Abstract
The results of x-ray diffraction and Fe-57 and Eu-151 Mossbauer spectroscopy measurements, supplemented with ab initio hyperfine-interaction parameter calculations, on the new 35.5 K superconductor Rb1-delta EuFe4As4 are presented. The superconductor crystallizes in the tetragonal space group P4/mmm with the lattice parameters a = 3.8849(1) angstrom and c = 13.3370(3) angstrom. It is shown that there is no magnetic order of the Fe magnetic moments down to 2.1 K and that the ferromagnetic order is associated solely with the Eu magnetic moments. The Curie temperature T-C = 16.54(8) K is determined from the temperature dependence of both the hyperfine magnetic field at Eu-151 nuclei and the transferred hyperfine magnetic field at Fe-57 nuclei that is induced by the ferromagnetically ordered Eu sublattice. The Eu magnetic moments are demonstrated to be perpendicular to the crystallographic c axis. The temperature dependence of the principal component of the electric field gradient tensor, at both Fe and Eu sites, is well described by a T-3/2 power-law relation. Good agreement between the calculated and measured hyperfine-interaction parameters is observed. The Debye temperature of Rb1-delta EuFe4As4 is found to be 391(8) K.