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 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 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.
