CRYSTAL-STRUCTURE AND CATION-TRANSPORT PROPERTIES OF THE LAYERED MONODIPHOSPHATES - LI9M3(P2O7)(3)(PO4)(2) (M = AL, GA, CR, FE)

A new series of isotypic monodiphosphates of general formula Li9M3(P2O 7)(3)(PO4)(2) with M = Al, Ga, Cr, Fe was synthesized by flux methods. The crystal structure of the aluminum and iron members is described h ere, They crystallize in the trigonal space group P (3) over bar c1 wi th a 9,553(1), c = 13,492(2) Angstrom (M = Al), a = 9.726(1), c = 13.6 15(2) Angstrom (M = Fe) and Z = 2, The structure consists of (2)(alpha ) [(MP2O7)(3)(PO4)(2)](9-) corrugated layers, parallel to (001), separ ated by lithium ions. The layers are built up of MO6 octahedra sharing corners with PO4 tetrahedra and P2O7 groups. Three nonequivalent lith ium ions are present according to the structural formula 1)(1)Li(2)(2) Li(3)(6)[MP(2)(2)O-7](3)[P(1)O-4](2). The crystals exhibit a lithium i on conduction mainly parallel to (001) but with rather low conductivit y values: sigma(//(001)) = 1.3 x 10(-4) and 3.0 x 10(-6) Ohm(-1) cm(-1 ) at 300 degrees C for M = Fe and M = Al, respectively. In the presenc e of acidic aqueous solutions, Li9Fe3(P2O7)(3)(PO4)(2) undergoes an io n-exchange reaction between Li+ and He with the introduction of water molecules, which causes a one-dimensional expansion of the crystals pe rpendicular to the layers. (C) 1998 Academic Press.