Structural phase separation in K _0.8Fe _1.6+xSe ₂ superconductors

Structural investigations on the K _0.8Fe _1.6+xSe ₂ 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 K _0.8Fe _1.6+xSe ₂, and q ₂ = 1/2[a ^* + b ^*], the superconducting phase K _yFe ₂Se ₂, respectively. These stripe patterns also show visible correlation with the anisotropic assembly of superconducting particles along the [110] and [11̄0] 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.