Origin of the monoclinic distortion and its impact on the electronic properties in KO₂

Potassium superoxide KO₂ is a p-electron system in which the interplay between spin, orbital and lattice degrees of freedom leads to a complex phase diagram. We use the density functional theory and lattice dynamics calculations to reveal the mechanism of formation of the low-temperature monoclinic phase [1]. We show dynamical instability of the high-temperature tetragonal structure and identify a soft phonon mode leading to the monoclinic C2/c symmetry and thus demonstrate a displacive character of the structural transition. The origin of enhanced insulating gap in the monoclinic phase is discussed within the model including the Hubbard U in the valence orbitals of the O₂^- ions. In the tetragonal phase without an orbital order, a small gap of 0.5 eV appears only if the spin-orbit interaction is taken into account. However, the lattice distortion present in the monoclinic structure induces orbital order via the Jahn-Teller effect, and the local Coulomb interactions increase the insulating gap to approximately 1.1 eV.

[1] O. Sikora, D. Goftryd, A. Ptok, M. Sternik, K. Wohlfeld, A. M. Oles, and P. Piekarz, Phys. Rev. B 102, 085129 (2020).