Defects and Transport in Ba-doped La27W5O55.5

Abstract

La6WO12 shows relatively high proton and mixed proton-electron conductivity and the stoichiometry is modified to La28-xW4+xO54+1.5xv2-1.5x (La/W=5.3-5.7) to synthesize single phase. Acceptor doping this material may help the formation of oxygen vacancies and thus increase the transport of proton. In this work, Ba as an acceptor is studied in attempt to substitute La site. 2% and 0.5% Ba-doped La27W5O55.5, donated as LBaWO-2% and LBaWO-0.5%, respectively, were synthesized by wet chemical method. Following sintering, XRD, SEM and EPMA were carried out to check impurities and to study the composition. The solubility of Ba in La27W5O55.5 was observed 0.4%. The conductivity was studied by AC impedance measurements in temperature range from 300 C to 1000 C. It shows that the conductivity of LBaWO-2% is dominated by grain boundary. The defect structure was investigated by measuring the conductivity as a function of pH2O and pO2. For LBaWO-0.5%, the conductivity is dominated by proton conductivity below 700 C under oxidizing condition, and the maximum proton conductivity is observed to be 2.310-3 Scm-1 at 700 C. Above 800 C, it exhibits mixed ionic and electronic conductivity under oxidizing condition. The effect on proton conductivity from acceptor doping is barely seen, in the contrast, the conductivity has a steeper pO2 dependency, indicated n-type conductivity under reducing condition due to inherent W_La^•••. Impedance spectroscope was conducted in the temperature from 200 C to 1000 C. LBaWO-2% (with secondary phase) and LBaWO-0.5% (relatively pure) both demonstrated huge resistive grain boundaries. Except for impurity blocking, it was suspected to be the presence of inherent space layer, which depleted the positively charged charge carriers. Based on thermogravimetry (TG), the proton concentration as a function of the inverse temperature were modeled to obtain the standard hydration enthalpy and entropy change. The water uptake of the two samples with different doping level did not show significant change. The thermodynamics parameters are in agreement with the values from the literature, with Hhydr=-100 kJ/mol and Shydr=-125 J/molK for LBaWO-0.5% and with Hhydr=-125 kJ/mol and Shydr=-140 J/molK for LBaWO-2%. Both the standard hydration enthalpy and entropy were observed to be more negative with increasing Ba doping level.