Coupled Redox and Structural Evolution of Mo in LaNb _0.9 Mo _0.1 O _4.05 : Implications for Ionic-Electronic Transport

High Resolution Image Download MS PowerPoint Slide Understanding the redox behavior and structural stability of aliovalent substituents in ionic conductors is critical, as their variable oxidation states can inadvertently introduce electronic conductivity and alter transport mechanisms under different atmospheric conditions. Here, we report the atmosphere-dependent redox behavior and local coordination of Mo in LaNb 0.9 Mo 0.1 O 4.05, focusing on its influence on phase transition and transport properties, where the as-sintered LaNb 0.9 Mo 0.1 O 4.05 was systematically annealed under pure O 2, pure N 2, vacuum (∼1.6 × 10 –8 mbar), and 5% H 2 /N 2 at 800 °C for different dwell times. Electron paramagnetic resonance (EPR) spectroscopy results demonstrate the emergence of Mo 5+ under 5% H 2 /N 2 . In situ X-ray absorption near edge structure (XANES) measurements reveal the reversible redox behavior of Mo, where Mo 5+ formed under 5% H 2 /N 2 reoxidizes to Mo 6+ upon exposure to static air, while complementary in situ extended X-ray absorption fine structure (EXAFS) analysis shows that the Nb coordination environment also transitions from prototypical LaNbO 4 structure under reducing conditions back to the Mo-substituted LaNbO 4 structure upon reoxidation. This change of the oxidation states of Mo could correspondingly alter the band structure of the sample, which further enhances charge transport: the sample annealed in 5% H 2 /N 2 for 24 h exhibits a reduced activation energy and increased electronic conductivity. These results highlight a strong coupling among substituent redox flexibility, local structure, and transport properties, providing an understanding of tailoring the properties of ionic conductors through controlled redox environments.