Non-centrosymmetrical phases in alkali metal oxyfluoroniobates

Abstract

Abstract Among the known ferroelectrics most common are niobium compounds such as double oxides crystallized in the type of perovskite, pyrochlorine, tangsten bronzes, LiNbO3, etc.1 Double oxides containing niobium (Y) are characterized by the structures with all NbO7 6- octahedrons forming a solid framework. Substitution of an oxide ion by its steric analogue-fluoride ion opens up greater possibilities for us to synthesize the compounds. Alongside the compounds isostructural of the corresponding double oxides with the framework-type structures, one may obtain new ones, which structure differs in the manner of octahedral junction. The increase in F/O ratio leads to the decrease in the number of common octahedral components for Nb(0, F)6 to produce the structures of a layered-, chain- and isIand-type.2.3 On the other hand, a change in the F/O ratio affects the length and degree of covalency in the bond Nb-(0, F) inside the niobium-bearing element of the structure, thus determining the dielectric properties...