In situ observation of phase separation and hierarchical microstructure of KxFe2-ySe2 single crystals

TL;DR

This study uses advanced microscopy to reveal that phase separation in KxFe2-ySe2 single crystals occurs via nucleation and growth, forming a hierarchical superconducting microstructure that influences synthesis and phase transformation understanding.

Contribution

It provides the first detailed 3D visualization of the hierarchical microstructure and clarifies the phase separation mechanism in KxFe2-ySe2 superconductors.

Findings

Phase separation is nucleation and growth, not spinodal decomposition.

Superconducting phase is a remnant of high-temperature phase.

Hierarchical microstructure influences synthesis and phase behavior.

Abstract

The nature of the phase separation in KxFe2-ySe2 single crystals is not well understood. Here, temperature dependent SEM images provide compelling evidence that phase separation corresponds to a nucleation and growth process rather than a spinodal decomposition. The superconducting phase is the remnant of high temperature phase after iron vacancy order-disorder transition. Three-dimension (3D) spatial distribution of superconducting phase in the iron vacancy ordered matrix was revealed by using focused ion beam scanning electron microscopy (FIB-SEM). Our results clearly show that superconducting phase forms a hierarchical structure. Understanding the formation of this hierarchical structure not only can guide the synthesis of bulk superconductors in the future, but also greatly enrich our knowledge about the interplay between phase separation and solid-state phase transformations.