Difference in Oxide Ion Conduction Properties of Oxide Ion Excess and Cation Deficient PbWO4s and Oxide Ion Deficient CaWO4

Abstract

Introduction When lanthanum ions are partly substituted for lead in PbWO4 with sheelite-type structure, high oxide ion conduction appeared at elevated temperature [1]. From the density measurement, it has been reported that oxide ion interstitials are formed for the substitution as Pb1-xLaxWO4+x/2, while Pb1-xLa2/3WO4 shows the defect structure of cation deficiency. While the former system exhibits a little higher oxide ion conduction, both systems show approximately unity of the oxide ion transport number. In recent years, it is also found that CaWO4 with the mineral name of scheelite also exhibit the oxide ion conduction by partly substituting potassium ions for calcium site as Ca1-xKxWO4-x/2. In addition to the electrochemical measurements, we have investigated the high-temperature neutron diffraction on these experiments to clarify the oxide ion conduction paths at high-temperatures. We have reported these results separately and have not compared with concerning defect structure. In the present study, we compare the conduction paths between three types of scheelite-type structured oxide ion conductors from the results of high-temperature neutron diffraction. Experimental Pb1-xLaxWO4+x/2 (x = 0.2), while Pb1-xLa2/3WO4 (x = 0.1) and Ca1-xKxWO4-x/2 (x = 0.2) were prepared by conventional solid-state reaction method started from PbO, La2O3, H2WO4, CaCO3 and K2CO3. The sintering temperatures were selected as 900ºC for PbWO4-based materials and 1000ºC for CaWO4s. The crystalline phase of the obtained samples was confirmed by X-ray diffraction. The sintered samples with the amount of c.a. 10 g are set into a vanadium holder with a thin quartz tube to prevent the reaction of sample with holder, which was mounted to the neutron diffractometer SHRPD in J-PARC. Diffracted neutrons were collected at various temperatures up to 800ºC. Structure analyses were performed by using Z-Rietveld, and the trace of nuclear density was calculated by Z-MEM. BVS calculations were also carried out based on the refined structure data. Results and Discussions Fig. 1 exhibits the nuclear density map of (a) Pb1-xLaxWO4+x/2, (b) Pb1-xLa2/3WO4 and (c) Ca1-xKxWO4-x/2 deduced by maximum entropy method based on the neutron diffraction data collected at 800ºC. For Pb1-xLaxWO4+x/2 with the oxide ion interstitials, nuclear density seems to be distributed to the interstitial site, while the latter two systems without oxide ion interstitials, much less amount of nuclear density is observed at the interstitial sites. Therefore, Pb1-xLaxWO4+x/2 system possess the three-dimensional conduction path allowing higher oxide ion conductivity at elevated temperatures. We will also discuss the conduction path in terms of the bond valence sum.