AntiferromagneticS=1/2Spin Chain Driven byp-Orbital Ordering inCsO2

We demonstrate, using a combination of experiment and density functional theory, that orbital ordering drives the formation of a one-dimensional (1D) $S=1/2$ antiferromagnetic spin chain in the 3D rocksalt structure of cesium superoxide (${\mathrm{CsO}}_{2}$). The magnetic superoxide anion (${\mathrm{O}}_{2}^{\ensuremath{-}}$) exhibits degeneracy of its $2p$-derived molecular orbitals, which is lifted by a structural distortion on cooling. A spin chain is then formed by zigzag ordering of the half-filled superoxide orbitals, promoting a superexchange pathway mediated by the ${p}_{z}$ orbitals of ${\mathrm{Cs}}^{+}$ along only one crystal direction. This scenario is analogous to the $3d$-orbital-driven spin chain found in the perovskite ${\mathrm{KCuF}}_{3}$ and is the first example of an inorganic quantum spin system with unpaired $p$ electrons.