  Much remains unknown of the microscopic origin of superconductivity when it
materializes in atomically disordered systems as in amorphous alloys (1) or in
crystals riddled with defects(2). A manifestation of this conundrum is
envisaged in the highly defective iron chalcogenide superconductors of
KxFe2-ySe2 (3-6). How can superconductivity survive under such crude conditions
that call for strong electron localization (7)? With vacancies present both at
the K and Fe sites, superconductivity is bordering a semi-metallic region below
x ~ 0.7 and an insulating and antiferromagnetic region above x ~ 0.85 (8,9).
Here, we report on the bulk local atomic structure and show that the Fe
sublattice is locally distorted in a way that it accommodates two kinds of Fe
valence environments giving rise to a bimodal bond distribution. While the bond
length distribution is driven by K and Fe contents, the superconducting state
is characterized by the coexistence of both short (metallic) and long
(insulating) Fe bond environments and is not phase separated. In contrast to
other Fe-based materials in which only one kind of Fe to Fe bond is present,
the dual nature of the Fe correlations explains why superconductivity is
intertwined with magnetic order. Such a hybrid state is most likely present in
cuprate superconductors as well (10,11) while our results point to the
importance of the local atomic symmetry by which the exchange interactions
between local moments can materialize (12).
