Buckle-Layered, Decorated Square-Kagome System KCu₇BiO₄(SO₄)₅

We report the gas-transport synthesis, crystal structure, and magnetic properties of the buckle-layered KCu₇BiO₄(SO₄)₅, which enriches few in number the family of decorated square-kagome systems consisting of the rare mineral nabokoite KCu₇TeO₄(SO₄)₅Cl and synthetic Na₆Cu₇BiO₄(PO₄)₄(Cl,OH)₃. The title compound crystallizes in the tetragonal space group P4/ncc with unit cell parameters a = 9.7731(1) and c = 20.4094(6) Å, Z = 4, and differs from nabokoite-like phases in the arrangement of Cu-centered polyhedra within the square-kagome-like layer, as well as in their connection with decorating ions. Although the square-kagome motif is clearly visible in the crystal structure of KCu₇BiO₄(SO₄)₅, from the magnetic perspective, it is a much more complex network. It evidences no indications on the formation of the short-range order, but KCu₇BiO₄(SO₄)₅ experiences a long-range order at Neel temperature T_N = 12 K. In comparison with negative Weiss temperature Θ = -219 K, it gives the frustration ratio F = |Θ|/T_N ≥ 18. This very large value points to both fierce competition of antiferromagnetic exchange interactions and the low dimensionality of the magnetic subsystem. Density functional theory calculations identify pathways of leading exchange interactions between Cu²⁺ ions in both buckled layers and between them.