Structural signatures of the insulator-to-metal transition in BaCo_1-<i>x</i>Ni<i>_x</i>S₂

The solid solution BaCo_1-xNi_xS₂ exhibits an insulator-to-metal transition close to x = 0.21. The questions of whether this transition is coupled with structural changes remain unresolved. X-ray diffraction data reveal significant basal sulfide anion displacements occurring preferentially along the CoS₅ pyramidal edges comprising the edge-connected bond network in BaCo_1-xNi_xS₂. These displacements decrease in magnitude as x increases and are nearly quenched in x = 1 BaNiS₂. Here we present density functional theory-based electronic structure calculations on x = 0 BaCoS₂ and show that these displacements arise as a dynamic first-order Jahn-Teller effect owing to partial occupancy of nominally degenerate Co²⁺ d_xz and d_yz orbitals. This Jahn-Teller instability leads to local structural symmetry breaking associated with opening a band gap that is further strengthened by electronic correlation. The Jahn-Teller effect is reduced upon increased electron filling as x → 1, indicating local structure and band filling cooperatively induce the observed insulator-to-metal transition.^1
1 Schueller, E. C. et al. Phys. Rev. Mater. 2020, 4 (10), 104401. 10.1103/PhysRevMaterials.4.104401