Role of Antisite Disorder, Rare-Earth Size, and Superexchange Angle on Band Gap, Curie Temperature, and Magnetization of R₂NiMnO₆ Double Perovskites

Homogeneous solid solutions of sol–gel-prepared R2NiMnO6 (R = La, Pr, Nd, Sm, Gd, Tb, Dy, Y, and Ho) double perovskites crystallize in a B-site-ordered monoclinic structure (P21/n space group). Monoclinic distortion enhances with decreasing R3+ ionic radii (rR3+). The magnetic ordering temperature (TC) decreases from 270 K for La2NiMnO6 to 80 K for Ho2NiMnO6 as rR3+ decreases from 1.16 Å (La3+) to 1.02 Å (Ho3+). An additional magnetic anomaly is observed in Nd2NiMnO6, Sm2NiMnO6, Tb2NiMnO6, and Dy2NiMnO6 at lower temperatures, which originates from the 3d–4f coupling between Mn–Ni and Nd3+/Sm3+/Tb3+/Dy3+ magnetic moments. Further, high saturation magnetization is achieved for all samples, indicating that they are atomically ordered and have less antisite disorders. Upon a decrease in the size of R3+, the local structure shows an expansion of NiO6 octahedra and almost unchanged of MnO6 octahedra. X-ray-absorption near-edge spectroscopy reveals a majority of Ni2+ and Mn4+ ions in all samples. Softening of phonon modes results in the elongation of the Ni/Mn–O bond length. Finally, a correlation among lattice parameters, structural distortion, octahedral tilting, superexchange angle, electronic band gap, Curie temperature, and the rare-earth ionic radius is established.