Structure and magnetic properties of LnBaFe₄O₇ oxides: Ln size effect

The investigation of the “114” ferrites has allowed seven new compounds of LnBaFe₄O₇ formula to be synthesized: the compounds LnBaFe₄O₇ (Ln = Dy, Ho, Er, Yb, Lu) with a cubic symmetry (a ≈ 8.9 Å) and the oxides TbBaFe₄O₇ and GdBaFe₄O₇ with hexagonal symmetry (a ≈ 6.34 Å and c ≈ 10.38 Å). Moreover, two other oxides, out of the lanthanide series, InBaFe₄O₇ and ScBaFe₄O₇, were also obtained with cubic symmetry. The relative stability of one structure with respect to the other is explained in terms of the relative sizes of the Ln³⁺ (or In³⁺, Sc³⁺) cations with respect to the Fe²⁺/Fe³⁺ cations for tuning the triangular and kagome layers in two different ways. The magnetic properties of these oxides show that all the compounds with cubic symmetry can be described as spin-glass like in agreement with their [Fe₄]_∞ tetrahedral framework similar to pyrochlores, whereas the hexagonal phases, (Tb/Gd)BaFe₄O₇, exhibit ferrimagnetism, like CaBaFe₄O₇ but with a much smaller T_C = 160 K. It is found that these symmetries are responsible for the two different types of magnetic frustration.