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We study the crystalline and electronic properties of the$\textrm{Fe}_{1-x}\textrm{Co}_x\textrm{Se}$ system ($x=0$, 0.25, 0.5, 0.75, and1.0) using X-ray diffraction, X-ray spectroscopy and density functional theory.We show that the introduction of Co $3d$ states in FeSe relaxes the bondstrengths and induces a structural transition from tetragonal to hexagonalwhose crossover takes place at $x\approx0.38$. This structural transition inturn modifies the magnetic order which can be related to the spin state. Usingresonant inelastic X-ray spectroscopy we estimate the spin state of the system;FeSe is found to be in a high spin state (S=2), but Fe is reduced to a low spinstate upon Co substitution of $x \le 0.25$, well below the structuraltransition. Finally, we show evidence that FeSe is a moderately correlatedsystem but the introduction of Co into the host lattice weakens the correlationstrength for $x\ge0.25$. These novel findings are important to unravel themechanisms responsible for the superconducting state in iron-chalcogenidesuperconductors.

The electronic structure of Co-substituted $\textrm{Fe}\textrm{Se}$ superconductor probed by soft X-ray spectroscopy and density functional theory