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Abstract

Magnetic and crystallographic properties of the mineral langite Cu-4(OH)(6)SO4 center dot 2H(2)Oare reported. Thermodynamic measurements combined with a microscopic analysis, based on density-functional bandstructure calculations, identify a quasi-two-dimensional (2D), partially frustrated spin-1/2 lattice resulting in the low Neel temperature of T-N similar or equal to 5.7 K. This spin lattice splits into two parts with predominant ferro-and antiferromagnetic (AFM) exchange couplings, respectively. The former, ferromagnetic (FM) part is prone to the long-range magnetic order and saturates around 12 T, where the magnetization reaches 0.5 mu(B)/Cu. The latter, AFM part features a spin-ladder geometry and should evade long-range magnetic order. This representation is corroborated by the peculiar temperature dependence of the specific heat in the magnetically ordered state. We argue that this separation into ferro-and antiferromagnetic sublattices is generic for quantum magnets in Cu2+ oxides that combine different flavors of structural chains built of CuO4 units. To start from reliable structural data, the crystal structure of langite in the 100-280 K temperature range has been determined by single-crystal x-ray diffraction, and the hydrogen positions were refined computationally.