Abstract
In situ neutron diffraction experiments of 2% Ca-doped and nominally undoped lanthanum tungstate (La28−xW4+xO54+3x/2, with x = 0.85) have been carried out under controlled pD2O and pO2 at elevated temperatures. All the diffraction patterns could be refined using an average cubic fluorite-related structure, in accordance with recent reports. The material exhibits disorder of the oxygen and the cation sublattices. Splitting of the oxygen sites around tungsten from the 32f to 96k Wyckoff position in the Fmm space group improves the model and can better represent the oxygen disorder. No phase transition was detected from room temperature up to 800 °C under any of the studied conditions. Expansion of the unit cell constants in the presence of water at intermediate and low temperatures was correlated with the formation of protonic defects. The thermal expansion coefficient for lanthanum tungstate is rather linear under all studied conditions (∼11 × 10−6 K−1). The in situ diffraction studies are correlated with dilatometry investigations and conductivity measurements.
Highlights
Lanthanum tungstates, represented by the general formula La28ÀxW4+xO54+dv2Àd,[1] are attractive for practical applications
One of the purposes of this contribution is to determine possible structural differences between a nominally undoped and a 2% Ca-doped LWO56 to rationalise the results reported in ref. 2
The absence of clear bands in the 500–700 cmÀ1 region is consistent with the essentially isolated character of the polyhedra in the structure,[31,32] an additional, very weak band at $670 cmÀ1 is present. This is well in accordance with the model drawn in sites, i.e. tungstate sits in lanthanum sites (Fig. 1b)
Summary
Lanthanum tungstates, represented by the general formula La28ÀxW4+xO54+dv2Àd (where d 1⁄4 3x/2),[1] are attractive for practical applications. These materials are predominantly proton conductors at low and intermediate temperatures[2,3,4] with conductivity slightly above 10À3 S cmÀ1 at 600 C.2–5. Last but not the least, these tungstates are chemically stable in CO2 and H2S containing environments above 650 C.16
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