(3 + 2)D modulation governs vacancy ordering and oxide-ion transport in γ-type BIMEVOX conductors

Abstract

Incommensurate structural modulation is a defining yet poorly understood feature of several functional solids, particularly regarding its impact on defect dynamics in fast-ion conductors. Here, using the model oxide-ion conductor Bi₂V_0.9Cu_0.1O_5.35 (BICUVOX.10), we achieve the first full determination of a (3 + 2)D incommensurately modulated structure in the well-known γ-type BIMEVOX family. Combined single-crystal and powder X-ray diffraction, neutron total scattering with reverse Monte Carlo modelling, and ab initio molecular dynamics (AIMD) reveal that the γ′-phase exhibits short-range oxygen-vacancy ordering that intrinsically causes its modulation behavior. Upon heating, this vacancy ordering transforms into the dynamically disordered γ-phase, establishing the structural origin of fast-ion conduction. The modulation waves strongly constrain oxygen motion within the vanadate layers, elevating the activation energy, while the coupled apical–equatorial positional modulations generate versatile V/Cu coordination geometries and a zig-zag oxide-ion diffusion network. In addition, Cu atoms were found to act as local vacancy traps. These findings identify vacancy-driven modulation as the mechanism governing the reversible γ′ ↔ γ transition and establish a general framework linking incommensurate structural modulation, defect organization, and ionic transport in complex solid-state conductors.