Rich crystal chemistry and magnetism of "114" stoichiometric LnBaFe4O7.0 ferrites

Stoichiometric LnBaFe4O7.0 oxides with Ln = Dy to Lu have been synthesized and protected in order to prevent oxidation at room temperature. The structural study of these compounds, using laboratory and synchrotron X-ray as well as neutron powder diffraction, shows the extraordinary flexibility of the tetrahedral [Fe4] sublattice of these compounds, which exhibit various distortions. At room temperature they all are tetragonal (I4), and at higher temperature (T > 580 K) they exhibit a cubic symmetry (F43m). Moreover, the low-temperature structures of these oxides are dependent on the nature of the Ln(3+) cation. At 110 K, compounds with Ln = Dy and Ho adopt the same monoclinic (P12(1)1) structure as YBaFe4O7.0, whereas YbBaFe4O7.0 possesses a new centered monoclinic cell (I121), and members with Ln = Er and Lu keep the tetragonal (I4) symmetry. Neutron diffraction patterns evidence long-range magnetic ordering only for the most distorted structures (Ln = Dy and Ho), showing that the geometric frustration generated by the tetrahedral [Fe4]∞ sublattice can be lifted only with the most severe distortions. The other oxides (Ln = Er, Yb, and Lu) with weakly distorted [Fe4]∞ sublattices do not exhibit magnetic ordering down to 4 K, demonstrating the importance of magnetic frustration. The behavior of these "114" iron oxides is compared to the cobalt family, showing in both cases a striking underbonding of barium.