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

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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