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

1.首次从山上采集的样品中确定了低位黑盐的结构。哈密顿(Ca)，然后用四圈衍射仪测量的强度数据进行修正。低相结构是高温立方相结构的一种超结构，它导致了相变的一阶。用X射线单晶技术研究了在-50~200℃的较宽温度范围内的结构变化，得到了约70℃处的相变点。对于原子的平均位置，Si-O距离和Si-O-Si角都表现出初步的相关性：在低态范围内，Si-O显著减小，而Si-O-Si增大。一些氧原子在原子平均位移中表现出显著的各向异性，这可能是一种无序。用最大熵方法分析了高态电子密度分布，结果表明，五角形十二面体的笼子全部被CH4占据，可能存在六种情况。黑云母以高达1,000℃的温度释放其客体分子。加热至1000℃的无客体样品与天然样品一样属于立方空间群，且不随温度变化而发生结构转变。这一结果否认了基于核磁共振研究的报道。对一个被CH4分子占据的五角形十二面体笼子模型结构的MD模拟实验，成功地再现了从立方结构到(2×2×1)四方超结构的结构转变。这表明客体分子在黑金云母的结构转变中起着重要的作用。

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.