Structure refinement of rubidium- and thallium niobyl diphosphates. Comparison with related compounds

Abstract The crystal structures of ANbOP 2 O 7 (A = Rb, Tl) compounds (space group P 2 1 n ) were refined, showing three dimensional [NbOP 2 O 7 − ] n frameworks more or less closely related to that of KNbOP 2 O 7 . The Tl one is significantly distorted. These frameworks, built up from NbO 6 octahedra and P 2 O 7 diphosphate groups sharing corners form large tunnels wherein the A + ions are situated. A discussion of all structures, including the bidimensional CsNbOP 2 O 7 one, presents evolutions related to the size and polarizability of the A + ion.