High-temperature phase stability of Ruddlesden-Popper barium stannate under fuel-processing-relevant atmospheres to proton ceramic cells

The Ruddlesden-Popper oxide Ba 2 SnO 4- δ (BSO) has attracted interest for energy-related applications due to its layered structure and redox-active Sn cations. However, its stability under fuel-processing-relevant atmospheres remains poorly understood. In this work, the structural evolution, defect chemistry, and phase stability of BSO under reducing and carbonaceous conditions are investigated using powder X-ray diffractometry, bond valence analysis, and thermodynamic calculations. Under nominally dry H 2 , BSO undergoes topotactic reduction up to 700 °C, accommodating oxygen-ion vacancy formation and partial reduction of Sn 4+ to Sn 2+ while retaining the Ruddlesden–Popper structure. Occupancy-weighted bond valence sums reveal a progressive decrease in the average Sn oxidation state accompanied by anisotropic structural relaxation, including shortening of apical Sn–O bonds and contraction along the c -axis. Tolerance factor analysis explains the observed stability window and the onset of structural collapse at higher reduction temperatures. In contrast, the presence of water vapour or CO 2 strongly destabilises the layered structure: wet H 2 induces partial decomposition at 600 °C, while CO 2 leads to complete transformation into BaSnO 3 and BaCO 3 . By comparison, BSO remains stable in nominally dry CH 4 at the same temperature. These results demonstrate that the stability of BSO is governed not only by redox conditions but also by hydration- and carbonation-driven reactions associated with the BaO rock-salt layers, imposing intrinsic limitations on the use of layered barium stannate oxides in humid and carbonaceous environments relevant to proton ceramic technologies.