Coupled dimerized alternating-bond quantum spin chains in the distorted honeycomb-lattice magnet Cu 5 SbO 6

We analyze powder-averaged inelastic neutron scattering and magnetization data for the distorted honeycomb compound Cu 5 SbO 6 using a first-order dimer expansion calculation and quantum Monte Carlo simulations. We show that, in contrast to the previously proposed honeycomb-lattice model, Cu 5 SbO 6 accommodates interacting dimerized spin chains with alternating ferromagnetic-antiferromagnetic couplings along the chain. Moreover, unlike the typical couplings observed in other Cu 2 + -based distorted honeycomb magnets, the spin chains in Cu 5 SbO 6 primarily couple through an antiferromagnetic coupling that arises between the honeycomb layers, rather than the expected interchain coupling in the layers. This finding reveals a different magnetic coupling scheme for Cu 5 SbO 6 . In addition, utilizing x-ray spectroscopy and transmission electron microscopy, we also refine the crystal structure and stacking-fault model of the compound.