Two new orthophosphates having a tunnel structure were synthesized. Both of them belonged to the orthorhombic space-group (Pmnn, Z = 4), with a = 0.6152, b = 1.6214, c = 0.9484nm and a = 0.6450, b = 1.6730, c = 0.9870nm, respectively, for Ni4K(PO4)3 and Mn4Rb(PO4)3. Their 3-dimensional frameworks, [M4P3O12]x, consisted of edge-sharing tri-octahedral units, [M3O13], and square pyramidal groups, M2O8; all of which were joined via isolated PO4 tetrahedra. This 3-dimensional lattice contained large pseudo-hexagonal tunnels which ran along the c-direction where the alkali cations were located. The ionic conductivity was measured by means of impedance spectroscopy, and was found to depend upon the alkali-ion mobility through the tunnels. The results were explained in terms of the structural characteristics and alkali-cation size.

Structure Features and Ionic Conductivity of the AM4(PO4)3 Orthophosphates: A.Daidouh, C.Pico, M.L.Veiga: Solid State Ionics, 1999, 124[1-2], 109-17