Mineralogy inspires the search for functional magnetic materials. Fumarolic copper oxysalts, with the Cu²⁺ spin-1/2 state, are prime candidates for quantum magnetism studies. Recent focus has been on minerals featuring a square kagome lattice of copper ions, such as nabokoite, KCu₇TeO₄(SO₄)₅Cl. Here we report a new compound structurally related to nabokoite.
Green plate crystals of KCu₇BiO₄(SO₄)₅ were synthesized via chemical vapor transport. A stoichiometric mixture of K₂SO₄, CuSO₄, CuO, and Bi₂O₃ was sealed in a fused silica ampoule with I₂, evacuated, and heated in a 580–640 °C gradient for 30 days. Resulting crystals were characterized by scanning electron microscopy and single-crystal X-ray diffraction. The crystal structure of KCu7BiO4(SO4)5 was found to be tetragonal with the space group P4/ncc and a = 9.7731(1) Å, and c = 20.4094(6) Å, and Z = 4. KCu₇BiO₄(SO₄)₅ crystallizes in the same tetragonal space group as KCu₇TeO₄(SO₄)₅Cl nabokoite. However, these compounds are not isostructural. The substitution of Te⁴⁺ by Bi³⁺ is accompanied by the absence of chloride ions, as is evident from a comparison of their formulas. The key structural difference lies in the arrangement of Cu-centered polyhedra within the kagome-like layers. In KCu₇BiO₄(SO₄)₅, the Cu–O layer is formed by Cu1O₆ octahedra and Cu2O₅ square pyramids sharing trans-edges, creating four-membered windows. In contrast, analogous layers in nabokoite phases are built from CuO₆ octahedra and CuO₅Cl distorted octahedra, where the latter share common chlorine vertices. Furthermore, as seen in projection along the layers, the decorating Cu atoms in nabokoite-like phase are in mixed coordination and form CuO₄Cl pyramids that share a chlorine vertex with the polyhedra of the square kagome lattice. In our compound, the decorating Cu atoms are in CuO4 square coordination and connect to the SKL exclusively via sulfate tetrahedra. KCu₇BiO₄(SO₄)₅ is promising for studyingmagnetic behavior.
