Phase Equilibria in the In 2 O 3 -WO 3 System
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Phase Equilibria in the In2O3-WO3 System Annette P. Richard and Doreen D. Edwards School of Ceramic Engineering and Material Science, Alfred University, Alfred, NY, 14802, U.S.A. ABSTRACT The subsolidus phase relationships in the In2O3-WO3 system at 800 – 1400oC were studied by X-ray diffraction. Two binary oxide phases – In2(WO4)3 and In6WO12 – are stable in air over the temperature range of 800 – 1200oC. Preferential volatilization of WO3 prevented the determination of phase equilibria above 1300oC. INTRODUCTION Numerous phases have been reported in the In2O3-WO3 system, including several tungsten-bronze phases [1-5], In2(WO4)3[6-8], and In6WO12 [8-11]. Interest in many of these materials has arisen because of their similarity to rare-earth tungstates and indium-containing defect-fluorites and their potential use as optical and electronic materials [6-11]. In2(WO4)3 is a trivalent ion conductor that is isostructural with Sc2(WO4)3.[7] In6WO12 has been investigated as possible electrochromic material.[8] While numerous phases have been reported in this system, the phase relationships in the system are not well understood. This report summarizes our efforts to determine the phase stability of the high-indium content compounds in this system. EXPERIMENTAL Samples were prepared with compositions ranging from xIn = 0.16 to xIn = 0.98, where xIn is defined on a cation basis, i.e. xIn = [In]/ ([In]+[W]). Most samples were prepared by solidstate reaction from commercially obtained powders: In2O3 (Indium Corporation of America) and WO3 (>99.99% purity, Aldrich Chemical Co.). Pre-weighed amounts of powders were moistened with acetone and mechanically mixed in an alumina mortar and pestle. Samples with xIn = 0.4 were also produced using a solution method in which a 1M aqueous solution of In(NO3)3 (Indium Corporation of America) and a 1M aqueous solution of H2WO4 (Kodak) were mixed in a porcelain crucible at a cation ratio of 2In:3W. Samples prepared by the solution method were calcined at 750°C to remove water and nitrate. Powders prepared by both methods ‘////were ground in an alumina mortar and pestle and then unniaxially pressed into 12.5 mm diameter pellets. Pellets were fired in alumina crucibles between 800°C and 1200°C at 100°C increments for 50 hours. Pellets were placed atop a sacrificial pellet and covered with either a second sacrificial pellet or powders of the same composition to limit volatilization of the constituent oxides. After firing, the samples were quenched in dry air, massed and ground for phase identification through x-ray diffraction analysis. The samples were then re-pressed and refired until consecutive X-ray diffraction patterns were identical. Select samples were heated to 1400oC in an attempt to identify the upper limits of phase stability. X-ray diffraction analysis was conducted using a Phillips XRG 3100 diffractometer (Phillips Inc., USA) with CuΚα radiation (40kV, 20mA). Reacted powders were mounted on a
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zero-background holder for phase identification and lattice-parame
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