Low-temperature crystal and magnetic structures of the chain-ladder composite material Sr0.4Ca13.6CU24+yO41+z: Hole redistribution and antiferromagnetic order

The low-temperature crystal structure of a one-dimensional chain-ladder com posite material Sr0.4Ca13.6Cu24+yO41+z Was determined by thr Rietveld analy sis 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 min imum of the distance between ladder-copper and chain-oxygen atoms [Cu(1)-O( 3)] was revealed to expand with lowering temperature. The BVS calculation i ndicated that such a structural change corresponds to a redistribution of h oles from the ladder to the chain and that almost all of the holes are loca lized in the chain below or near the Neel temperature. By assuming reasonab le magnetic interactions between hole-unoccupied Cu sites on the chain plan e, we propose a possible magnetic structure model taking into account the d istribution of holes and observed magnetic neutron Bragg reflections. The r esults suggest the presence of spin dimers, spin trimers, and ''lone'' spin s in the chain, of which the latter two have effective magnetic moments. Th ese moments may be an origin of staggered antiferromagnetic spin modulation onto the spin-liquid state.