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

Abstract

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 = 1 BaNiS2. Density functional theory-based electronic structure calculations on x = 0 BaCoS2 suggest that these displacements arise as a dynamic first-order Jahn-Teller effect owing to partial occupancy of nominally degenerate Co 2+ 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 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 local structure and band filling cooperatively result in the observed insulator-to-metal transition.

Two-dimensional fluctuations and competing phases in the stripe-like antiferromagnet BaCoS$_{2}$

“…The Rietveld refinements in the orthorhombic Cmma symmetry explain well the data not only at room temperature, as previously, [14,15] but at all temperatures in the whole 100-400 K range studied. Namely, using the above symmetry, the refinements yield reliability R-Bragg factors less than 10% for all spectra and, contrary to previous reports [14,19,20], we find no indications of any structural distortion or modulation that would lower the crystal symmetry. In Figure 1, we plot the lattice parameters of the Cmma cell as a function of temperature.…”

Order from disorder phenomena in BaCoS2

“…A mixing between x and y orbitals is instead allowed by the Pba2 space group proposed in ref. 27 as an alternative scenario for BaCoS 2 at room temperature. As a matter of fact, the two symmetry-lowering routes, P4/ nmm → Cmme and P4/nmm → Pba2, correspond to different Jahn-Teller-like distortions involving the d xz -d yz doublet and the E g phonon mode of the P4/nmm structure at the M point, which is found to have imaginary frequency by ab initio calculations 27 .…”

“…27 as an alternative scenario for BaCoS 2 at room temperature. As a matter of fact, the two symmetry-lowering routes, P4/ nmm → Cmme and P4/nmm → Pba2, correspond to different Jahn-Teller-like distortions involving the d xz -d yz doublet and the E g phonon mode of the P4/nmm structure at the M point, which is found to have imaginary frequency by ab initio calculations 27 . However, latest high-accuracy X-ray diffraction data 18 confirm the Cmme orthorhombic structure even at room temperature, thus supporting our assumption.…”

Order from disorder phenomena in BaCoS$_2$

“…A mixing between x and y orbitals is instead allowed by the P ba2 space group proposed in Ref. [36] as an alternative scenario for BaCoS 2 at room temperature. As a matter of fact, the two symmetry-lowering routes, P 4/nmm → Cmme and P 4/nmm → P ba2, correspond to different Jahn-Teller-like distortions involving the d xz -d yz doublet and the E g phonon mode of the P 4/nmm structure at the M point, which is found to have imaginary frequency by ab initio calculations [36].…”