Oxygen-induced formation of a new hexagonal BaMoO3±δphase

Hexagonal perovskites possess unique advantages in stabilizing novel quantum states due to their hexagonal or trigonal symmetrical lattice geometries. However, their scarcity compared to cubic perovskites makes it crucial to understand their structural emergence criteria and develop reliable structure modulation methods. In this work, a new hexagonal perovskite phase ofBaMoO3±δwith a six-layered (6H) structure was discovered near the cubic-hexagonal phase boundary for the first time, verified by x-ray diffraction and high-resolution electron microscope. This phase exhibits metallic conductivity across 7 K-300 K and classical linear magnetoresistance. The emergence of this new hexagonal perovskite phase is directly attributed to a slight oxygen enrichment during epitaxial growth, as revealed by a combination of experiments and calculations. Intriguingly, similar oxygen-dependent cubic-to-hexagonal transitions have been retrospectively identified in prior studies of 3dperovskite systems (e.g. BaFeO3-δ, BaMnO3-δ), suggesting a potential universal mechanism. An explanation is proposed: oxygen content modulates B-O bond lengths, thereby adjusting the tolerance factor to favor hexagonal stacking. This study not only expands the family of hexagonal perovskites but also offers guidance for discovering other novel hexagonal polymorphs.