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 symmet ry analysis and the temperature behavior of the probability density fu nction of Cu2+ ions reveal a degenerate ground orbital state in the Eu 2CuO4 crystal (tetragonal doublet d(xz), d(yz)). The pattern and anhar monicity of the local displacements are found to be determined not onl y by lattice vibrations but by 2D antiferromagnetic spin fluctuations in the CuO2 sheet as well. By contrast, in the La2CuO4 crystal the orb ital ground state of the Cu2+ ion is a singlet (d(z)2). The pattern of Eu3+ local displacements and their effect on the properties of the Cu subsystem in the Eu2CuO4 crystal are also studied. (C) 1997 American Institute of Physics.