Structural properties and phase transition of hole-orbital-ordered(C2H5NH3)2CuCl4studied by resonant and non-resonant x-ray scatterings under high pressure

Abstract

The effects of hydrostatic pressure on a structure and the corresponding resonant x-ray scattering (RXS) spectrum have been investigated for the hole-orbital-ordered compound ${({\mathrm{C}}_{2}{\mathrm{H}}_{5}\mathrm{N}{\mathrm{H}}_{3})}_{2}\mathrm{Cu}{\mathrm{Cl}}_{4}$. We have found a structural phase transition at ${P}_{\mathrm{c}}\ensuremath{\sim}4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, as suggested by the Raman scattering measurement, by observing a peak splitting resulting from an orthorhombic-to-monoclinic symmetry breaking. The reduction of the Jahn-Teller distortion (JTD) toward ${P}_{\mathrm{c}}$ is also ascertained by structural analyses. The gradual change in the color of the crystal is also observed in connection with the structural change near ${P}_{\mathrm{c}}$. The red-colored transparency of the crystal indicates that the system is still insulative above ${P}_{\mathrm{c}}$. The observed RXS and fluorescence spectrum at ambient pressure were precisely analyzed. We experimentally confirmed that the RXS intensity comes from the polarization of the density of states of ${p}_{\mathrm{x}}$ and ${p}_{\mathrm{y}}$ symmetries, which is mainly dominated not by the on-site $3d\text{\ensuremath{-}}4p$ Coulomb interaction but by the JTD of the $\mathrm{Cu}{\mathrm{Cl}}_{6}$ octahedron. charged Cl ions neighboring the Cu site. It is also expected that the RXS intensity is proportional to JTD; however, our RXS study under a high pressure shows no striking change as JTD is suppressed by the application of pressure. On the other hand, the RXS intensity becomes zero above ${P}_{\mathrm{c}}$. The results indicate that the local environment of the ${\mathrm{Cu}}^{2+}$ ion markedly changes. Two possible structures of ${\mathit{EA}}_{2}\mathrm{Cu}{\mathrm{Cl}}_{4}$ above ${P}_{\mathrm{c}}$ are proposed.