Structural signatures of the insulator-to-metal transition in BaCo1-xNix S2

The solid solution BaCo1-xNixS2 exhibits an insulator-to-metal transition close to x=0.21. Questions of whether this transition is coupled with structural changes remain open. Here we follow the structural evolution as a function of the Ni content x using synchrotron powder x-ray diffraction and pair distribution function analyses to reveal significant basal sulfide anion displacements occurring preferentially along the CoS5 pyramidal edges comprising the edge-connected bond network in BaCo1-xNixS2. These displacements decrease in magnitude as x increases and are nearly quenched in x=1BaNiS2. Density-functional-theory-based electronic structure calculations on x=0BaCoS2 suggest that these displacements arise as a dynamic first-order Jahn-Teller effect owing to partial occupancy of nominally degenerate Co2+dxz and dyz orbitals, leading to local structural symmetry breaking in the xy-plane of the Co-rich phases. The Jahn-Teller instability is associated with the opening of a band gap that is further strengthened by electronic correlation. The Jahn-Teller effect is reduced upon increased electron filling as x→1, indicating that the local structure and band filling cooperatively result in the observed insulator-to-metal transition.