Antiferromagnetic and structural transitions in the superoxide KO ₂ from first principles: A 2p -electron system with spin-orbital-lattice coupling

KO₂ exhibits concomitant antiferromagnetic (AFM) and structural transitions, both of which originate from the open-shell 2p electrons of O ₂^- molecules. The structural transition is accompanied by the coherent tilting of O₂^- molecular axes. The interplay among the spin-orbital-lattice degrees of freedom in KO₂ is investigated by employing the first-principles electronic structure theory and the kinetic-exchange interaction scheme. We have shown that the insulating nature of the high-symmetry phase of KO₂ at high temperature (T) arises from the combined effect of the spin-orbit coupling and the strong Coulomb correlation of O2p electrons. In contrast, for the low-symmetry phase of KO₂ at low T with the tilted O₂^- molecular axes, the band gap and the orbital ordering are driven by the combined effects of the crystal field and the strong Coulomb correlation. We have verified that the emergence of the O2p ferro-orbital ordering is essential to achieve the observed AFM structure for KO₂.