Effect of Ca doping on the electrical conductivity of the high temperature proton conductor LaNbO ₄

The sintering properties, crystal structure and electrical conductivity of La _1-xCa _xNbO _4-δ (x = 0, 0.005, 0.01, 0.015, 0.02 and 0.025), prepared by a solid-state reaction, have been investigated using powder X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), and electrochemical impedance spectroscopy (EIS). In 2.5% Ca-doped samples, a small amount of impurities Ca ₂Nb ₂O ₇ were observed from the XRD patterns. Impedance spectra show that the grain boundary resistance increases with increasing Ca content, while the bulk resistance remains essentially constant below 550°C. Despite the higher degree of grain growth observed for higher Ca doping levels, the total conductivity of the La _1-xCa _xNbO _4-δ series decreases with increasing Ca content from 0.5 to 2.0 mol%. The activation energy for the total conductivity decreases with increasing Ca content from 0.71 eV (x = 0) to 0.54 eV (x = 0.01) for the high temperature tetragonal phase, then it increases to 0.60 eV for x = 0.02. For the monoclinic phase, the activation energy exhibits similar trend except La _0.995Ca _0.005NbO _4-δ shows the lowest value of 1.26 eV. The Ca and Nb content present at the grain boundaries for La _0.99Ca _0.01NbO _4-δ are much higher than that on the grain surface, as determined from the EDS analysis. These results imply that the solubility of CaO in LaNbO ₄ is in the range from 0.5 to 1.0 mol%. By increasing the sintering temperature from 1500°C to 1550°C, the proton conductivity of the Ca-doped LaNbO ₄ was improved with enlarged grain size due to a reduction in the resistive grain boundary contribution.