Transport studies on the superionic system 4AgI-Ag3VO4: conductivity comparison with the corresponding superionic glass

Abstract

Superionic conductors have attracted much attention because of their unique transport properties and technological application in solidstate batteries [1], electrochromic displays [2], etc. Solid silver ion conductors showing high ionic conductivity at ambient temperature have been obtained by either cationic or anionic substitution of AgI [3]. Unlike most of the cationsubstituted AgI-type solid electrolytes, anionsubstituted ones are not only inert towards moisture and iodine, but can also be prepared in the glassy state [4]. Once in the glassy state, these materials have many advantages over the materials in the polycrystalline state, such as isotropic properties, ease of forming, no grainboundary disks, formation of thin films, higher conductivities [5, 6], etc. In the present work ionic conductivity and thermoelectric powder studies on the 4AgI-Ag3VO4 system has been undertaken and its conductivity compared with the corresponding glassy system. Silver vanadate was prepared [7] by the addition of an aqueous solution of AgNO 3 to a saturated solution of ammonium meta-vanadate containing sodium acetate and concentrated ammonia liquid. A yellow precipitate was formed which turned into brown after settling. This was filtered out and dried at l l 0 ° C in a nitrogen atmosphere for 24h. Silver vanadate, thus prepared, was vacuum sealed in glass ampoules with Analar-grade silver iodide in the molar ratio 1:4 and heated at 400°C for 48h; the product was then cooled to room temperature. X-ray analysis of this compound indicated that the most intense lines of AgI and m g 3 V O 4 w e r e not present in this new compound formed, which may be tentatively written as Ag7IaVO 4. Total conductivity was measured using a 1-kHz impedance bridge (GR1650) on pellets of the compound formed under a pressure of 4000kgcm -2 with electrodes of a mixture of silver and the electrolyte, to minimize contact polarization. Fig. 1 shows a typical plot of log aT against reciprocal of absolute temperature for a pellet annealed at 100°C in vacuum for 12h. From the slope of the graph, the activation energy for Ag ÷ ion conduction has been found to be 0.185eV. The conductivity at room temperature (300K) was 5.3 x 10-3Q -1 cm -1. The high ionic conductivity and low activation energy suggest that silver ions in this new compound may be statistically distributed in the lattice taking an averaged structure as in other silver ion conductors [8]. A home-built apparatus has been used for thermoelectric power (TEP) measurements. The thermocell was of the following configuration: Ag(T) I Ag714VO41Ag(T + A T). The expression for thermoelectric power, 0, of a superionic conductor [8] can be written as