Structure and magnetism across the Sr2−xBaxNiOsO6 phase diagram: Understanding magnetic superexchange between first and third row transition metal ions

The crystal chemistry and magnetism of compositions in the Sr2−xBaxNiOsO6 phase diagram have been investigated to better understand the superexchange interactions between 3d and 5d transition metal ions. Compositions with x ​< ​1.35 crystallize with the double perovskite structure, while those with x ​> ​1.83 crystallize in a trigonal structure that is a cation ordered variant of the 6H hexagonal perovskite structure. The Sr-rich double perovskites undergo a cubic to tetragonal distortion upon cooling that is driven by out-of-phase octahedral tilting. The tetragonal distortion occurs upon cooling below 650 ​K in Sr2NiOsO6. The temperature of this transition decreases as the barium content increases and is completely suppressed for x ​≥ ​1.2. All compositions in the double perovskite region become spin glasses below Tg ​≈ ​40 ​K, due to the competition between ferromagnetic Ni–Os and antiferromagnetic Ni–Ni and Os–Os superexchange interactions. The double perovskite compositions that are rich in barium, like Sr0.8Ba1.2NiOsO6 show some signs of magnetic ordering in small clusters, but the presence of Ba/Sr disorder inhibits long-range magnetic order. Trigonal Ba2NiOsO6 orders ferrimagnetically below TC ​= ​108 ​K. The magnetic structure of this compound contains ferromagnetic ∼180° Ni–O–Os interactions and antiferromagnetic ∼90° Ni–O–Os interactions, both in agreement with the Goodenough-Kanamori rules.