Monoclinic isotypic compounds, where M = Al, Cr, Fe or Ga, were synthesized by crystallization in a flux of sodium phosphates. They exhibited high ionic conductivities (0.001 to 0.01S/cm at 300C) and good cation exchange properties; particularly with regard to Ag ions. It was found that, at temperatures ranging from -20 to 240C (depending upon the compound), they underwent a reversible    phase transition which was associated with a loss of long-range order of the Na ions and resulted in an increase in the conductivity. A model was proposed for the Na diffusion paths in the crystal structure of -Na7Fe3(P2O7)4. This was based upon 2-dimensional conduction within planes which were parallel to (001). Conductivity studies of monocrystals confirmed this 2-dimensional model by revealing a ||/ ratio of about 2000 (table 174).

C.Masquelier, F.D'Yvoire, E.Bretey, P.Berthet, C.Peytour-Chansac: Solid State Ionics, 1994, 67[3-4], 183-9

Table 174

Conductivities and Activation Energy of Na7Fe3(P2O7)4