Effects of A-site disorder in the properties of A2CoMnO6 (A = La, Tb).

We have studied the structural and physical properties of the La2-xTbxCoMnO6 series. The crystal and magnetic structures of these compounds were determined by x-ray and neutron diffraction techniques. All samples belong to the family of double perovskites with space group P21/n, but the Co/Mn ordering is not perfect, and antisite defects are formed. The concentration of these defects increases for intermediate compositions, indicating that La/Tb disorder influences the Co/Mn arrangement. A ferromagnetic ground state is established due to the strong Mn4+-O-Co2+ superexchange interaction. For the intermediate compositions and at low temperature, the Co/Mn ordering is accompanied by the ordering of Tb3+ moments in the ab-plane, indicating a mutual polarization between both sublattices. Macroscopic magnetic properties reveal that Curie temperature decreases as Tb content increases in correlation with the increase of the structural distortion. All samples show semiconducting behaviour, and overall the electrical resistivity increases with decreasing La-content. The dielectric constant (?') has a value of around 12 at low temperatures for all samples, revealing the lack of permanent dipoles. The temperature dependence of ?' on warming exhibits a strong increase that depends heavily on the frequency of the electric field. This effect is ascribed to non-intrinsic effects such as contacts or internal barrier-layers.