Enhancing the Néel temperature in 3d/5d R2NiIrO6 (R=La, Pr and Nd) double perovskites by reducing the R3+ ionic radii

Double perovskites containing Ir4+ were synthesised by a citrate technique, followed by an annealing treatment in air at 1100 °C. The crystal structure of the three compounds, with formula R2NiIrO6 (R= La, Pr and Nd), were determined using a combined refinement against neutron powder diffraction (NPD) and synchrotron x-ray powder diffraction (SXRPD) data sets. At room temperature, all the samples were indexed in the space group P21/n and the monoclinic symmetry remains in the 300 to 1273 K temperature range. Magnetization measurements suggest competitive antiferromagnetic and ferromagnetic interactions, with an unexpected increment of the ordering temperature (TN) along the series. The magnetic structures of all the samples were defined with the propagation vector k = 0; the Ni2+ and Ir4+ moment arrangement, different for each compound, shows a strong dependence on the nature of the rare-earth ion.