Magnetic properties, Mössbauer, and specific-heat studies of RBa2Fe3O8 (R=Y,Eu) compounds

Complete replacement of Cu by Fe in RBa2Cu3O7 leads to RBa2Fe3O8 (R=Y and Eu). We investigated YBa2Fe3O8 and EuBa2Fe3O8 by several complementary experimental techniques. These compounds crystallize in a tetragonal structure with the configuration of the atoms in the unit cell very similar to that in the superconductor YBa2Cu3O7. Magnetic-susceptibility and Mössbauer- spectroscopy measurements of Fe57 and Eu151 in RBa2Fe3O8 (R=Y and Eu) at temperatures from 4.2 to 800 K have been performed. In both compounds the iron moments order antiferromagnetically at the same Néel temperature TN700 K. In YBa2Fe3O8 the Mössbauer spectra reveal two inequivalent iron sites, probably corresponding to iron in the Fe(2) site (fivefold oxygen coordination) and in the Fe(1) site (distorted octahedral oxygen coordination). The quadrupole splitting of Fe(2) is zero. Specific-heat (Cp) measurements at different magnetic fields have been carried out between 1.5 and 30 K. The Cp(T,H) curves can be resolved into a contribution of the form Ce(T)=*T with finite * and a lattice contribution that consists of Debye terms. The pronounced high *=10.5 mJ/mol K2 for YBa2Fe3O8 is discussed.