Oxide ion conductors have been attracted great interests for the application of electrolytes of solid oxide fuel cells (SOFC). Most of the oxide ion conductors employ oxide ion vacancies to perform ionic conduction, while very few possess oxide ion interstitials for the oxide ion migration. When lanthanum ions are partly substituted into lead site in scheelite-type structured PbWO₄, oxide ion conduction appears at elevated temperatures [1,2]. In that case, oxide ion interstitials are formed as Pb_1‑xLa_xWO_4+x/2 for the oxide ion conduction. On the other hand, CaWO₄ with the isostructure with PbWO₄ does not show such an oxide ion conduction enhancement due to the lanthanum substation. Recently, we have reported that alkaline-ion substituted CaWO₄ also exhibit high oxide ion conduction [3]. Although the defect structure of the latter system is oxide ion vacancy as Ca_1-xK_xWO_4-x/2, activation energies of the oxide ion conduction for both systems are relatively close at high-temperature.

In this study, oxide ion conduction paths of these two systems, Pb_1-xLa_xWO_4+x/2 and Ca_1-xK_xWO_4-x/2, are evaluated by means of high-temperature neutron diffraction experiments coupled with maximum entropy method and compared based on the defect structure.

Experimental

Pb_1-xLa_xWO_4+x/2 and Ca_1-xK_xWO_4-x/2 (x = 0.2 for both systems) were prepared by solid state reaction method from the starting oxides. Calcining and sintering temperatures are selected as 800^oC and 900^oC, respectively for Pb_1-xLa_xWO_4+x/2, and 800^oC and 1000^oC for Ca_1-xK_xWO_4-x/2. The phase singularities of the obtained compounds are confirmed by X-ray diffraction.

About 10 g of the sample pellets were inserted into a vanadium holder through a thin quartz tube and neutron powder diffraction experiments were performed in the temperature range between 200°C and 800°C by using SHRPD diffractometer in J-PARC. After the Rietveld refinements using Z-Rietveld code, oxide ion conduction paths at high temperatures were estimated by using maximum entropy method (MEM) using Z-MEM. Typically, 50 reflections are served for the MEM analyses.

Results and Discussions

Figs.1 and 2 represent the nuclear density distribution for Pb_1-xLa_xWO_4+x/2 and Ca_1-xK_xWO_4-x/2, respectively, obtained by MEM based on the neutron diffraction data collected at 800^oC. Regular atomic positions obtained by the Rietveld analysis are indicated by spheres in the figures. For Pb_1-xLa_xWO_4+x/2, nuclear density seems to extend from regular site to interstitial ones, suggesting the oxide ion diffusion path occurs parallel to the a-b plane. On the other hand, Ca_1-xK_xWO_4-x/2 without oxide ion interstitials did not show such diffusion path parallel to the a-b plane, but the neutron density distribution extends toward c-directions. Defect structure would largely attribute to the diffusion mechanism in these two scheelite systems.

References

[1] T. Esaka, T. Mina-ai and H. Iwahara, Solid State Ionics, 52, 319-325, (1992).

[2] S. Takai, S. Touda, K. Oikawa, K. Mori, S. Torii, T. Kamiyama and T. Esaka, Solid State Ionics, 148, 123-133, (2002).

[3] S. Takai, Y. Morhishita, Y. Kondo, T. Yao, T. Yabutsuka, T. Esaka, J. Ceram. Soc. Jpn., 124, 819-822, (2016).