Abstract: We have investigated the crystal structure of Cs2CuCl4 in the 0-20 GPa range as a function of pressure and how pressure affects its electronic properties by means of optical absorption spectroscopy. In particular, we focused on the electronic properties in the low-pressure Pnma phase, which are mainly related to the tetrahedral CuCl4 2- units distorted by the Jahn-Teller effect. This study provides a complete characterization of the electronic structure of Cs2CuCl4 in the Pmna phase as a function of the cell volume and the Cu?Cl bond length, RCu?Cl. Interestingly, the opposite shift of the charge-transfer band-gap and the Cu2+ d-d crystal-field band shift with pressure are responsible for the strong piezochromism of Cs2CuCl4. We have also explored the high-pressure structure of Cs2CuCl4 above 4.9 GPa yielding structural transformations that are probably associated with a change of coordination around Cu2+. Since the high-pressure phase appears largely amorphized, any structural information from X-ray diffraction is ruled out. We use electronic probes to get structural information of the high-pressure phase.

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