The high pressure behaviour of molecular diatomic iodine is perhaps one of the most fascinating, with a gradual shortening of the intramolecular I-I contacts until they become near in value to the intermolecular I-I bonding distances.  At ~18 GPa metallization occurs^1,2 even though the system remains a molecular solid.³  With continuing pressure the distances continue to approach a similar value, finally at ~30 GPa the iodine bonds break and an extended monoatomic solid forms.⁴  To date, there have been no reports of the high pressure chemistry of related simple polyhalide salts, such as the well known AI₃ (A = Rb, Cs, Tl, NH₄) systems, to explore the possibilities of similar metallisation and/or phase transitions.  We have recently collected synchrotron X-ray and neutron powder diffraction data on these materials and observe phase transitions from orthorhombic to trigonal systems at very low pressures, in the range of 0.7-3 GPa.  We have solved the structure of the high pressure phase, it is compared to the ambient pressure phase in Figure 1.  Although the phase transition does not involve loss of triiodide units, there are drastic changes in the packing of the cations and polyhalides precluding a simple correspondence between the two polymorphs.  It also appears that the phases are approaching a simple cubic AX₃ system with the A15 (Cr₃Si) structure type.  In this talk, details of the novel high pressure phases and correlations between the nature of the A cation and phase transition pressure and measured bulk moduli  will be presented.

Fig. 1.Views of the ambient (left) and high (right) pressure phases

1. Balchan, A. S. & Drickamer, H., J. Chem. Phys. 34, 1948 (1961).
2. Riggleman, B. M. & Drickamer, H. G., J. Chem. Phys. 38, 2721 (1963).
3. Shimomura, O. et al., Phys. Rev. B 18, 715 (1978).
4. Takemura, K., Minomura, S., Shimomura, O. & Fujii, Y., Phys. Rev. Lett. 45, 1881 (1980).

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2. In-situ high-pressure powder diffraction studies of layered metal phosphates
3. Powder diffraction studies of mixed titanium zirconium phosphates
4. The high pressure crystal structure of the NLO compound BiB₃O₆ from 2D powder diffraction data