B-Site-Ordered and Disordered Structures in A-Site-Ordered Quadruple Perovskites RMnNiMnO with R = Nd, Sm, Gd, and Dy.

ABO perovskite materials with small cations at the A site, especially with ordered cation arrangements, have attracted a lot of interest because they show unusual physical properties and deviations from general perovskite tendencies. In this work, A-site-ordered quadruple perovskites, RMnNiMnO with R = Nd, Sm, Gd, and Dy, were synthesized by a high-pressure, high-temperature method at about 6 GPa. Annealing at about 1500 K produced samples with additional (partial) B-site ordering of Ni and Mn cations, crystallizing in space group -3. Annealing at about 1700 K produced samples with disordering of Ni and Mn cations, crystallizing in space group -3. However, magnetic properties were nearly identical for the -3 and -3 modifications in comparison with ferromagnetic double perovskites RNiMnO, where the degree of Ni and Mn ordering has significant effects on magnetic properties. In RMnNiMnO, one magnetic transition was found at 26 K (for R = Nd), 23 K (for R = Sm), and 22 K (for R = Gd), and two transitions were found at 10 K and 36 K for R = Dy. Curie-Weiss temperatures were close to zero in all compounds, suggesting that antiferromagnetic and ferromagnetic interactions are of the same magnitude.