Structural signatures of the insulator-to-metal transition in Ba Co 1 − x Ni x S 2

The solid solution $\\mathrm{Ba}{\\mathrm{Co}}_{1\\ensuremath{-}x}{\\mathrm{Ni}}_{x}{\\mathrm{S}}_{2}$ 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 ${\\mathrm{CoS}}_{5}$ pyramidal edges comprising the edge-connected bond network in $\\mathrm{Ba}{\\mathrm{Co}}_{1\\ensuremath{-}x}{\\mathrm{Ni}}_{x}{\\mathrm{S}}_{2}$. These displacements decrease in magnitude as $x$ increases and are nearly quenched in $x=1\\phantom{\\rule{4pt}{0ex}}{\\mathrm{BaNiS}}_{2}$. Density-functional-theory-based electronic structure calculations on $x=0\\phantom{\\rule{4pt}{0ex}}{\\mathrm{BaCoS}}_{2}$ suggest that these displacements arise as a dynamic first-order Jahn-Teller effect owing to partial occupancy of nominally degenerate ${\\mathrm{Co}}^{2+}\\phantom{\\rule{4pt}{0ex}}{d}_{xz}$ and ${d}_{yz}$ 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 $x\\ensuremath{\\rightarrow}1$, indicating that the local structure and band filling cooperatively result in the observed insulator-to-metal transition.