Ionic transport and luminescence properties in sodium- and fluorine-co-doped rare-earth molybdates NaLn4Mo3O15F (Ln = Sm–Tb)

Abstract

Halogenated rare-earth molybdates of the NaLn₄Mo₃O₁₅F (Ln = Sm–Tb) nominal composition were synthesized via a solid-state route and investigated for their structural, thermal, IR spectroscopic, luminescent, and ionic transport properties. The study demonstrated that lanthanide cation size critically governs structural symmetry. Guided by symmetry analysis, Sm and Eu were prioritized for investigation as they stabilize the cubic fluorite-like framework, which exhibits superior ionic conductivity, whereas smaller cations (Gd, Tb) form low-conductivity monoclinic oxymolybdates. Fluorine incorporation and oxidative annealing significantly modulated oxygen interstitial content, as evidenced by lattice parameter variations and luminescence spectroscopy, which confirmed europium's oxidation state transition (Eu²⁺ ↔ Eu³⁺). Here, the conductive properties of NaSm₄Mo₃O₁₅F (NSMF) and NaEu₄Mo₃O₁₅F (NEMF) were studied for the first time. Crystal chemical analysis of ionic conductivity revealed approximately equal oxygen migration barriers, with an estimated value of about 0.5 eV for 3D oxygen diffusion. The results of kinetic Monte Carlo simulations demonstrated oxygen ionic conductivity of about 10⁻²–10⁻³ S cm⁻¹ at 800 °C. To estimate the electronic contribution, density functional theory calculations were performed for band gaps calculations, which turned out to be ca. 0.6 eV. Conductivity measurements demonstrated a similar order of anionic conductivity, with enhanced electronic contributions under reducing conditions. Thermal analysis linked suppressed phase transitions in NSMF/NEMF to lattice rigidity caused by smaller lanthanides, contrasting with flexible lattices in MLn₄Mo₃O₁₅F (M = Li, Na; Ln = La, Pr, Nd). These findings highlight fluorine's role in tuning oxygen mobility and underscore the interplay between cation size, lattice dynamics, and conductivity for intermediate-temperature solid oxide fuel cell applications.