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Pressure-induced amorphous-amorphous phase transition in molecular solids with tetrahedral molecules

四面体分子固体中压力诱导的非晶-非晶相变

基本信息

批准号：
14540296
负责人：
HAMAYA Nozomu
金额：
$ 2.3万
依托单位：
Ochanomizu University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2002
资助国家：
日本
起止时间：
2002 至 2003
项目状态：
已结题

项目摘要

When the high-pressure amorphous state induced by compression of molecular crystal SnI_4 is decompressed from 30 GPa, a discontinuous shift of the position of the first diffraction peak occurs at 3 GPa. Subsequent compression yields the shift in the reverse direction at 6 GPa. These previous observations suggest an occurrence of structural change in the amorphous state. The objective of the present work is to study change of amorphous structure quantitatively and to obtain knowledge of phase transition in a disordered state.We first carried out synchrotron radiation x-ray measurements and studied amorphous structure near the amorphous-amorphous phase transition. The position, width and shape of the first peak in the structure factor, S(Q), were confirmed to change discontinuously at 3 GPa. The reduced radial distribution function G(r) was obtained from Fourier transform of S(Q) at each pressure. Examination of G(r) showed that the distances corresponding to the one between the Sn and I … More ions and the I ions within the tetrahedral molecule are missing in G(r) above 3 GPa, while below 3 GPa distinct peaks appearing at these intramolecular atomic distances clearly indicate the formation of the tetrahedral molecules in the low-pressure amorphous state. Upon the second compression the reverse process took place at 7 GPa with a large pressure-hysteresis. Those observations suggest that the structural change in pressure-induced amorphous SnI_4 is of first-order phase transition. A characteristic of the amorphous-amorphous phase transition in SnI_4 is that the formation/dissociation of the tetrahedral molecules occurs. In this respect this phase transition has a stronger similarity with that in liquid phosphor at high pressure and high temperature rather than the pressure-induced low density-high density transition in amorphous ice.The density of amorphous solid has been a physical parameter difficult to measure under pressure. We demonstrated that the density can be obtained from G(r) with precision high enough to distinguish the difference between the low and high pressure amorphous states. We showed that the amorphous-amorphous phase transition in SnI_4 is accompanied by a pressure-hysteresis of the density and by an anomalous hardening above the transition pressure upon decompression. Our recent Raman spectroscopic measurements showed that a new vibration mode is produced at the transformation form the high-pressure to the low-pressure amorphous state.Part of the results of this study was presented at the 19th IUCr Congress of General Assembly (Geneva) in Aug., 2002 and the 19^<th> International Conf.on High Pressure Sci.and Technology (Bordeaux) in July, 2003. Less

当分子晶体SnI_4压缩引起的高压非晶态从30 GPa减压时，在3 GPa处第一衍射峰的位置发生不连续的移动。随后的压缩会在 6 GPa 下产生相反方向的偏移。这些先前的观察表明在无定形状态下发生了结构变化。本工作的目的是定量研究非晶结构的变化，获得无序状态下相变的知识。我们首先进行了同步辐射X射线测量，研究了非晶-非晶相变附近的非晶结构。结构因子 S(Q) 中第一个峰的位置、宽度和形状被证实在 3 GPa 下发生不连续变化。简化的径向分布函数 G(r) 是通过每个压力下 S(Q) 的傅里叶变换获得的。对 G(r) 的检查表明，在高于 3 GPa 的 G(r) 中，与 Sn 和 I 离子之间以及四面体分子内的 I 离子之间的距离相对应的距离缺失，而在低于 3 GPa 时，这些分子内原子距离处出现的明显峰清楚地表明在低压非晶态下四面体分子的形成。第二次压缩时，反向过程在 7 GPa 下发生，具有较大的压力滞后。这些观察结果表明压力引起的非晶 SnI_4 的结构变化是一级相变。 SnI_4中非晶-非晶相变的一个特征是发生四面体分子的形成/解离。在这方面，这种相变与高压高温下液体荧光粉的相变具有更强的相似性，而不是非晶冰中压力引起的低密度-高密度转变。非晶固体的密度一直是在压力下难以测量的物理参数。我们证明了可以从 G(r) 获得密度，其精度足以区分低压和高压非晶态之间的差异。我们发现 SnI_4 中的非晶-非晶相变伴随着密度的压力滞后以及减压时高于转变压力的异常硬化。我们最近的拉曼光谱测量表明，在从高压到低压非晶态的转变过程中产生了一种新的振动模式。这项研究的部分结果已在2002年8月第19届IUCr大会（日内瓦）和2003年7月第19届国际高压科学与技术会议（波尔多）上发表。