  Temperature dependent single-crystal x-ray diffraction (XRD) in transmission
mode probing the bulk of the newly discovered K0.8Fe1.6Se2 superconductor (Tc =
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
vacancy ordering and (ii) a minority non-magnetic phase having an in-plane
compressed lattice volume and a weak superstructure, is reported. The XRD peaks
due to the Fe vacancy ordering in the majority phase disappear by increasing
the temperature at 580 K, well above phase separation temperature confirming
the order-disorder phase transition. The intrinsic phase separation at 520K
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 reconstruction and charge density) 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 multi-gaps superconductor tuned a shape resonance.
