Structure features and ionic conductivity of the AM4(PO4)3 orthophosphates:: (A = Na, K, Rb; M = Ni, Mn)

Two new orthophosphates KNi4(PO4)(3) and RbMn4(PO4)(3), with tunnel structure, have been synthesized. Both compounds crystallize in the orthorhombic space group Pmm, Z = 4 whose parameters are a = 6.152(2), b = 16.214(2), c = 9.484(3) for KNi4(PO4)(3) and a = 6.450(4), b = 16.730(3), c = 9.870(3) for RbMn4(PO4)(3). Their three-dimensional framework [M4P3O12](infinity) consists of edge-sharing trioctahedral units [M3O13] and square pyramidal groups M2O8, and all of them are joined by the isolated PO, tetrahedra. This 3D lattice possesses large pseudo-hexagonal tunnels running along the c-direction where the alkali cations are located. Ionic conductivity in these materials as well as in the isostructural phases NaNi4(PO4)(3) and KMn4(PO4)(3), was undertaken by impedance spectroscopy, and this behaviour is due to the alkali ions mobility through the tunnels. These results are analyzed on the basis of the structural characteristics and alkali cations sizes. (C) 1999 Elsevier Science B.V. All rights reserved.