X-ray diffraction and niolecular dynamics studies of the phase transitions in selica clathrote miuerals

Selica 笼形分子相变的 X 射线衍射和分子动力学研究

基本信息

批准号：
12640466
负责人：
KIHARA Kuniaki
金额：
$ 2.24万
依托单位：
Kanazawa University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2000
资助国家：
日本
起止时间：
2000 至 2001
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-12640466/
关键词：
silica mimeral melanophlogite low-high phase trasition melecalar dynamics simulation X-ray single crystal anaiysis シリカ包摂鉱物

项目摘要

1. The structure of low melanophalohite was firstly determined using specimens from Mt. Hamilton (Ca), and thenrefined with intensity data measuread on a 4-circle diffractometer. The low form, which is a superstructure of the high temperature cubic one, results in the first order phase trasition from the cubic form.2. The structural changes in a wide range of temperature from -50 to 200℃ were studied with X-ray single crystal techniques, showing the trasition point at about 70℃. Si-O distances and Si-O-Si angles, both for the atomic mean positions, show a begative correlation : Si-O decreases significantly in the range of the low form, while Si-O-Si increases.3. Some of oxygen atoms show remarkable anisotropy in atomic mean swuare displacements, suggesting a possible disorder.4. The maximum entropy method was applied to analyze electron density distribution in the high form, indicating that all the cages of pentagon dodecahedron are occupied by CH4, which are possibly in six oreintations.5. Melanophlohite releases its guest molecules at a temperature as high as 1000℃. The guest-free specimen obtained by heating up to 1000℃ belongs to the cubic space group as the same as that of natural specimens, and show no structural transition with varying temperatures. This result denies a reported one based on a NMR study.6. Some trials of MD simulations for a model structure with pentagon dodecahedral cages, fully occupied by CH4 molecules, succeeded to reproduce the structure transition from the cubic form to the (2x2x1) tetragonal superstructure. This suggests that guest molecules play important roles in the structural transitions in melanophlogite.

1. 首先使用来自汉密尔顿山 (Ca) 的样品确定了低黑绿石的结构，然后使用 4 环衍射仪测量的强度数据进行了细化，得出了低黑绿石的结构，它是高温立方体的上层结构。 2．采用X射线单晶技术研究了其在-50~200℃宽温度范围内的结构变化。 70℃左右的Si-O距离和Si-O-Si角均呈负相关：Si-O在低态范围内显着降低，而Si-O-Si在低态范围内显着降低。 3. 一些氧原子的原子平均位移表现出显着的各向异性，表明可能存在无序。4. 采用最大熵方法分析了高态电子密度分布，表明所有笼子都存在缺陷。五角十二面体的五角星十二面体被CH4占据，可能有六种排列。 5．黑绿石在1000℃高温下释放出客体分子，加热至1000℃得到的无客体样品属于立方空间群。与天然样本相同，并且没有显示随温度变化的结构转变。该结果否定了基于 NMR 研究的模型结构的 MD 模拟的一些试验。完全被 CH4 分子占据的五边形十二面体笼成功地再现了从立方形式到（2x2x1）四方超结构的结构转变，这表明客体分子在黑金矿的结构转变中发挥着重要作用。