Synthesis and HRTEM Investigation of EuRbFe₄As₄ Superconductor

In the stoichiometric iron-based superconductor EuRbFe₄As₄, superconductivity coexists with a long-range magnetic ordering in Eu layers. Using high-resolution transmission electron microscopy (HRTEM), we observed an atomic structure of as-grown EuRbFe₄As₄ crystals. HRTEM shows that crystals have two-dimensional intrinsic nanoinclusions established to be the RbFe₂As₂ (122) phase with a volume fraction of ~5.6%. In contrast with the CaKFe₄As₄ compound, similar inclusions are not superconducting down to 2 K, and no second magnetization peak was observed in the magnetization measurements at low temperature with B ‖ ab. We show that the non-superconducting 122 phase nanoinclusions could act as 2D pinning centers.