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

当分子晶体SnI4压缩产生的高压非晶态从30 GPa解压缩时，第一衍射峰的位置在3 GPa处发生了不连续的移动。随后的压缩产生了6 GPa时相反方向的位移。这些先前的观察结果表明，非晶态发生了结构变化。本工作的目的是定量研究非晶态结构的变化，了解无序状态下的相变。我们首先进行了同步辐射x射线测量，研究了非晶态-非晶态相变附近的非晶态结构。结构因子的第一个峰S(Q)的位置、宽度和形状在3 GPa时发生了不连续的变化。通过对S(Q)的傅里叶变换，得到了各压力下的折合径向分布函数G(R)。对G(R)的检验表明，对应于Sn和I…之间的距离在G(R)大于3 GPa时，四面体分子中缺少更多的离子和I离子，而在分子内原子距离低于3 GPa时出现明显的峰，这清楚地表明四面体分子处于低压无定形状态。在第二次压缩时，在7 GPa时发生了相反的过程，具有很大的压力滞后。这些观察结果表明，压力诱导的非晶态SnI_4的结构变化是一级相变。SnI_4的非晶态-非晶态相变的一个特征是发生了四面体分子的形成/解离。在这一点上，这种相变与高压高温下液态荧光粉的相变有更强的相似性，而不是压力诱导的无定形冰的低密度-高密度相变。非晶态固体的密度一直是一个在压力下难以测量的物理参数。我们证明了从G(R)可以得到足够高的精度来区分低压和高压非晶态的密度。结果表明，SnI_4的非晶态-非晶态相变伴随着密度的压力滞后和减压时高于转变压力的反常硬化。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，产生了一个新的振动模式。这项研究的部分结果发表在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大会(波尔多)上。较少

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