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 (X2O7)4 have been synthesized by crystallization in a flux of sodium phosphates or arsenates. They exhibit high ionic conductivity (σ≈10−3-10−2ω−1cm−1 at 300°C) and good cation exchange properties particularly with Ag+ ions. At temperatures varying from −20°C to 240°C, depending on the particular compound, they undergo a reversible phase transition, α⇌β, which is associated with a loss of long range ordering of the sodium ions and results in an increase of the conductivity. The crystal structures of β-Na7Fe3 (As2O7)4 and α-Na7Fe3 (P2O7)4 have recently been determined and a model of sodium diffusion paths has been proposed. This model indicates a two-dimensional conduction within planes parallel to (001). Conductivity measurements performed on single crystals confirm this two-dimensional character with a σ|/σ⊥ ratio of about 2×103.