Accurate Determination of Composite Crystal Structure of Incommensurately Modulated Sr14Cu24O41 Using the Akaike Information Criterion

The composite crystal structure of the spin-ladder compound Sr14Cu24O41 has been reinvestigated using the Akaike information criterion (AIC) to solve the possible overfitting of atomic positions in our previous work [Phys. Rev. B 68. 224108 (2003)]. The minimizing AIC method using the same data as before removes the splitting behavior of the Cu-O bonds along the one-dimensional Cu-O chain in the two-legged Cu2O3 ladder and reveals the importance of the Cu-O-Cu rung with a strong Cu-O bond. Nonsymmetric paths of hole transfer from the O atom in the CuO2 chain to the Cu-O-Cu rung in the ladder have been elucidated by the resulting new modulated structure. Bond-valence sum analysis of the modulated CuO2 substructure of Sr(14)Cu(24)O(41 )shows the role of large displacive modulation of the O atom in the CuO2 chain and the valence fluctuation of Cu atom with a periodicity almost 200 times that of the average CuO2 lattice. There is no discommensuration of the < Cu2+>-< Cu3+>-< Cu2+> arrangements in the CuO2 chain. The incommensurability between the average substructures is characterized as sigma = (0 0 0.6995) and the chemical formula should be exactly expressed as (Sr2Cu2O3)(0.6995)CuO2.