A new series of isotypic monodiphosphates of general formula Li 9 M 3 (P 2 O 7 ) 3 (PO 4 ) 2 with M ؍ Al, Ga, Cr, Fe was synthesized by flux methods. The crystal structure of the aluminum and iron members is described here. They crystallize in the trigonal space group P3 c1 with a ؍ 9.553(1), c ؍ 13.492(2) A s (M ؍ Al), a ؍ 9.726(1), c ؍ 13.615(2) A s (M ؍ Fe) and Z ؍ 2. The structure consists of 2 [(MP 2 O 7 ) 3 (PO 4 ) 2 ] 9 \ corrugated layers, parallel to (001), separated by lithium ions. The layers are built up of MO 6 octahedra sharing corners with PO 4 tetrahedra and P 2 O 7 groups. Three nonequivalent lithium ions are present according to the structural formula Li(1
The crystals exhibit a lithium ion conduction mainly parallel to (001) but with rather low conductivity values: // (001) ؍ 1.3؋10 ؊4 and 3.0؋10\ 6 \ 1 cm\ 1 at 300°C for M ؍ Fe and M ؍ Al, respectively. In the presence of acidic aqueous solutions, Li 9 Fe 3 (P 2 O 7 ) 3 (PO 4 ) 2 undergoes an ion-exchange reaction between Li ؉ and H ؉ with the introduction of water molecules, which causes a one-dimensional expansion of the crystals perpendicular to the layers.