Structure features and ionic conductivity of the AM(4)(PO4)(3) orthophosphates: (A = Na, K, Rb; M = Ni, Mn)

Two new orthophosphates KNi4(PO4)(3) and RbMn4(PO4)(3), with tunnel structu re, have been synthesized. Both compounds crystallize in the orthorhombic s pace 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) fo r RbMn4(PO4)(3). Their three-dimensional framework [M4P3O12](infinity) cons ists of edge-sharing trioctahedral units [M3O13] and square pyramidal group s M2O8, and all of them are joined by the isolated PO, tetrahedra. This 3D lattice possesses large pseudo-hexagonal tunnels running along the c-direct ion where the alkali cations are located. Ionic conductivity in these mater ials 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 th e basis of the structural characteristics and alkali cations sizes. (C) 199 9 Elsevier Science B.V. All rights reserved.