Synthesis, Structural, and Magnetic Characterization of New Hg2Ba2LnCu2O8−δPhases withLn=Nd–Gd, Dy–Lu

Abstract

New phases of general formula Hg2Ba2LnCu2O8−δwithLn=Nd–Gd, Dy–Lu were synthesized by high-pressure, high-temperature technique. All the compounds crystallize in space group I 4/m m m and the lattice parameters were found to increase with the ionic radii, ranging froma=3.8460(3) Å,c=28.853(3) Å for Lu up toa=3.9021(3) Å,c=28.986(3) Å for Nd. Hg-2212 phases were not obtained with La, due to the high ionic radius of La3+, or with cations having possible valences higher than (+3), such as Tb, Pr, and Ce. Energy dispersive spectroscopy (EDS) analysis performed on samples with Lu, Yb, Er, and Gd showed significant Hg-deficiency as well as rare earth overstoichiometry. This allows us to suggest that lanthanide cations partially substitute for Hg and that these phases are better described as (Hg,Ln)2Ba2LnCu2O8−δ. The substitution rate depends on the ionic radii of the lanthanide cation, varying between 25% for Lu (0.977 Å) or Er (1.004 Å) to 15% for Gd (1.053 Å). In the case of Nd-sample it was found that Nd does not replace Hg but Ba instead. The large ionic radius of Nd (1.109 Å) can probably account for this effect and this phase would be described as Hg2(Ba, Nd)2NdCu2O8−δ. The susceptibility of Sm, Eu, Dy, and Ho samples exhibits a sharp cusp at a temperatureTLbetween 25 and 28 K. These temperatures are attributed to the antiferromagnetic binding of Cu spins, because they are too high to be due to rare earth ordering. Specific heat and neutron diffraction experiments are planned to establish an eventual cooperative ordering of Cu and rare earth moments.