# Optimization of the Crystal Growth of the Superconductor CaKFe$_{4}$As$_{4}$ from Solution in the FeAs-CaFe$_{2}$As$_{2}$-KFe$_{2}$As$_{2}$ System

## TL;DR

This paper details an optimized method for growing high-quality single crystals of CaKFe$_{4}$As$_{4}$ from a complex FeAs-rich solution, improving purity and understanding phase relationships in the quaternary system.

## Contribution

It introduces a refined growth protocol for CaKFe$_{4}$As$_{4}$ crystals, emphasizing impurity suppression and phase purity, based on phase equilibrium principles and comprehensive measurements.

## Key findings

- Optimized growth parameters yield phase-pure CaKFe$_{4}$As$_{4}$ crystals.
- Temperature, resistance, and magnetization measurements detect impurity phases.
- Insights into the four-component phase diagram of the FeAs-CaFe$_{2}$As$_{2}$-KFe$_{2}$As$_{2}$ system.

## Abstract

Measurements of the anisotropic properties of single crystals play a crucial role in probing the physics of new materials. Determining a growth protocol that yields suitable high-quality single crystals can be particularly challenging for multi-component compounds. Here we present a case study of how we refined a procedure to grow single crystals of CaKFe$_{4}$As$_{4}$ from a high temperature, quaternary liquid solution rich in iron and arsenic ("FeAs self-flux"). Temperature dependent resistance and magnetization measurements are emphasized, in addition to the x-ray diffraction, to detect inter-grown CaKFe$_{4}$As$_{4}$, CaFe$_{2}$As$_{2}$ and KFe$_{2}$As$_{2}$ within, what appear to be, single crystals. Guided by the rules of phase equilibria and these data, we adjusted growth parameters to suppress formation of the impurity phases. The resulting optimized procedure yielded phase-pure single crystals of CaKFe$_{4}$As$_{4}$. This optimization process offers insight into the growth of quaternary compounds and a glimpse of the four-component phase diagram in the pseudo-ternary FeAs-CaFe$_{2}$As$_{2}$-KFe$_{2}$As$_{2}$ system.