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

In the present work, we report on a chemical phase separation in crystalline superconducting K0.65Fe1.74Se2, 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 < 100 > with an approximated volume fraction of about 30% in the surrounding < 001 > 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 K0.8Fe1.6Se2 matrix, and KxFe2-y Se-2 platelets inducing superconductivity. This time-driven, chemical phase separation is therefore responsible for various coexistent magnetic and electrical properties measured in KxFeySe2 samples. DOI: 10.1103/PhysRevB.86.224502