Structural effects of Sc doping on the multiferroicTbMnO3

Abstract

The new ${\text{TbMn}}_{1\ensuremath{-}\text{x}}{\text{Sc}}_{\text{x}}{\text{O}}_{3}$ series has been synthesized and the structural properties have been characterized by x-ray diffraction, neutron diffraction, and x-ray absorption spectroscopy. The whole series belongs to the family of $AB{\text{O}}_{3}$ perovskites. All of the samples are isostructural to the parent compound ${\text{TbMnO}}_{3}$, crystallizing in an orthorhombic cell with space-group $Pbnm$ symmetry. The homovalent substitution of ${\text{Mn}}^{3+}$ with the larger ${\text{Sc}}^{3+}$ ion leads to an increase in both the unit-cell volume and the $B{\text{O}}_{6}$ octahedra tilting. The analysis of neutron and x-ray diffraction patterns suggest a continuous evolution from a Jahn-Teller distorted $B{\text{O}}_{6}$ octahedron in ${\text{TbMnO}}_{3}$ into a nearly regular one in ${\text{TbScO}}_{3}$. However, x-ray absorption measurements at the $\text{Mn}\text{ }K$ edge reveal that the local geometry around the ${\text{Mn}}^{3+}$ cation remains distorted in the series even for high values of $x$. This result is in opposition to previous findings in similar compounds such as ${\text{LaMn}}_{1\ensuremath{-}\text{x}}{(\text{Ga}/\text{Sc})}_{\text{x}}{\text{O}}_{3}$ and suggests that the strong orthorhombic distortion of the unit cell due to the small ${\text{Tb}}^{3+}$ cation size favors the stability of the distorted ${\text{MnO}}_{6}$ octahedron in diluted systems. Long-range magnetic ordering is not found for $x\ensuremath{\ge}0.3$ samples and there is not sign of ferromagnetism for $x=0.5$ in opposite to the behavior observed in the La-based compounds. Since Sc substitution induces a minor perturbation on the local structure of the Mn sublattice but magnetic ordering of both Mn and Tb sublattices disappear for a small degree of substitution, we conclude that the involved magnetic interactions should have a strong directional anisotropy and the Tb magnetic ordering strongly depends on the existence of a long-range coherent Mn ordering.