Synthesis and Characterization of the Orthorhombic Sn₃ O₄ Polymorph

An unexplored tin oxide crystal phase (Sn₃ O₄ ) was experimentally synthesized via a facile hydrothermal method. After tuning the often-neglected parameters for the hydrothermal synthesis, namely the degree of filling of the precursor solution and the gas composition in the reactor head space, an unreported X-ray diffraction pattern was discovered. Through various characterization studies, such as Rietveld analysis, energy dispersive X-ray spectroscopy, and first-principles calculations, this novel material was characterized as orthorhombic mixed-valence tin oxide with the composition Sn^II ₂ Sn^IV O₄ . This orthorhombic tin oxide is a new polymorph of Sn₃ O₄ , which differs from the reported conventional monoclinic structure. Computational and experimental analyses showed that orthorhombic Sn₃ O₄ has a smaller band gap (2.0 eV), enabling greater absorption of visible light. This study is expected to improve the accuracy of hydrothermal synthesis and aid the discovery of new oxide materials.