Structural modulation, hole distribution, and hole-ordered structure of the incommensurate composite crystal(Sr2Cu2O3)0.70CuO2

Abstract

Modulated structure of incommensurate composite crystal (Sr 2 Cu 2 O 3 ) 0 . 7 0 CuO 2 Sr 1 4 Cu 2 4 O 4 1 , has been investigated by single-crystal x-ray-diffraction method using centrosymmetric (3+1)-dimensional superspace group. In (Sr 2 Cu 2 O 3 ) 0 . 7 0 CuO 2 displacive modulation of O atom in the CuO 2 chain is fairly large. Considering the modulation of bond angles, it has been found that the Cu-O bond in the CuO 2 chain is tilting toward the Cu 2 O 3 ladder in order that the O atom in the chain plays as apical oxygen for the CuO 4 square in the ladder. The bond-valence sum (BVS) method has been applied to investigate the hole distribution in the modulated structure of (Sr 2 Cu 2 O 3 ) 0 . 7 0 CuO 2 . It is indicated that the valence of Cu atom in the Cu 2 O 3 ladder is +2.04, where about 0.03 holes are certainly transferred from the CuO 2 chain through the modulated O atom in the CuO 2 . The BVS calculation has demonstrated that almost all of the holes are prepared in the CuO 2 chain by the large modulation of the Cu-O bond. Cu atoms in the modulated CuO 2 chain have been proved to form hole-ordered structure with next-nearest-neighbor Cu 2 + ions separated by Cu 3 + ion on the Zhang-Rice singlet site. The periodicity of the hole-ordered structure is five times of the average CuO 2 lattice along the crystallographic c axis, which is compatible with the spin-dimerized state at low temperature. The new model of the two-dimensional hole-ordered structure in the CuO 2 plane has been obtained by the BVS calculation. Furthermore, the two-dimensional configuration of the spin dimers has been successfully derived from the hole-ordered structure in the CuO 2 plane. It has been concluded that the valences of Cu atoms both in the Cu 2 O 3 ladder and in the CuO 2 chain are well controlled by the modulated O atom in the CuO 2 chain.