Tailoring the Structural and Transport Properties of Ba ₂ In ₂ O ₅ through Cr ⁶⁺ Substitution for Enhanced Oxygen Permeation

This work reveals the structural evolution and transport behavior of chromium-substituted Ba 2 In 2 O 5 (BIO) as a mixed ionic electronic conductor for oxygen transport membranes. Controlled substitution of In 3+ by Cr 6+ induces a transition from an orthorhombic brownmillerite to an on average cubic defect-perovskite ( AB O 3−δ ) phase while suppressing the high-temperature phase transformations typical of undoped BIO. A comprehensive set of structural and spectroscopic techniques confirms the stabilization of Cr 6+ in the lattice and its function as a donor dopant. The aliovalent substitution introduces additional electrons while reducing the oxygen-vacancy concentration in the lattice, resulting in increased electronic and decreased ionic conductivities. The composition with x = 0.1 achieves a well-balanced contribution from ionic and electronic carriers, yielding the highest ambipolar conductivity and oxygen permeation flux among the studied samples. At higher substitution levels (e.g., x = 0.2), where In 3+ and Cr 6+ coexist on the B -site of the perovskite framework, a coupled donor/acceptor system (Cr 6+ /In 3+ ) is formed, giving rise to complex charge compensation mechanisms and mixed electronic conduction. These findings provide fundamental insights into the crystal structure, defect chemistry, and charge transport mechanisms in Cr-substituted BIO, offering a rational design strategy for efficient oxygen transport membranes.