Metal-Dependent Antiferromagnetic Interactions in Ba₂MCl₆ (M²⁺ = Co, Fe, Mn)

Exploring the wide array of metal-halide structures offers the opportunity for examining structure-property relationships in magnetic solids. Here, we report the magnetism of Ba₂CoCl₆, which has a crystal structure based on edge-sharing octahedral transition-metal-halide dimers. Furthermore, the previously unreported analogues Ba₂FeCl₆, Ba₂MnCl₆, and Ba₂MgCl₆ are synthesized and their structures are determined by single-crystal X-ray diffraction. Using Ba₂MgCl₆ as a diamagnetic control, we measure their temperature-dependent magnetizations and heat capacities. Ba₂CoCl₆ and Ba₂FeCl₆ exhibit short-range antiferromagnetic ordering near 1 K, while Ba₂MnCl₆ displays a transition to long-range magnetic ordering at 0.63 K that is not observed down to 0.5 K in Ba₂CoCl₆ and Ba₂FeCl₆. We conclude that the short-range intradimer and long-range interdimer magnetic ordering temperatures depend on both the ionic radius and electronic configuration of the transition metal.