Partially substituting lanthanum ions for lead site of the Scheelite-type structured PbWO₄, oxide ion interstitials are formed as Pb_1-xLa_xWO_4+x/2 and high oxide ion conduction appears at high temperature [1]. While most of the oxide ion conductors including zirconias, cerias, or LSGM cause high oxide ion conduction by oxide ion vacancies, the present system utilizes oxide ion interstitials. For the vacancy diffusion, conduction paths can be predicted by connecting regular oxide ions by straight or curved lines. On the other hand, for interstitial diffusion as the present system, conduction path would be complicated. We have conducted high-temperature neutron diffraction on the oxide ion conductor Pb_1-xLa_xWO_4+x/2 (x = 0.2) with oxide ion interstitials to estimate the conduction path by means of the Rietveld analysis and Maximum Entropy Method (MEM) [2]. As a result, two types of oxide ion interstitial sites O_i(1) and O_i(2) were detected. The former can be detected even at room temperature, while the latter appears only at high-temperatures. We have concluded that the oxide ion conduction is performed through the regular oxide ion sites and these two interstitial sites. However, it is still uncertain whether the interstitial site is simply a localized position or surely contributes to the oxide ion conduction. In this study, high-temperature neutron diffraction has been carried out on Pb_1-xLa_2x/3WO₄ (x = 0.1), which has the isostructure with the Pb_1-xLa_xWO_4+x/2 but without apparent interstitials. Comparing the occupancy of the interstitials in these systems, we evaluate the contribution of interstitials to the oxide ion conduction.

Experimental

Pb_1-xLa_2x/3WO₄ (x = 0.1) sample has been prepared by conventional solid-state reaction method starting from PbO, H₂WO₄ and La₂O₃. Before the weighing, La₂O₃ has been calcined at 900 °C for 6 hours. Stoichiometric mixtures of the starting materials were fired at first at 800 °C in air for 10 hours, finely ground, pressed into pellets under a hydrostatic pressure of 100 MPa for 3 minutes, and finally sintered at 900 °C in air for 10 hours. Crystalline phase of sintered samples was identified by X-ray diffraction measurement. About 10 g of the sample pellets were set in vanadium holder through a thin quartz tube and the neutron powder diffraction experiments were performed in the temperature range between 200 °C and 800 °C by using SHRPD diffractometer in J-PARC. The Rietveld refinements were carried out by using Z-Rietveld code, and the oxide ion conduction path at high temperatures was estimated with Maximum Entropy Method using Z-MEM.

Results and Discussions

Fig. 1 shows the temperature dependence of the lattice constant obtained by the Rietveld analysis. Both a and c-lengths increase linearly with increasing temperature. The a-length of Pb_1-xLa_2x/3WO₄ is close to that of PbWO₄, while Pb_1-xLa_xWO_4+x/2 shows a smaller a-length. On the other hand, c-lengths were not largely varied within all the systems. Fig. 2 shows the temperature dependence of the W-O bond length. In Pb_1-xLa_xWO_4+x/2, it decreases with temperature, whereas it increased in Pb_1-xLa_2x/3WO₄ as PbWO₄. In the Pb_1-xLa_xWO_4+x/2 system, it is supposed that WO₄ tetrahedra shrink with temperature for the activation of the movement of the oxide ions localized at the interstitial sites. For Pb_1-xLa_2x/3WO₄ system, due to the absence of excess oxide ions, WO₄ tetrahedra do not shrink with temperature.

Considering the possibility of the oxide ion conduction through the oxide ion interstitials, the Rietveld refinements were carried out on Pb_1-xLa_2x/3WO₄ assuming O_i(1) and O_i(2) of the two types of interstitials as represented in Fig. 3. The refined occupation factor of O_i(1) and O_i(2) for Pb_1-xLa_2x/3WO₄ system are 0.0343 and 0.0129, respectively, while those for Pb_1-xLa_xWO_4+x/2 are 0.0316 and 0.0092, respectively. Since the similar occupation factors are obtained between these two systems even for no apparent interstitial, the deduced oxide ion interstitial position would be attributed to the oxide ion conduction.

References

[1] T. Esaka et al. Solid State Ionics 52, 319-325 (1992). [2] S. Kaji et al., 230th Meet. Electrochem. Soc., 4002 (2016).