Structural Phase Separation in K0.8Fe1.6+xSe2 Superconductors

Structural investigations on the K0.8Fe1.6+xSe2 superconducting materials have revealed microstripes arising from phase separation. Two coexisting structural phases can be characterized by modulations of q(1) = 1/5[3a* + b*], the antiferromagnetic phase K0.8Fe1.6Se2, and q(2) = 1/2[a* + b*], the superconducting phase KyFe2Se2, respectively. These stripe patterns also show visible correlation with the anisotropic assembly of superconducting particles along the [110] and [1 (1) over bar0] directions. In addition to the notable stripe structures, a nanoscale phase separation also appears in the present superconducting system as clearly observed by high resolution transmission electron microscopy. Certain nanoscale structural features in this heterogeneous system can be qualitatively understood by the percolation scenario.