The monoclinic isotypic compound was synthesized by crystallization in a flux of sodium arsenate. It exhibited high ionic conductivities (0.001 to 0.01S/cm at 300C) and good cation exchange properties; particularly with regard to Ag ions. It 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(As2O7)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 104).

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

 

 

 

 

Table 104

Conductivities and Activation Energy of -Na7Fe3(As2O7)4

 

Specimen

Direction

T(C)

 (S/cm)

E (eV)

polycrystalline

-

100

2.3 x 10-5

0.47

polycrystalline

-

300

2.3 x 10-3

0.47

monocrystalline

||

100

5.9 x 10-4

0.39

monocrystalline

 

100

2.4 x 10-7

0.38

monocrystalline

||

300

2.6 x 10-2

0.39

monocrystalline

 

300

9.8 x 10-6

0.38