A NEW FAMILY OF SODIUM-ION CONDUCTORS - THE DIPHOSPHATES AND DIARSENATES NA7M3 (X2O7)4 - (M=AL, GA, CR, FE, X=P, AS)

Seven monoclinic isotypic compounds with the general formula Na7M3(X2O 7) 4 have been synthesized by crystallization in a flux of sodium phos phates or arsenates. They exhibit high ionic conductivity (sigma almos t-equal-to 10(-3)-10(-2) OMEGA-1 cm-1 at 300-degrees-C) and good catio n exchange properties particularly with Ag+ ions. At temperatures vary ing from -20-degrees-C to 240-degrees-C, depending on the particular c ompound, they undergo a reversible phase transition, alpha half arrow right over half arrow left beta, which is associated with a loss of lo ng range ordering of the sodium ions and results in an increase of the conductivity. The crystal structures of beta-Na7Fe3 (AS2O7)4 and alph a-Na7Fe3(P2O7)4 have recently been determined and a model of sodium di ffusion paths has been proposed. This model indicates a two-dimensiona l conduction within planes parallel to (001). Conductivity measurement s performed on single crystals confirm this two-dimensional character with a sigma(parallel-to)/sigma(perpendicular-to) ratio of about 2 x 1 0(3).