首页> 外文期刊>Acta Crystallographica, Section B. Structural Science >Second-degree twinning and dynamic disorder in the crystal structure of deca-dodecasil 3R
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Second-degree twinning and dynamic disorder in the crystal structure of deca-dodecasil 3R

机译:十十二碳三烯3R晶体结构的二级孪生和动态无序

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The structure of deca-dodecasil 3R (DD-3R), Si120O240, a very well suited material for the synthesis of inorganic/organic composites structured on a nanometer level, has been investigated in detail. So far, a highly complicated twinning has hampered its structure description at a desirable level of accuracy. This twinning has now been resolved and a new structure determination is presented. Structure refinement in the R (3) over bar space group revealed a large, unusually shaped atomic displacement ellipsoid for oxygen-bridging units (tetrahedra), bridging Si-O bonds shorter than expected and the linear Si-O-Si' bond angle dictated by special positions at a threefold axis. A structure model based on a statistically disordered bridging O atom improved the accuracy of the Si-O bonds of interest, but provided unacceptable O-O contacts. To solve this dilemma, ab initio NVT molecular dynamics calculations were performed to study the possible configurations. Wavelet analysis of the time variations of selected Si-O distances pointed to a synchronous shift of the whole building units (tetrahedra). Low-frequency features of the calculated phonon density of states agree well with the published INS (inelastic neutron scattering) spectra of several silica polymorphs, indicating that the nature of the disorder in DD-3R is dynamic rather than static.
机译:已详细研究了十-十二碳三硅(DD-3R)Si120O240的结构,这是一种非常适合合成纳米级无机/有机复合材料的材料。迄今为止,高度复杂的孪晶已将其结构描述妨碍了所需的精度水平。现在已经解决了这种孪生问题,并提出了新的结构确定方法。 R(3)上棒空间群上的结构细化揭示了氧桥键单元(四面体)的大而异形的原子位移椭圆体,桥连的Si-O键比预期的短,并且线性Si-O-Si'键的角度决定了通过三重轴上的特殊位置。基于统计上无序的桥接O原子的结构模型提高了目标Si-O键的准确性,但提供了不可接受的O-O接触。为了解决这个难题,从头开始进行NVT分子动力学计算以研究可能的构型。对选定的Si-O距离的时间变化的小波分析表明整个建筑单元(四面体)的同步移动。计算得出的声子密度的低频特征与几种二氧化硅多晶型物的已发布INS(非弹性中子散射)光谱非常吻合,这表明DD-3R中无序的性质是动态的,而不是静态的。

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