Crystal chemistry and magnetic properties of mineral-like sulfates with copper square-kagome lattice

The minerals elasmochloite, Na₃Cu₆BiO₄(SO₄)₅, nabokoite, KCu₇TeO₄(SO₄)₅Cl, and atlasovite, KCu₆FeBiO₄Cl(SO₄)₅, are rare fumarolic minerals. In their crystal structures, Cu-centered polyhedra connect via edges and vertices, forming Cu–O/Cl layers with a kagome square topology. Interest in compounds featuring such architectures has grown in recent years due to the potential for realizing a quantum spin liquid state.

Given the scarcity of these minerals, analogs of elasmochloite were synthesized using a chemical vapor transport method, which simulates the fumarol conditions. It was established that, in laboratory experiments, a polymorphic variety crystallizes with the analog of the mineral, Na₃Cu₆BiO₄(SO₄)₅. Interestingly, both polymorphs belong to the same sp. gr., P2₁/n, and have similar cell parameters. Single-crystal XRD analysis revealed that the key structural difference between the polymorphs lies in the degree of distortion and a different relative arrangement of the Cu–O layers alternating with Na–O 2D fragments. In the mineral's structure, the Cu–O layers lie parallel to the monoclinic axis, whereas in the synthetic compound, they are perpendicular to the monoclinic axis. The Na–O sublattice adapts to the copper–oxygen layers, forming unique structural motifs.

In an attempt to substitute sodium ions with larger potassium ions, a new sulfate, KCu₇BiO₄(SO₄)₅, was synthesized. In its crystal structure, the magnetic subsystem of the square kagome type is decorated with additional copper cations compared to the copper subsystem in elasmochloite. The crystal structure of KCu₇BiO₄(SO₄)₅ is related to that of nabokoite, with both crystallizing in the tetragonal space group P4/ncc. The structural difference arises from the substitution of Te⁴⁺ by Bi³⁺, which is compensated by the absence of chlorine atoms in the new phase.

Magnetic susceptibility and heat capacity measurements indicate that both compounds are frustrated antiferromagnets with magnetic ordering transitions at 18 and 12 K for the Na- and K-sulfates, respectively.