Structural and electrical study of the alluaudites (Ag_(1-x)Na_x)_2FeMn_2(PO_4)_3 (x = 0, 0.5 and 1)

摘要

Three compounds with the alluaudite-type structure (Ag_(1-x)Na_x)_2FeMn_2(PO_4)_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 FeO_6 and MnO_6 octahedra linked together via common corners of PO_4 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.