Mechanism of pressure-induced amorphization, superconductivity and molecular dissociation in tetrahedral-molecular crystals

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The objective of the present work is to study structural properties of tetrahedral-molecular crystals SnIィイD24ィエD2 and SnBrィイD24ィエD2 at low temperature and at ultrahigh pressure. Major goals are (1) to determine crystal structure of the nonmolecular phase of SnIィイD24ィエD2 crystallizing from the amorphous phase at 60 Gpa at room temperature and (2) to establish a P-T phase diagram down to 10K and up to 100 Gpa. In the first fiscal year, we designed and constructed two pieces of apparatus crucial for those experiments. One is a low-temperature cryostat equipped with a diamond anvil cell (DAC) device for high-pressure generation. Another is a spectrometer capable of measuring fluorescence from ruby micro-chips encapsulated in a sample chamber in a DAC. Duplicates of them were later installed at the BL10XU station at the SPring-8 synchrotron radiation experimental facility.In the second fiscal year, we carried out synchrotron x-ray diffraction experiments at the SPring-8, using the apparatu … More s described above. A low temperature condition of 18K and a high pressure condition of 89 GPa were reached simultaneously. The following results were obtained:(1) The nonmolecular phase of SnIィイD24ィエD2 has the same crystal structure as a substitutional, disordered alloy with the fcc lattice. The lattice sites are occupied by the Sn and I atoms at random.(2)At 18K, the amorphous phase persists to a very high pressure of 80 Gpa, suggesting that a thermal activation process plays an important role in the amorphous-nonmolecular phase transition. No new crystal structures were observed in this study. Results of previous Mossbauer studies at low temperature and high pressure need to be reexamined on the basis of the phase relationship determined in the present study.(3) A new crystal structure of SnBrィイD24ィエD2 stable between 3 Gpa and 9 Gpa was determined to be cubic. Effect of pressure on disordering of crystal structure is much more insensitive in SnBrィイD24ィエD2 than in SnIィイD24ィエD2.The results of this study were presented at the IUCr Congress and General Assembly (Glasgow), the fall meeting of the Physical Society of Japan (Morioka), and the annual meeting of the Japan Society of High Pressure Science and Technology (Fukuoka) in 1999. Less