Low-temperature crystal and magnetic structures of the chain-ladder composite material Sr_0.4Ca_13.6Cu_24+yO_41+z: Hole redistribution and antiferromagnetic order

The low-temperature crystal structure of a one-dimensional chain-ladder composite material Sr_0.4Ca_13.6Cu_24+yO_41+z was determined by the Rietveld analysis of neutron diffraction data using a superspace group approach. The hole distribution between the chain and ladder planes was estimated by the bond-valence sum (BVS) calculation based on Cu-O interatomic distances. The minimum of the distance between ladder-copper and chain-oxygen atoms [Cu(1)-O(3)] was revealed to expand with lowering temperature. The BVS calculation indicated that such a structural change corresponds to a redistribution of holes from the ladder to the chain and that almost all of the holes are localized in the chain below or near the Néel temperature. By assuming reasonable magnetic interactions between hole-unoccupied Cu sites on the chain plane, we propose a possible magnetic structure model taking into account the distribution of holes and observed magnetic neutron Bragg reflections. The results suggest the presence of spin dimers, spin trimers, and "lone" spins in the chain, of which the latter two have effective magnetic moments. These moments may be an origin of staggered antiferromagnetic spin modulation onto the spin-liquid state.