Antiferromagnetic $S=1/2$ Spin Chain Driven by $p$-Orbital Ordering in ${\mathrm{CsO}}_2$

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 (${\mathrm{O}}_2^{-}$) 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.

Figure 1

(a) Magnetic susceptibility versus temperature for polycrystalline ${\mathrm{CsO}}_2$, measured on warming in a field of 10 kOe after cooling in the same field. Fits to the models of Keith et al. [21] (green solid line) and Feyerherm et al. [22] (red dashed line) are shown. The arrow indicates an anomaly at 70 K. (b) Low-temperature magnetic susceptibility showing $T_N$ and a fit of the low-temperature susceptibility tail to the Curie law. A Curie-Weiss fit to the inverse susceptibility above 150 K is shown in the inset.

Figure 2

(a),(b) 200 and 103 XRPD peaks in the tetragonal and orthorhombic regimes. (c) Evolution of lattice parameters and (d) selected microstrain peak-broadening parameters with temperature.

Figure 3

Raman spectra of ${\mathrm{CsO}}_2$ at (a) 110 K and (b) 4.5 K. (c) Raman modes associated with the interlayer motion of Cs and (d) librational mode of ${\mathrm{O}}_2^{-}$ as a function of temperature.

Figure 4

(a) Partial oxygen DOS for anions $A$ and $B$ along the $b$ axis in the $z=0$ layer below 70 K. Because of the AFM configuration, the spin-up and spin-down DOS for anions in the $z=1/4$ layer is reversed compared to the DOS shown here. (b) $ab$-plane view of the optimized structure of ${\mathrm{CsO}}_2$ below 70 K showing anion tilting and ordering of the half-occupied ${\pi }_x^{*}$ and ${\pi }_y^{*}$ orbitals.