Origin of the piezochromism in Cs2CuCl4 : Electron-phonon and crystal-structure correlations

Abstract

By using optical absorption and Raman spectroscopy, we have investigated the effects of pressure in the structural, electronic, and vibrational properties of ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$ in the 0--20 GPa range. Our study has provided a complete characterization of the electronic and vibrational structures of ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$ in the low-pressure Pnma phase as a function of cell volume and the Cu-Cl bond length, ${R}_{\text{Cu-Cl}}$. We have focused on the electronic structure in the Pnma phase, which is mainly related to the tetrahedral ${\mathrm{CuCl}}_{4}^{2\ensuremath{-}}$ units distorted by the Jahn-Teller effect. The strong piezochromism of ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$ is caused by the opposite shift of the charge-transfer band gap and the ${\mathrm{Cu}}^{2+}$ d-d crystal-field transitions with pressure. The high-pressure structure of ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$ above 4.9 GPa yields structural transformations that we ascribe to a change of coordination around ${\mathrm{Cu}}^{2+}$. The high-pressure phase appears largely amorphized, therefore any structural information from x-ray diffraction is ruled out. Here we use electronic and vibrational probes to get inside the structural information.