Structural effects of Sc doping on the multiferroic TbMnO3

Abstract

The new TbMn1−xScxO3 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 ABO3 perovskites. All of the samples are isostructural to the parent compound TbMnO3, crystallizing in an orthorhombic cell with space-group Pbnm symmetry. The homovalent substitution of Mn3+ with the larger Sc3+ ion leads to an increase in both the unit-cell volume and the BO6 octahedra tilting. The analysis of neutron and x-ray diffraction patterns suggest a continuous evolution from a Jahn-Teller distorted BO6 octahedron in TbMnO3 into a nearly regular one in TbScO3. However, x-ray absorption measurements at the Mn K edge reveal that the local geometry around the Mn3+ 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 LaMn1−x(Ga/Sc)xO3 and suggests that the strong orthorhombic distortion of the unit cell due to the small Tb3+ cation size favors the stability of the distorted MnO6 octahedron in diluted systems. Long-range magnetic ordering is not found for x≥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.