We report a molecular crystal that exhibits four successive phase transitions under hydrostatic pressure, driven by aurophilic interactions, with the ground-state structure re-emerging at high pressure. The effect of pressure on two polytypes of tris(μ2-3,5-diisopropyl-1,2,4-triazolato-κ2 N 1:N 2)trigold(I) (denoted Form-I and Form-II) has been analysed using luminescence spectroscopy, single-crystal X-ray diffraction and first-principles computation. A unique phase behaviour was observed in Form-I, with a complex sequence of phase transitions between 1 and 3.5 GPa. The ambient C2/c mother cell transforms to a P21/n phase above 1 GPa, followed by a P21/a phase above 2 GPa and a large-volume C2/c supercell at 2.70 GPa, with the previously observed P21/n phase then reappearing at higher pressure. The observation of crystallographically identical low- and high-pressure P21/n phases makes this a rare example of a re-entrant phase transformation. The phase behaviour has been characterized using detailed crystallographic theory and modelling, and rationalized in terms of molecular structural distortions. The dramatic changes in conformation are correlated with shifts of the luminescence maxima, from a band maximum at 14040 cm-1 at 2.40 GPa, decreasing steeply to 13550 cm-1 at 3 GPa. A similar study of Form-II displays more conventional crystallographic behaviour, indicating that the complex behaviour observed in Form-I is likely to be a direct consequence of the differences in crystal packing between the two polytypes.
Bibliographical noteFunding Information:
We are grateful to the EPSRC for financial support of the project (EP/K012576/1 and EP/K004956/1) and for studentship funding for CHW (EP/F021151/1). JMS gratefully acknowledges funding from the EPSRC Programme (grant No. EP/K004956/1). Parts of this work were carried out using the HeCTOR and ARCHER supercomputers through membership of the UK HPC Materials Chemistry Consortium, which is funded by EPSRC grant No. EP/L000202. We would like to thank the ALS, LBNL, for the beamtime to perform these measurements. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under Contract No. DE-AC02-05CH11231. Additional thanks go to COMPRES, the consortium for Materials Properties Research in Earth Sciences under NSF Cooperative Agreement EAR 11-57758. CR gratefully acknowledges funding from the Natural Sciences and Engineering Research Council of Canada.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Condensed Matter Physics