Crystal and magnetic structure of cesium superoxide

${\mathrm{CsO}}_2$ is a member of the family of alkali superoxides (formula ${A\mathrm{O}}_2$ with $A=\mathrm{Na}, \mathrm{K}, \mathrm{Rb}, \mathrm{and} \mathrm{Cs}$) that exhibit magnetic behavior arising from open $p$-shell electrons residing on ${\mathrm{O}}_2^{-}$ molecules. We use neutron diffraction to solve the crystal and magnetic structures of ${\mathrm{CsO}}_2$ and observe a complex series of structures on cooling from room temperature to 1.6 K. These include an incommensurate modulation along the $a$ axis of the structure at intermediate temperatures, which then locks into a commensurate modulation that doubles the unit cell compared to the previously supposed orthorhombic unit cell. In both incommensurate and commensurate phases, our structural solution involves a staggering of the cesium ion positions along the $b$ axis, in contrast to studies of other alkali superoxides in which staggered tilts of the ${\mathrm{O}}_2^{-}$ dimers relative to the $c$ axis are seen. Below $T\simeq 10 \mathrm{K}$, we observe magnetic Bragg reflections arising from an antiferromagnetically ordered structure with a wave vector of $\mathbf{k}=(0,0,0)$ (relative to the doubled crystallographic unit cell), with moments that point predominantly along the $b$ axis with a small component along the $a$ axis that hints at possible anisotropic exchange coupling (consistent with the crystal structure). Measurements of the magnetic Bragg reflections in an applied magnetic field suggest a spin-flop transition takes place between 2 T and 4 T in which moments likely flop to point along the crystallographic $a$ axis. Our measurements indicate that ${\mathrm{CsO}}_2$ is an interesting example of magnetic properties being inherently linked to the crystal structure, in that the staggered displacement of the cesium ions activates antisymmetric exchange, which then permits the observed spin canting.