Larisa Victorovna Shvanskaya

The ANa3M7(PO4)6 series unites isostructural alkali A and transition metal M phosphates, arranged from diamond-like chains combined into a triperiodic metallic substructure. Two members of the family, a monoclinic, partially ordered solid solution KNa3Co5.74Cu1.26(PO4)6 and the first triclinic Cu variety KNa3Cu7(PO4)6, were obtained by spontaneous crystallization. Their morphology, chemical composition, and crystal structure were studied by means of electron microscopy, EDS analysis, and single-crystal X-ray diffraction. The symmetry lowering from monoclinic to triclinic in KNa3Cu7(PO4)6 is associated with Jahn–Teller distortion of Cu2+ polyhedra. Thermodynamic measurements have revealed that KNa3Co5.74Cu1.26(PO4)6 exhibits ferrimagnetic-like behavior below TG = 6.4 K. KNa3Cu7(PO4)6 exhibits long-range antiferromagnetic order at TN = 8 K, even though its Curie–Weiss temperature is very high in magnitude, Θ = −93 K. The analysis of the spin exchange interactions in KNa3Cu7(PO4)6 indicates that it is not caused by geometrical spin frustration, but by the fact that its spin–lattice has a layered character with weak interlayer interactions.

Obtained in a flux, K ₂ CaCu ₆ O ₂ (PO ₄ ) ₄ represents a new centrosymmetric modification of fumarolic arsenate shchurovskyite. The new oxocuprate phosphate orders antiferromagnetically in two steps, and theoretically, it can facilitate K-ion migration.

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.

KCu₂BiO₂(SO₄)₂ has been grown by chemical vapor transport and characterized by single-crystal X-ray diffraction. It crystallizes in the orthorhombic Pnma space group with a = 22.8761(8), b = 5.1961(2), c = 7.2083(2) Å, V = 856.83(5) ų, Z = 4. Its peculiar crystal structure is organized by heteropolyhedral layers built by edge- and vertex-sharing CuO₅ and BiO₈ polyhedra. Potassium cations are embedded between the layers, which are additionally reinforced by the SO₄ tetrahedra. The temperature dependence of magnetic susceptibility evidences a broad hump at T_max ∼122 K, accompanied by the Curie tail at lowest temperatures, while the temperature dependence of specific heat features no anomalies ascribable to the phase transition in the whole temperature range. Ab initio calculations demonstrate that the exchange interaction pattern in this material is highly nontrivial. A specific orbital ordering results in the strongest coupling being that of the third-nearest neighbors. This leads to the formation of a spin ladder, a structure that is not directly apparent from crystallographic considerations. There are strong antiferromagnetic interactions on both the rung (J' = 264 K) and leg (J = 140 K) of the ladder, which result in an energy gap of Δ = 161 K in the magnetic excitation spectrum. The Pade approximation of the experimental data provides somewhat lower values of J' = 205 and J = 117 K, yielding an energy gap of Δ = 121 K. The ground state of KCu₂BiO₂(SO₄)₂ is a robust quantum spin liquid.

As a structural and magnetic bridge between V- and P-based averievites, (CsCl)Cu ₅ As ₂ O ₁₀ reveals a 310 K trigonal–monoclinic transition followed by canted antiferromagnetic order at 21 K.

Hydrothermally synthesized Na ₂ Mn ₃ (Si ₃ O ₁₀ ) shows a canted antiferromagnetic ground state with two phase transitions; its hypothetical Li-analogue suggests Li ⁺ ion mobility.