MAGNETIC-PROPERTIES, MOSSBAUER, AND SPECIFIC-HEAT STUDIES OF RBA2FE3O8(R=Y,EU) COMPOUNDS

Complete replacement of Cu by Fe in RBa2CU307 leads to RBa2Fe3O8 (R=Y and Eu). We investigated YBa2Fe3O8 and EuBa2Fe3O8 by several complemen tary experimental techniques. These compounds crystallize in a tetrago nal structure with the configuration of the atoms in the unit cell ver y similar to that in the superconductor YBa2CU307. Magnetic-susceptibi lity and Mossbauer-spectroscopy measurements of Fe-57 and Eu-151 in RB a2Fe3O8 (R=Y and Eu) at temperatures from 4.2 to 800 K have been perfo rmed. In both compounds the iron moments order antiferromagnetically a t the same Noel temperature T(N) approximately 700 K. Iri YBa2Fe3O8 th e Mossbauer spectra reveal two inequivalent iron sites, probably corre sponding to iron in the Fe(2) site (fivefold oxygen coordination) and in the Fe(1) site (distorted octahedral oxygen coordination). The quad rupole splitting of Fe(2) is zero. Specific-heat (C(p)) measurements a t different magnetic fields have been carried out between 1.5 and 30 K . The C(p)(T,H) curves can be resolved into a contribution of the form C(e)(T)=gammaT with finite gamma* and a lattice contribution that co nsists of Debye terms. The pronounced high gamma=10. 5 mJ/mol K2 for YBa2Fe3O8 is discussed.