Structure and Mixed Proton-Electronic Conductivity in PR-Substituted La5.4MoO12- Ceramics

Lanthanide molybdates, with the general formula Ln5.4MoO11.1-δ, are materials known for their mixed proton-ionic conductivity. This study investigated the effects of Pr content and Nb-doping on the crystal structure and electrical properties of the La5.4-xPrxMo1-yNbyO12-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 depends significantly on the Pr content. In general, compositions with higher Pr content crystallize in a cubic fluorite-type structure, whereas lower Pr content stabilizes a rhombohedral polymorph. However, detailed TEM studies reveal a more complex local crystal structure characterized by nanodomains and incommensurate modulations. The highest conductivity values are observed in a N2 atmosphere for compositions with elevated Pr content, 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-temperatures electrochemical applications.