Jahn-Teller effect and the orbital ground state of Cu2+ ions in Eu2CuO4 and La2CuO4 crystals

Local ion displacements and their anharmonicity in Eu2CuO4 and La2CuO4 crystals are studied by high-precision x-ray diffractometry. A symmetry analysis and the temperature behavior of the probability density function of Cu2+ ions reveal a degenerate ground orbital state in the Eu2CuO4 crystal (tetragonal doublet dxz, dyz). The pattern and anharmonicity of the local displacements are found to be determined not only by lattice vibrations but by 2D antiferromagnetic spin fluctuations in the CuO2 sheet as well. By contrast, in the La2CuO4 crystal the orbital ground state of the Cu2+ ion is a singlet (dz2). The pattern of Eu3+ local displacements and their effect on the properties of the Cu subsystem in the Eu2CuO4 crystal are also studied.