Structural and electrical study of the alluaudites (Ag1-xNax)2FeMn2(PO4)3 (x=0, 0.5 and 1)

Three compounds with the alluaudite-type structure (Ag1-xNax)(2)FeMn2(PO4)(3), with x = 0, 0.5 and 1 have been synthesised by solid state reactions. These phases crystallise in the monoclinic system (space group C2/c, and Z = 4), and the structure is formed by layers of edge-sharing FeO6 and MnO6 octahedra linked together via common corners of PO4 tetrahedra, yielding to 1D channels in which monovalent cations are located. Complex impedance measurements were made in the frequency range from 1 Hz to 10 MHz, at temperatures between 373 and 873 K, and reveal the predominately ionic character of the electrical conductivity in all these compounds. The activation energies show that the silver derivatives (x = 0 and 0.5) are better conductors than the sodium one (x = 1) and a phase transition about 625 K is observed for the first of them. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.