Direct evidence of chemical and crystallographic phase separation in K0.65Fe1.74Se2

Abstract

In the present work, we report on a chemical phase separation in crystalline superconducting K${}_{0.65}$Fe${}_{1.74}$Se${}_{2}$, investigated by means of magnetization experiments, scanning electron microscopy, electron backscatter diffraction, and energy-dispersive x-ray spectrometry. It is shown that the crystal consists of platelets oriented in $\ensuremath{\langle}100\ensuremath{\rangle}$ with an approximated volume fraction of about 30$%$ in the surrounding $\ensuremath{\langle}001\ensuremath{\rangle}$ oriented matrix. The platelets (the matrix) are depleted in K (Fe) and enriched in Fe (K). Chemical phase separation is demonstrated by a stable, antiferromagnetic K${}_{0.8}$Fe${}_{1.6}$Se${}_{2}$ matrix, and K${}_{x}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ platelets inducing superconductivity. This time-driven, chemical phase separation is therefore responsible for various coexistent magnetic and electrical properties measured in K${}_{x}$Fe${}_{y}$Se${}_{2}$ samples.