  AeAFe4As4 (Ae = Ca, A = K; 1144) having a transition temperature of 35 K is a
stoichiometric family of iron-based superconductors (FBS). Here, we present a
detailed study of a high-pressure synthesis of CaKFe4As4 bulks to investigate
the impact of these conditions on the superconducting properties of the 1144
family. Additionally, these samples are also prepared by conventional synthesis
method at ambient pressure (CSP) and studied the influence of impurities on the
superconducting properties of CaKFe4As4. Structural, microstructural, transport
and magnetization measurements have been performed to reach the final
conclusions. Interestingly, the high-pressure synthesis of the parent CaKFe4As4
compound enhances the transition temperature (Tc) by 2 K and the critical
current density (Jc) by one order of magnitude in the whole magnetic field
range of 9 T than that of the 1144 bulks prepared by CSP. It suggests the
improvement of the pinning centers and grain connections by the high-pressure
synthesis approach. Interestingly, our results depict the superconducting onset
transition temperature (Tconset) of this stoichiometric 1144 family is robust
with the presence of the common 122 (CaFe2As2 or KFe2As2) impurity phases,
which is a different behavior compared to other FBS families. However, these
impurity phases reduce the grain connections and lead to the phase separation
during the formation of the superconducting (1144) phase.
