Structure analysis of pressure-induced amorphous state of a molecular crystal with tetrahedral molecules

本工作的目的是研究由四面体分子组成的分子晶体SnI_4的压力非晶化过程的机理。主要目标是建立一种从高压下收集的同步辐射X射线衍射数据确定非晶态结构的实验技术，并在较宽的压力范围内获得金属非晶态SnI_4的详细结构知识。在第一个财政年度，我们设计和建造了一个新的金刚石顶压室(DAC)装置，并在BL04B2上准备了一些仪器用于本研究。首次测量了金属非晶态SnI_4在35、45和55 GPa时的衍射图。通过对可靠散射强度的估计，我们最终得到了结构因子S(Q)和各压强下的约化径向分布函数G(R)。考察G(R)，我们发现SnI4分子在35-55 Gpa之间不再以非晶态存在，其G(R)表现出许多由硬球的致密随机堆积形成的非晶态结构的特征。对SnI_4的衍射谱研究表明，在2.5 Gpa的减压下，SnI_4发生了非晶态-非晶态的相变，并伴随着SnI_4分子的重新生成。其他研究包括高温高压下非晶态的再结晶、静水条件下的非晶化和利用核前向散射的高压穆斯堡尔谱。通过考察非晶态的热力学稳定性、原子尺度结构和电子结构，我们现在讨论了SnI_4中压力诱导非晶化过程的机制。这项研究的部分结果已在2001年8月第18届AIRAPT国际会议(北京)和2001年9月第11届国际液态和非晶态金属会议(横滨)上公布

The objective of the present work is to study a mechanism of amorphization process induced by pressure in SnI_4, a molecular crystal consisting of tetrahedral molecules. Major goals are to establish an experimental technique for determination of amorphous structure from synchrotron x-ray diffraction data collected at high pressure and to obtain detailed structural knowledge of metallic amorphous SnI_4 in a wide pressure range.In the first fiscal year, we designed and constructed a new diamond anvil cell (DAC) device and prepared some instruments at BL04B2 at SPring-8 for the present study. Diffraction patterns of metallic amorphous SnI_4 at 35, 45 and 55 GPa were measured first. Elaborating an estimation of reliable scattering intensities, we finally obtained the structure factor S(Q) and the reduced radial distribution function G(r) at each pressure. Inspecting G(r) we found that the SnI4 molecules are no longer existent in the amorphous state between 35 and 55 GPa and that its G(r) shows many features of the amorphous structure formed by the dense random packing of hard spheres.In the second fiscal year, various studies were conducted. Diffraction study of SnI_4, revealed that at 2.5 GPa on decompression there is an amorphous-amorphous phase transition accompanied by the re-formation of the SnI_4 molecules. Other studies include the recrystallization from the amorphous state at high temperature and high pressure, the amorphization under hydrostatic condition and the Mossbauer spectroscopy at high pressure utilizing the nuclear forward scattering. Examining thermodynamic stability of the amorphous state, its atomic-scale structure and its electronic structure, we now discuss the mechanism of the pressure-induced amorphization process in SnI_4.Part of the results of this study were presented at the 18th AIRAPT International Conference (Beijin) in Aug., 2001 and the 11th International Conference on Liquid and Amorphous Metals (Yokohama) in Sep., 2001