - K2-XNB4O3(F,O)(3)F

The structure of a new low-valence niobium phase K2 - xNb4O3(F,O)(3)F is investigated by single-crystal X-ray diffraction analysis. The comp ound is obtained by electrocrystallization on a cathode from molten sa lts that contain both the Nb(IV) and Nb(V) fluoride and oxyfluoride co mplexes 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 P 4/mmm are a = 4.137, c = 21.860 Angstrom (I) and a = 4.148, c = 21.582 Angstrom (II). The refinement in the anisotropic approximation of ato mic 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 altern ate along the four-fold axis. The general crystal chemical formula is [Nb4O3(O,F)(2)].[KF][(K,square)(F,O]. Structures I and II differ in th e 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 f ragments. It is shown that the new phase is a structural homologue of compounds MNb4O6 (M = Ba or K). The m''NbO''. ''MX'' and ''NbO''.n''MX '' homologous series are compared. The occurrence of the m''NbO''.n''M X'' series (M = Ba or K; X = O or F; and n and m are integers) is pred icted.