Alkali Bismuth Niobates, RbBi2Nb5O16 and CsBi2Nb5O16, with a Pyrochlore Structure

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Abstract
A phase equilibrium study of the ternary system A2O-Bi2O3-Nb2O5 (A=K, Rb or Cs) has been made. An extended range of a solid solution with the pyrochlore-type structure was found in the respective system. The composition of this new phase is described by a representative formula ABi2Nb5O16, which exists mainly along a composition range BiNbO4-ANb3O8. Phase diagrams of BiNbO4-ANb3O8 systems with A=K, Rb and Cs were determined. Large single crystals of RbBi2Nb5O16 and CsBi2Nb5O16 were grown by the pulling method. They exhibit dielectric anomalies at 120-130 K.