METALLIZATION AND PHASE-TRANSITIONS OF THE 3-DIMENSIONAL HALOGEN-BRIDGED MIXED-VALENCE COMPLEX CS2AU2I6 UNDER HIGH-PRESSURE

We have recently shown that insulating mixed-valence complex Cs2Au2I6 transforms gradually to a metallic state between ambient pressure and 4.5 GPa at room temperature. Also shown that Cs2Au2I6 undergoes a metal-to-metal transition in a process of increasing temperature at 6.5 GPa. The present energy-dispersive X-ray diffraction study has revealed that Cs2Au2I6 undergoes a pressure-induced first-order tetragonal-to-tetragonal phase transition at about 5.5 GPa and room temperature. At the transition pressure, the Au-Au distance elongates along the c-axis, which is due to the 'band Jahn-Teller' transition. The Au(II) valence state is considered to be realized at this pressure. This phase transition would relate to the minimum in the pressure-resistance curve near 5.5 GPa, previously reported. It was also found that Cs2Au2I6 undergoes a first-order tetragonal-to-cubic phase transition at 6.8 GPa and T approximately 350 K.