Coexistence of different electronic phases in the K _0.8Fe _1.6Se ₂ superconductor: A bulk-sensitive hard x-ray spectroscopy study

We have studied electronic and magnetic properties of the K _0.8Fe _1.6Se ₂ superconductor by x-ray absorption and emission spectroscopy. Detailed temperature-dependent measurements along with a direct comparison with the binary FeSe system have revealed coexisting electronic phases: a majority phase with a high-spin 3 ⁺Fe state and a minority phase with an intermediate-spin 2 ⁺Fe state. The effect of high-temperature annealing suggests that the compressed phase with a lower-spin 2 ⁺Fe state is directly related to the high-T _c superconductivity in the title system. The results clearly underline the glassy nature of superconductivity in the electronically inhomogeneous K _0.8Fe _1.6Se ₂, similar to the superconductivity in granular phases.