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Mixed-valence compound Pb2Cu10O4(SeO3)4Cl7 has a complex structure consisting of one nonmagnetic Cu+ (S=0) ion and four nonequivalent magnetic Cu2+ (S=1/2) ions. It orders antiferromagnetically at TN = 10.2 K. At...

Compounds with the general formula Ln3+(SeO3)(HSeO3)·2H2O, where Ln = Sm3+, Tb3+, Nd3+ and Lu3+, are characterized by orthorhombic symmetry with space group P212121 and unit-cell parameters in the ranges a ∼ 6.473–6.999, b ∼ 6.845–7.101, c ∼ 16.242–16.426 Å. Light-purple irregularly shaped crystals of a new monoclinic polytype of neodymium selenite Nd(SeO3)(HSeO3)·2H2O have been obtained during a mild-condition hydrothermal synthesis. The monoclinic unit-cell parameters are: a = 7.0815 (2), b = 6.6996 (2), c = 16.7734 (5) Å, β = 101.256 (1)°, V = 780.48 (6) Å3; space group P21/c. The crystal structures of Nd(SeO3)(HSeO3)·2H2O polymorphs show order–disorder (OD) character and can be described using the same OD groupoid family, more precisely a family of OD structures built up from two kinds of non-polar layers (category IV). The first monoclinic maximum degree order (MDO) structure (MDO1-polytype) with space group P21/c can be obtained when the inversion centre is active in the L 2n -type layers, while the second MDO structure (MDO2-polytype) is orthorhombic with space group P212121 and can be obtained when the [21--] operation is active in the L 2n -type layers. The structural complexity parameters and DFT calculations of both polytypes show that the polytype structures are extremely close energy-wise and almost equally viable from the point of total energy of the structure.

A new family of compounds ACu7TeO4(SO4)5Cl (A=Na, K, Rb, Cs) isostructural to mineral Nabokoite (K species) was synthesized by solid state and gas transport reactions in sealed ampoules and characterized in measurements of magnetization and specific heat in a wide temperature range. These complex compounds are of the utmost interest as a testing playground to study the properties of quasi‐two‐dimensional magnets with a square kagome lattice geometry. A quantum ground state of such a corner‐sharing network is a spin liquid. Unlike idealized grid analyzed in numerous models, the square kagome lattice in nabokoites is wavy and distorted being composed of versatile triangles. Moreover, it contains “excessive” decorating magnetic ions, which makes magnetism of these objects even more complicated. The interaction of these decorating ions through virtual excitations of the square kagome lattice is accompanied by the formation of a long‐range magnetic order coexisting with the spin liquid.

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