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

当SnI_4分子晶体压缩产生的高压非晶态从30 GPa开始减压时，第一衍射峰的位置在3GPa处发生不连续的位移。随后的压缩产生了在6 GPa下的相反方向的位移。这些先前的观察表明在非晶态中发生结构变化。为了定量地研究非晶结构的变化，获得无序状态下相变的知识，我们首先利用同步辐射X射线测量技术研究了非晶-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、宽度和形状在3GPa下不连续地变化。通过对各压力下的S（Q）进行傅立叶变换，得到了约化径向分布函数G（r）。对G（r）的检验表明，对应于Sn和I之间的距离 ...更多信息 在高于3GPa的G（r）中，四面体分子内的离子和I离子缺失，而低于3GPa时，在这些分子内原子距离处出现的明显峰清楚地表明四面体分子形成为低压非晶态。在第二次压缩时，相反的过程发生在7 GPa，具有较大的压力滞后。这些结果表明，压力诱导非晶态SnI_4的结构变化是一级相变。SnI_4的非晶-非晶相变的一个特征是四面体分子的形成/解离。在这方面，这种相变与高压高温下的液态磷相变有更强的相似性，而不是压力诱导的非晶态冰的低密度-高密度相变。我们证明了密度可以从G（r）得到，精度高到足以区分低压和高压非晶态之间的差异。我们发现SnI_4的非晶-非晶相变伴随着密度的压力滞后和在高于相变压力时的反常硬化。我们最近的拉曼光谱测量表明，在高压到低压非晶态的转变过程中产生了一个新的振动模式。这项研究的部分结果已于8月在第19届IUCr大会（日内瓦）上发表。2002年和<th>2003年7月在法国波尔多举行的第19届国际高压科学与技术会议上，少

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