Structural signatures of the insulator-to-metal transition in BaCo1−xNixS2

Abstract

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.