Physical properties optimization of polycrystalline LiFeAs

Abstract

We present a study of parameter optimization for synthesizing truly stoichiometric polycrystalline LiFeAs. Stoichiometric LiFeAs has been prepared in a very broad range of synthesis temperature (200–900°C) under otherwise exactly the same conditions, and has been characterized by structural, magnetic, transport, nuclear quadrupole resonance (NQR), and specific heat measurements. Our study showed that the LiFeAs phase is formed at 200°C with a large amount of impurity phases. The amount of these impurity phases reduces with increasing synthesis temperature and the clean LiFeAs phase is obtained at a synthesis temperature of 600°C. Magnetic susceptibility and resistivity measurements confirmed that the superconducting properties such as the critical temperature Tc, and the upper critical field Hc2 do not depend on the synthesis temperature (≤700°C), remaining at almost the same value of ∼19K and ∼40T, respectively. However, the width ΔTc of the transition and the NQR line width decrease with increasing the synthesis temperature and reached to minimum value for the synthesis temperature of 600°C. Our careful analysis suggests that the best sample obtained at 600°C is optimal concerning the low resistivity, high residual resistivity ratio (RRR), low ΔTc, high Tc and Hc2, and a small NQR line width with values which are comparable to that reported for LiFeAs single crystals. Specific heat measurements confirmed the bulk superconducting nature of the samples. The Hc2 value estimated from the specific heat is consistent with that of the resistivity measurements. Concisely, 600°C synthesis temperature yields optimal high quality polycrystalline LiFeAs bulk samples. Further improvement of the quality of the sample prepared at 600°C could be obtained by a controlled slow cooling process. Microstructural analysis reveals that the abundance of micro-cracks becomes strongly reduced by the slow cooling process, resulting in an increase in clean and well-connected grain boundaries.