Tsvetkov D S, Ivanov I L, Malyshkin D A, Steparuk A S, Zuev A Yu
Department of Chemistry, Institute of Natural Sciences, Ural Federal University, Ekaterinburg, 620000, Russia.
Dalton Trans. 2016 Aug 9;45(32):12906-13. doi: 10.1039/c6dt02513g.
The defect structure of B-site ordered double perovskites Sr2BMoO6-δ was analyzed. The defect structure model was proposed and successfully verified using data on oxygen nonstoichiometry of Sr2MgMoO6-δ and Sr2FeMoO6-δ. As a result, equilibrium constants of the defect reactions involved were estimated. Fe and Mo in Sr2FeMoO6-δ were found to be in the mixed oxidation state close to +2.5 and +5.5, respectively. Chemical strain of the Sr2FeMoO6-δ double perovskite lattice was studied by in situ high temperature XRD at 1100 °C depending on pO2. Parameter a of the Sr2FeMoO6-δ cubic cell was found to increase with decreasing pO2 because of lattice chemical expansion. The tetragonal polymorph of Sr2FeMoO6-δ was shown to exhibit transversal isotropy with respect to chemical expansion. It was also found that its crystal lattice expands in the ab-plane and simultaneously contracts along the c-axis when the oxygen content in the double perovskite decreases. In order to describe the degree of anisotropy of chemical strain a new phenomenological coefficient was introduced. This coefficient was shown to affect both the magnitude and change direction of an oxide cell volume caused by its reduction/oxidation. Excellent agreement between the chemical expansion along the a-axis calculated for both polymorphs of Sr2FeMoO6-δ according to the model recently developed and that measured experimentally was shown. Chemical contraction observed along the c-axis with a decreasing oxygen content in the tetragonal polymorph was also found to coincide completely with that calculated using the approach developed in the present study.
对B位有序双钙钛矿Sr2BMoO6-δ的缺陷结构进行了分析。提出了缺陷结构模型，并利用Sr2MgMoO6-δ和Sr2FeMoO6-δ的氧非化学计量数据成功验证了该模型。结果，估算了所涉及缺陷反应的平衡常数。发现Sr2FeMoO6-δ中的Fe和Mo分别处于接近+2.5和+5.5的混合氧化态。通过在1100°C下根据pO2进行原位高温XRD研究了Sr2FeMoO6-δ双钙钛矿晶格的化学应变。发现Sr2FeMoO6-δ立方晶胞的参数a随着pO2的降低而增加，这是由于晶格化学膨胀。结果表明，Sr2FeMoO6-δ的四方多晶型在化学膨胀方面表现出横向各向同性。还发现，当双钙钛矿中的氧含量降低时，其晶格在ab平面内膨胀，同时沿c轴收缩。为了描述化学应变的各向异性程度，引入了一个新的唯象系数。结果表明，该系数会影响氧化物晶胞体积因还原/氧化而产生的大小和变化方向。根据最近开发的模型计算出的Sr2FeMoO6-δ两种多晶型沿a轴的化学膨胀与实验测量值之间显示出极好的一致性。还发现，在四方多晶型中，随着氧含量的降低沿c轴观察到的化学收缩与使用本研究中开发的方法计算出的结果完全一致。
