Structural properties and phase transition of hole-orbital-ordered (C2H5NH3)2CuCl4 studied by resonant and non-resonant x-ray scatterings under high pressure

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 (C{sub 2}H{sub 5}NH{sub 3}){sub 2}CuCl{sub 4}. We have found a structural phase transition at P{sub c}{approx}4 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{sub 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{sub c}. The red-colored transparency of the crystal indicates that the system is still insulative above P{sub 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{sub x} and p{sub y} symmetries, which is mainly dominated not by the on-site 3d-4p Coulomb interaction but by the JTD of the CuCl{sub 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 ismore » suppressed by the application of pressure. On the other hand, the RXS intensity becomes zero above P{sub c}. The results indicate that the local environment of the Cu{sup 2+} ion markedly changes. Two possible structures of EA{sub 2}CuCl{sub 4} above P{sub c} are proposed.« less