Neutron diffraction and multinuclear solid state NMR investigation into the structures of oxide ion conducting La9.6Si6O26.4 and La8Sr2Si6O26, and their hydrated phases.
Neutron diffraction and multinuclear solid state NMR investigation into the structures of oxide ion conducting La9.6Si6O26.4 and La8Sr2Si6O26, and their hydrated phases.
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中子衍射和多核固态核磁共振研究了导电 La9.6Si6O26.4 和 La8Sr2Si6O26 的氧化物离子结构及其水合相。
DOI：
10.1039/c5dt03190g
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发表时间：
2016
期刊：
影响因子：
--
通讯作者：
Corrie BJ
中科院分区：
文献类型：
--
作者：
Corrie BJ
磷灰石硅酸盐作为潜在的SOFC电解质材料正引起人们的极大兴趣。它们是非常规的氧化离子导体，因为关键缺陷是氧化离子间隙，而不是空位。在这项工作中，我们比较了La9.6Si6O26.4和La8Sr2Si6O26水化前后的结构，以收集有关间隙氧化离子位置的信息。中子衍射结构研究表明，在制备的La8Sr2Si6O26和水合La8Sr2Si6O26中，间隙氧化离子位置接近磷灰石通道中心。对于La9.6Si6O26.4，在靠近通道中心的位置观察到类似的位置，但在该特定样品的水化过程中，间隙位置显示出明显地从通道中心向SiO4单元偏移。这可以解释为需要从通道中心进行额外的位移，以容纳水合相中大量的间隙阴离子。固体29Si MAS NMR谱对La8Sr2Si6O26和La9.6Si6O26.4体系所表现出的不同形态非常敏感，前者以规则的SiO4骨架物种为主，后者以阳离子空位和间隙引起的网络中断为主。相应的17O MAS NMR研究从SiO4基团的O原子中识别出强信号，从而表明这些体系中的所有O种在非均相气相条件下都是可交换的O。此外，由于Si位置上的悬垂OH键而导致的间隙O物质被清楚地识别和解决，并在脱水时被去除。这一观察和分配被相应的1H MAS NMR测量证实。总的来说，中子衍射工作表明，这些磷灰石硅酸盐的间隙位置取决于阴离子含量，随着阴离子含量的增加，阴离子逐渐向SiO4四面体位移，而SiO4基团上交换O的观察结果与先前的模型预测一致，即硅酸盐单元在传导过程中的重要性。
Apatite silicates are attracting significant interest as potential SOFC electrolyte materials. They are non-conventional oxide ion conductors in the sense that oxide ion interstitials, rather than vacancies, are the key defects. In this work we compare the structures of La9.6Si6O26.4 and La8Sr2Si6O26, both before and after hydration in order to gather information about the location of the interstitial oxide ion site. Neutron diffraction structural studies suggest that in the as-prepared La8Sr2Si6O26 and hydrated La8Sr2Si6O26, the interstitial oxide ion sites are close to the apatite channel centre. For La9.6Si6O26.4, a similar site close to the channel centre is observed, but on hydration of this particular sample, the interstitial site is shown to be significantly displaced away from the channel centre towards the SiO4 units. This can be explained by the need for additional displacement from the channel centre to accommodate the large amount of interstitial anions in this hydrated phase. The solid state 29Si MAS NMR spectra are shown to be very sensitive to the different speciation exhibited by the La8Sr2Si6O26 and La9.6Si6O26.4 systems, with the former being dominated by regular SiO4 framework species and the latter being dominated by interruptions to this network caused by cation vacancies and interstitials. The corresponding 17O MAS NMR study identifies a strong signal from the O atoms of the SiO4 groups, thus demonstrating that all of the O species in these systems are exchangeable O under heterogeneous gas phase conditions. In addition, interstitial O species attributed to pendant OH linkages on the Si positions are clearly identified and resolved, and these are removed on dehydration. This observation and assignment is corroborated by corresponding 1H MAS NMR measurements. Overall the neutron diffraction work indicates that the interstitial site location in these apatite silicates depends on the anion content with progressive displacement towards the SiO4 tetrahedra on increasing anion content, while the observation of exchangeable O on the SiO4 groups is consistent with prior modelling predictions as to the importance on the silicate units in the conduction process.
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