Structural Evolution of One-dimensional Spin Ladder Compounds Sr14-xCaxCu24O41 with Ca doping and Related Hole Redistribution Evidence

TL;DR

This study investigates how calcium doping affects the crystal structure and charge distribution in Sr14-xCaxCu24O41 spin ladder compounds, revealing structural evolution and hole transfer mechanisms between chains and ladders.

Contribution

It provides detailed structural analysis and evidence of hole redistribution mechanisms in Ca-doped Sr14Cu24O41, advancing understanding of their electronic properties.

Findings

Ca doping makes CuO4 units more square-shaped

Charge transfer from chains to ladders increases with Ca doping

Structural refinements support hole transfer mechanisms

Abstract

Incommensurate crystal structures of spin ladder series Sr14-xCaxCu24O41 (x=3, 7, 11, 12.2) were characterized by powder neutron scattering method and refined using the superspace group Xmmm(00{\gamma})ss0 (equivalent to superspace group Fmmm(0,0,1+{\gamma})ss0); X stands for non-standard centering (0,0,0,0), (0,1/2,1/2,1/2), (1/2,1/2,0,0), (1/2,0,1/2,1/2)) with a modulated structure model. The Ca doping effects on the lattice parameters, atomic displacement, Cu-O distances, Cu-O bond angles and Cu bond valence sum were characterized. The refined results show that the CuO4 planar units in both chain and ladder sublattices become closer to square shape with an increase of Ca doping. The Cu bond valence sum calculation provided new evidence for the charge transfer from the chains to ladders (approximately 0.16 holes per Cu from x=0 to 12.2). The charge transfer was attributed to two different mechanisms: (a) the Cu-O bond distance shrinkage on the ladder; (b) increase of the interaction between two sublattices, resulting in Cu-O bonding between the chains and ladders. The low temperature structural refinement resulted in the similar conclusion, with a slight charge backflow to the chains.