Nonstoichiometry in Bixbyite-Type Vanadium Sesquioxide

Recently, a metastable bixbyite-type polymorph of vanadium sesquioxide V2O3 has been synthesized from vanadium trifluoride. During the preparation, a very low oxygen partial pressure was necessary to prevent oxidation to higher valent vanadium oxides. In order to provide a quantitative description of the oxidation process, periodic quantum-chemical calculations at density-functional theory level were performed to study the thermodynamics of oxygen incorporation into bixbyite-type V2O3. Different defect structures for nonstoichiometric phases with the general composition V2O3+x are discussed, obtained either by removing single atoms from their respective lattice positions or by introducing additional atoms into empty lattice sites. We show that the stoichiometric phase is likely to incorporate excess oxygen into the empty 16c Wyckoff position under ambient pressure. Taking into account the equilibrium of nonstoichiometric phases with the gas phase, we arrive at an estimate of 10–17 bar for the oxygen partial pressure as the upper limit for stabilizing the stoichiometric phase under reaction conditions.

近期，本研究以三氟化钒（vanadium trifluoride）为原料，成功合成出亚稳方铁锰矿型三氧化二钒（vanadium sesquioxide, V₂O₃）多晶型物。制备过程中需维持极低氧分压（oxygen partial pressure），以避免产物被氧化为高价钒氧化物（higher valent vanadium oxides）。为定量描述该氧化过程，我们采用密度泛函理论（density-functional theory）水平的周期性量子化学计算方法，探究了氧掺入方铁锰矿型V₂O₃的热力学行为。本文讨论了通式为V₂O₃₊ₓ的非化学计量相（nonstoichiometric phases）的多种缺陷结构（defect structures），这些结构可通过两种途径获得：一是从晶格对应位点移除单个原子，二是向空位晶格位点引入额外原子。研究表明，常压条件下化学计量相V₂O₃更易将过量氧掺入空位16c威科夫位置（Wyckoff position）。结合非化学计量相与气相的平衡关系，我们估算得到：在反应条件下稳定化学计量相所需的氧分压上限约为10⁻¹⁷巴。