Influence of the B-site ordering on the magnetic properties of the new La3Co2MO9 double perovskites with M = Nb or Ta

Double perovskites La3Co2NbO9 and La3Co2TaO9 have been prepared by both solid state and sol–gel synthesis. The crystal structures have been studied from X-ray and neutron powder diffraction data. Rietveld refinements show that the crystal structure is monoclinic (P21/n), with different degrees of ordering of B′ and B″ cations, with octahedra tilted according to the Glazer notation a−b−c+. Occupancy refinements show that the solid state materials are more B-site ordered than the sol–gel ones. Magnetization measurements show that these perovskites show two magnetic contributions, one with spontaneous magnetization and other with linear behaviour with the magnetic field associated to antiferromagnetic correlations. In the samples synthesized by solid state the spontaneous magnetization is more important than those synthesized by the sol–gel and present TC of 62 K for Nb and 72 K for Ta. On the other hand, materials prepared by sol–gel have TC 20 K for Nb and 40 K for Ta, respectively and major presence of the antiferromagnetic contribution. The competition between these magnetic behaviours is interpreted, by a microscopic point of view, as to be due to the different degrees of Co2+ ions disorder on the B site of the double perovskite structure. This disorder affects the ratio between the antiferromagnetic Co2+–O–Co2+ and the ferromagnetic Co2+–O–M5+–O–Co2+ couplings proposed for the system.