We find that structural distortions play a pivotal role in the insulator-to-metal transition (IMT), in BaCo_1-xNi_xS₂ despite the average structure remaining unchanged. We demonstrated that the Jahn-Teller (JT) effect, stemming from degenerate d_xz and d_yz orbitals sharing a single electron, is consistent with anomalous sulfide displacements observed in X-ray diffraction studies. The sulfide displacements, which relieve the JT instability, assist the opening of a band gap which exists at low Ni concentrations. Adding Ni to the system introduces extra electrons which suppress this JT effect, therefore assisting the IMT observed near 21% Ni.

Since IMTs are often driven cooperatively by structural, electronic, and magnetic interactions, understanding the contributions and interplay of these degrees of freedom is crucial to designing materials with desirable properties. Knowing that lattice distortions can play a role in structure-preserving IMTs enables us to functionalize such materials for use in emerging technologies.

Reverse Monte Carlo modeling of atomic positions showing that the sulfide ions (yellow) feature large displacements which decrease with higher Ni content. Green denotes the barium site and green denotes the cobalt/nickel site.

Illustrations of (a) the ideal CoS₅ pyramid with its energy level diagram, featuring the JT instability, and (b) the two possible distorted CoS₅ pyramids with their corresponding energy level diagram, featuring a resolved JT instability.