Vacancy Disorder in K_xFe_2-ySe₂ Iron Based Superconductor

The potassium doped iron selenide K_xFe_2-ySe₂ superconductor has been intensively studied since its discovery because of phase separation driven by the Fe vacancy order. Above the structural phase transition the system has randomly distributed Fe vacancies. Below, the system phase separates into the √5×√5×1 Fe vacancy order phase with space group I4/m, and the Fe vacancy free phase with space group I4/mmm. Post-annealing and quenching can increase the sample’s Meissner shielding fraction by orders of magnitude. We studied the structures of two K_xFe_2-ySe₂ single crystals, one quenched and superconducting and the other as-grown and non-superconducting, using X-ray diffraction carried out at 11-ID-C at the Advanced Photon Source. The structure of the quenched crystal shows a partially ordered I4/m phase while the as-grown sample has fully ordered I4/m phase. A Monte-Carlo simulation was performed to simulate the effects of thermal treatment on Fe vacancy distribution. The simulation revealed that the high temperature I4/mmm phase can be frozen during quenching, leaving the I4/m phase mixed with vacancy disorder I4/mmm phase. We propose that this vacancy disorder phase is the superconducting phase of K_xFe_2-ySe₂.