Non-stoichiometry as a structural lever: tailoring oxygen sublattice occupancy for enhanced oxide-ion transport in BICUVOX

Abstract

BICUVOX materials exhibit high oxide-ion conductivity at intermediate temperatures, making them attractive for IT-SOFC applications. However, the detailed influence of non-stoichiometry on the local coordination environment and macroscopic transport properties remains insufficiently explored. Herein, we systematically study the non-stoichiometric solid solution Bi₄(V_0.9Cu_0.1)_xO_6+2.35x, which stabilizes the highly conductive γ-phase down to room temperature. By employing a synergistic combination of single crystal X-ray diffraction (SXRD), synchrotron X-ray diffraction (SYXRD), time-of-flight powder neutron diffraction (TOF-PND), and extended X-ray absorption fine structure (EXAFS) spectroscopy, we unravel the atomic-scale structural evolution induced by cation-ratio variations. Our results demonstrate that varying the Bi:(V + Cu) ratio effectively tunes the lattice parameters and oxygen sublattice occupancies. EXAFS reveals a strongly distorted local vanadium environment with short apical V–O bonds, while Rietveld refinements from diffraction data show a systematic decrease in equatorial oxygen (O2) site occupancy with decreasing x. The grain conductivity of Bi₄(V_0.9Cu_0.1)_1.95O_10.5825 reaches 6.6 × 10⁻³ S cm⁻¹ at 350 °C, which is 28% higher than that of the x = 2 composition under the present measurement conditions. Bond valence site energy (BVSE) calculations further confirm a vacancy-mediated conduction mechanism, wherein reduced equatorial O2 occupancy facilitates the formation of robust two-dimensional percolation pathways for oxide-ion hopping. This study highlights non-stoichiometric engineering as a powerful structural tool for optimizing high-performance solid electrolytes in intermediate-temperature solid oxide fuel cells.