Intrinsic phase separation in superconducting K0.8Fe1.6Se2 (T-c=31.8 K) single crystals

Temperature dependent single-crystal x-ray diffraction (XRD) in transmission mode probing the bulk of the newly discovered K0.8Fe1.6Se2 superconductor (T-c = 31.8 K) using synchrotron radiation is reported. A clear evidence of intrinsic phase separation at 520 K between two competing phases, (i) a first majority magnetic phase with a ThCr2Si2-type tetragonal lattice modulated by the iron root 5 x root 5 vacancy ordering and (ii) a minority non-magnetic phase having an in-plane compressed lattice volume and a root 2 x root 2 weak superstructure, is reported. The XRD peaks due to the Fe vacancy root 5 x root 5 ordering in the majority phase disappear on increasing the temperature to 580 K, well above phase separation temperature, confirming the order-disorder phase transition. The intrinsic phase separation at 520 K between a competing first magnetic phase and a second non-magnetic phase in the normal phase both having lattice superstructures (that imply different Fermi surface topology reconstructions and charge densities) is assigned to a lattice-electronic instability of the K0.8Fe1.6Se2 system typical of a system tuned at a Lifshitz critical point of an electronic topological transition that gives a multigap superconductor tuned to a shape resonance.