Expanding the Family of Magnetic Vacancy-Ordered Halide Double Perovskites

Vacancy-ordered halide double perovskites, with the general formula A₂^IB^IVX₆, can accommodate a wide variety of tetravalent B-site cations. However, few examples containing trivalent B-site cations exist, limiting the variety of magnetic cations that can comprise this structure type. Here, we incorporate divalent A-site cations to form the vacancy-ordered double perovskites A^IA^IIB^IIICl₆ (A^I = Na, K, Rb, Cs; A^II = Sr, Ba; B^III = Ti, V, Cr, Ir) and Ba_1.5B^IIICl₆ (B^III = V, Cr). By tuning the radius of the A-site through cation substitution, we form four structure-types with these formulas, including a K₂PtCl₆-type structure, a low-temperature K₂SnCl₆-type structure, a novel derivative with ordered A¹⁺/A²⁺ cations, and a second novel derivative with ordered A-site vacancies. This structural diversity, which includes 22 unique compositions, allows us to study the effect of structure and composition on the magnetic properties of these solids, which show antiferromagnetic coupling of weak-to-moderate strength and signatures of frustrated long-range ordering. Furthermore, our studies of temperature-dependent magnetism and heat capacity reveal that the magnetic coupling strength decreases with octahedral tilting, consistent with expectations; in contrast, the coupling strength counterintuitively increases from B^III = Ti to Cr to V, which we speculate may be a result of competing antiferromagnetic and ferromagnetic interactions. By substituting divalent A-site cations into vacancy-ordered halide double perovskites, we further expand the already rich phase space of these structures to include magnetic trivalent transition metals and deepen our understanding of structure-magnetism relationships in metal halides.