Structure Matters: Expanding the Origin of the Insulator-metal Transition in BaCo1-xNixS2

We find that structural distortions play a pivotal role in the  insulator-to-metal  transition  (IMT),  in  BaCo1-xNixS2despite the average structure remaining unchanged. We demonstrated  that  the  Jahn-Teller  (JT)  effect,  stemming from  degenerate dxzand dyz  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.

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