Structural and magnetic properties of new rare-earth - antimony pyrochlore-type oxides Ln2BSbO7 (B = Sc, Ga, In)
Résumé
A number of new quaternary rare-earth pyrochlores Ln2BSbO7 are reported with Ln = Pr, Nd, Eu, Gd or Dy, B = Sc, Ga or In. These compositions extend the range of available rare-earth pyrochlores with diamagnetic B-site cations, especially for early lanthanides (Pr, Nd). Rietveld refinements and Mo¨ssbauer spectroscopy show that most compounds actually contain a fraction of Sb3+, e.g. have a composition $Ln_2(B^{3+}_{1-x}Sb_xSb^{3+}_x Sb^{3+})O_7$. In addition, the rather large size of $In^{3+}$ induces partial site exchange of Ln and In in $Ln_2InSbO_7$ phases. The cell parameters and interatomic distances are in good agreement with those in corresponding $Ln_2M^{4+}_2O_7$ pyrochlores. Magnetic measurements show dominant antiferromagnetic interactions and the absence of magnetic ordering down to 2 K, as expected in the geometrically frustrated lanthanide sublattice in the pyrochlore structure. Strong crystal field effects are present in all compounds excepted gadolinium ones. The europium magnetism (J = 0) is well described by a Van Vleck behaviour leading to a finite constant magnetic susceptibility at low temperature.