Expanding\nthe Family of Magnetic Vacancy-Ordered Halide\nDouble Perovskites

Abstract

Vacancy-ordered halide double perovskites, with the general\nformula\nA₂^IB^IVX₆, can accommodate\na wide variety of tetravalent B-site cations. However, few examples\ncontaining trivalent B-site cations exist, limiting the variety of\nmagnetic cations that can comprise this structure type. Here, we incorporate\ndivalent A-site cations to form the vacancy-ordered double perovskites\nA^IA^IIB^IIICl₆ (A^I = Na, K, Rb, Cs; A^II = Sr, Ba; B^III = Ti,\nV, Cr, Ir) and Ba_1.5B^IIICl₆ (B^III = V, Cr). By tuning the radius of the A-site through cation\nsubstitution, we form four structure-types with these formulas, including\na K₂PtCl₆-type structure, a low-temperature\nK₂SnCl₆-type structure, a novel derivative with\nordered A¹⁺/A²⁺ cations, and a second novel\nderivative with ordered A-site vacancies. This structural diversity,\nwhich includes 22 unique compositions, allows us to study the effect\nof structure and composition on the magnetic properties of these solids,\nwhich show antiferromagnetic coupling of weak-to-moderate strength\nand signatures of frustrated long-range ordering. Furthermore, our\nstudies of temperature-dependent magnetism and heat capacity reveal\nthat the magnetic coupling strength decreases with octahedral tilting,\nconsistent with expectations; in contrast, the coupling strength counterintuitively\nincreases from B^III = Ti to Cr to V, which we speculate\nmay be a result of competing antiferromagnetic and ferromagnetic interactions.\nBy substituting divalent A-site cations into vacancy-ordered halide\ndouble perovskites, we further expand the already rich phase space\nof these structures to include magnetic trivalent transition metals\nand deepen our understanding of structure–magnetism relationships\nin metal halides.