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.