Crystal structure flexibility and magnetic behavior of a new shchurovskyite-related phosphate K₂CaCu₆O₂(PO₄)₄

The unique conditions of fumaroles give rise to an exceptional diversity of mineral species due to a combination of key factors: high temperatures, atmospheric pressure, gas transport of chemical components, and the unusual geochemical setting of active volcano exhalations. Using high-temperature synthesis, we obtained single crystals of a new copper phosphate K₂CaCu₆O₂(PO₄)₄, which represents a new morphotropic modification of the fumarolic arsenate shchurovskyite, K₂CaCu₆O₂(AsO₄)₄. The K₂CaCu₆O₂(PO₄)₄ crystal structure and chemical composition were studied by single-crystal X-ray diffraction and EDS spectroscopy.

In this work, six structurally related oxysalts were grouped into the shchurovskyite family with the general formula A₂B[M₆O₂](TO₄)₄, (A is K⁺ or Rb⁺; B is Ca²⁺, Cu^2+, or K⁺; T is P⁵⁺ or As⁵⁺; and M is Cu²⁺, Cu^2+, or Al³⁺). In their structures, two-periodic blocks composed of TO₄ tetrahedra and Cu²⁺-centered polyhedra are interconnected into a heteropolyhedral framework with channels occupied by extra-framework cations. The geometric topology of the Cu-ion substructure is similar across the family. However, the anionic environment of the copper polyhedra systematically varies with symmetry changes, exhibiting a large diversity of Cu²⁺ coordination geometry. This structural flexibility arises from the Jahn–Teller effect of Cu²⁺ cations, resulting in structural rearrangement driven by temperature and compositional variations.

Calculations based on the topology of the framework channels indicate the potential for K⁺-ion migration through the K₂CaCu₆O₂(PO₄)₄ structure. Magnetic susceptibility measurements show that K₂CaCu₆O₂(PO₄)₄ undergoes a transition into a long-range ordered state with a spontaneous magnetic moment at T_C = 10 K. The fit of the χ(T) curve yields a Weiss temperature of -97.8 K, indicating dominant antiferromagnetic interactions at high temperatures. Compared to its Rb-analog (T_C = 25 K), K₂CaCu₆O₂(PO₄)₄ exhibits a lower ordering temperature and signs of magnetic frustration.