Structural correlations in $Cs_2CuCl_4$: Pressure dependence of electronic structures

We have investigated the crystal structure of $Cs_2CuCl_4$ 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 $CuCl_4^{2-}$ units distorted by the Jahn-Teller effect. This study provides a complete characterization of the electronic structure of $Cs_2CuCl_4$ in the Pmna phase as a function of the cell volume and the $Cu-Cl$ bond length, $R_{Cu-Cl}$. Interestingly, the opposite shift of the charge-transfer band-gap and the $Cu^{2+}$ d-d crystal-field band shift with pressure are responsible for the strong piezochromism of $Cs_2CuCl_4$. We have also explored the high-pressure structure of $Cs_2CuCl_4$ above 4.9 GPa yielding structural transformations that are probably associated with a change of coordination around $Cu^{2+}$. 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. Edited by: A. Goñi, A. Cantarero, J. S. Reparaz