Comparative crystal chemistry of KCu₇BiO₄(SO₄)₅ and nabokoite-like KCu₇TeO₄(SO₄)₅Cl sulfates

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.