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首页> 外文期刊>Ultramicroscopy >Direct electron crystallographic determination of zeolite zonal structures
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Direct electron crystallographic determination of zeolite zonal structures

机译：直接电子晶体学测定沸石的带状结构

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The prospect for improving the success of ab initio zeolite structure investigations with electron diffraction data is evaluated. First of all, the quality of intensities obtained by precession electron diffraction at small hollow cone illumination angles is evaluated for seven representative materials: ITQ-1, ITQ-7, ITQ-29, ZSM-5, ZSM-10, mordenite, and MCM-68. It is clear that, for most examples, an appreciable fraction of a secondary scattering perturbation is removed by precession at small angles. In one case, ZSM-10, it can also be argued that precession diffraction produces a dramatically improved 'kinematical' data set. There seems to no real support for application of a Lorentz correction to these data and there is no reason to expect for any of these samples that a two-beam dynamical scattering relationship between structure factor amplitude and observed intensity should be valid. Removal of secondary scattering by the precession mode appears to facilitate ab initio structure analysis. Most zeolite structures investigated could be solved by maximum entropy and likelihood phasing via error-correcting codes when precession data were used. Examples include the projected structure of mordenite that could not be determined from selected area data alone. One anomaly is the case of ZSM-5, where the best structure determination in projection is made from selected area diffraction data. In a control study, the zonal structure of SSZ-48 could be determined from selected area diffraction data by either maximum entropy and likelihood or traditional direct methods. While the maximum entropy and likelihood approach enjoys some advantages over traditional direct methods (non-dependence on predicted phase invariant sums), some effort must be made to improve the figures of merit used to identify potential structure solutions.
机译：评价了利用电子衍射数据提高从头算沸石结构研究成功率的前景。首先，对以下七个代表性材料评估了在小空心锥照射角下通过旋进电子衍射获得的强度质量：ITQ-1，ITQ-7，ITQ-29，ZSM-5，ZSM-10，丝光沸石和MCM -68。清楚的是，对于大多数示例，通过进动以小角度去除了相当一部分的二次散射扰动。在一种情况下，ZSM-10，也可以说旋进衍射产生了显着改善的“运动学”数据集。似乎没有真正的支持将Lorentz校正应用于这些数据，并且没有理由期望这些样本中的任何一个在结构因子振幅和观察到的强度之间的两束动态散射关系都是有效的。通过进动模式去除二次散射似乎有助于从头算结构的分析。当使用进动数据时，大多数研究的沸石结构都可以通过纠错码通过最大熵和似然定相来解决。示例包括无法仅从所选区域数据中确定的丝光沸石的预计结构。 ZSM-5是一种异常情况，其中从选定的区域衍射数据确定投影中的最佳结构。在对照研究中，SSZ-48的区域结构可以通过最大熵和似然性或传统直接方法从选定的区域衍射数据中确定。尽管最大熵和似然法相对于传统的直接方法具有一些优势（不依赖于预测的相位不变性总和），但必须做出一些努力来改进用于识别潜在结构解的品质因数。

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