Structural and Conducting Features of Niobium-Doped Lanthanum Tungstate, La₂₇(W_1–xNb_x)₅O_55.55−δ

Abstract

The most studied ceramic proton conductors are those based on the perovskite structure. However, these materials have some practical drawbacks including their poor tolerance to carbonation. Hence, proton-conducting ceramic materials with fluorite structure are currently under investigation. One of the most studied materials is the lanthanum tungstate, “Ln6WO12”. Here, we report a new series of compounds La27W5–5x Nb5x O55.5–5x/2□8.5+5x/2 obtained by niobium doping to optimize/increase the amount of oxygen vacancies. The limiting composition has been established as La27NbW4O55.0□9.0 with an astonishing 14% of oxygen vacancies. The materials have been studied by Rietveld analysis of high-resolution laboratory X-ray powder diffraction data and electron microscopy. Thermal analysis measurements in a wet atmosphere indirectly confirm the increase of oxygen vacancies as the amount of incorporated protons increases with the niobium content. A thorough electrical characterization has been carried out including overall conductivity measurements in wet and dried atmospheres, conductivity as a function of the oxygen partial pressure, and electronic contribution by the Hebb–Wagner polarization method. The data collected suggest that the proton conductivity is dominant below 600 °C. However, above 800 °C the conductivity values are almost independent of the water partial pressure which indicates that the oxide ion is the main charge carrier. The highest conductivity value was measured for La27NbW4O55, i.e., 0.01 S·cm–1 at 800 °C compared to 0.004 S·cm–1 for the nonsubstituted material La27W5O55.5. At temperatures below 800 °C, these materials are nearly pure ionic conductors with transport numbers higher than 0.98, while at higher temperatures these compounds are mixed ionic–electronic conductors displaying both n- and p-type electronic contributions.

Colloidal processing and characterisation of lanthanum tungstate sheets, La5.5WO11.25, prepared by tape casting and reaction sintering

“…These conductivity values are somewhat lower than those previously reported for dense pellets prepared from nanosized powders obtained by freeze-drying precursors and uniaxial pressing into disks, i.e. 4 Á 10 À 3 S cm À 1 at 800 1C (wet atmosphere) [14]. These differences are clearly related to the much reduced particle size and better densification of the ceramics by applying high pressures.…”

“…Fig. 6c 2θ (º) S10-10BP 1600ºC/10h S10-20BP 1600ºC/10h S10-10BP 1600ºC/20h S10-20BP 1600ºC/20h [7,14] where the same composition was heated at 1600 1C and W losses were not observed either.…”

“…For instance, the partial substitution of W 6 þ by Mo 6 þ , La 28-y (W 1 À x Mo x ) 4 þ y O 54 þ δ (x=0-1, y=0.923), [11,12] increases significantly the ambipolar conductivity, improving the performance as hydrogen gas separation membrane [13]. Other substitutions, such as Nb 5 þ , Ca 2 þ and Sr 2 þ , increases the oxygen vacancy concentration and enhances the ionic conductivity [14][15].…”

A CO2‐stable hollow‐fiber membrane with high hydrogen permeation flux

“…[23][24][25] Escolástico et al 26 found that the H 2 permeation flux through a La 5.5 WO 11.25-δ membrane only changed slightly over 72 h when using a mixture of 15% CO 2 in Ar as a sweep gas at 800°C.…”

Improved protonic conductivity and Vickers hardness for lanthanum tungstate with potassium doping (La,K)28−W4+O54+

“…In this report, the synthesis and study of a new series of materials (La 1−x K x ) 27.08 W 4.92 O 55.38−θ (y = 0.92, the (La,K)/W ratio is 5.5) prepared with K + partial substitution for La 3+ to improve the conductivity. Besides, mechanical property is regarded as an important characteristic in practical applications besides their electrical property.…”