Structure and Mixed Proton-Electronic Conductivity in Pr and Nb-Substituted La_5.4MoO_12-δ Ceramics

Lanthanide molybdates are materials known for their mixed proton-ionic conductivity. This study investigates the effects of Pr content and Nb-doping on the crystal structure and electrical properties of the La_5.4-xPr_xMo_1-yNb_yO_12-δ (x = 0, 1.35, 2.7, 4.05, 5.4; y = 0, 0.1) series. The research focuses on two primary objectives: (i) enhancing the electronic conductivity through the use of Pr⁴⁺/Pr³⁺ redox pairs and (ii) increasing the ionic conductivity through Nb⁵⁺ aliovalent doping. The materials were thoroughly characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission and scanning electron microscopy (TEM and SEM), and complex impedance spectroscopy. The average crystal structure of the materials depended significantly on the Pr content. In general, compositions with a higher Pr content crystallize in a cubic fluorite-type structure, whereas those with a lower Pr content stabilize a rhombohedral polymorph. However, detailed TEM studies reveal a more complex local crystal structure characterized by nanodomains and incommensurate modulations. The highest conductivity values were observed in a N₂ atmosphere for compositions with an elevated Pr content, with values of 0.17 and 204.4 mS cm^-1 for x = 0 and x = 5.4, respectively, at 700 °C, which is attributed to electronic conduction mediated by the Pr⁴⁺/Pr³⁺ redox pair, as confirmed by XPS. These findings highlight the potential of tailored doping strategies to optimize the conducting properties of lanthanide molybdates for specific high-temperature electrochemical applications.