La- and Lu-agardite – preparation, crystal structure, vibrational and magnetic properties

Polycrystalline samples of La- and Lu-agardite with the composition RECu₆(OH)₆(AsO₄)₃ · n H₂O (RE = La, Lu; n≈3) have been prepared and the structure of the products was determined by X-ray powder diffraction studies. The characterization has been complemented by Raman and UV/Vis spectroscopic, magnetic and TGA investigations. DFT calculations support the conclusions drawn from the experiments. The arsenates RECu₆(OH)₆(AsO₄)₃ · n H₂O (RE = La, Lu; n≈3) are isostructural with the mineral mixite and crystallize with a hexagonal structure which contains ribbons of edge-sharing [CuO₅] square-pyramids extending along the hexagonal axis. They are interconnected via (AsO₄)³⁻ groups to form hexagonal tubes of about 10 A inner diameter. Such zeolite-like tubes host water molecules, which can be reversibly removed at moderate temperatures (T≈100°C). Like in mixite and YCu₆(OH)₆(AsO₄)₃ · 3 H₂O, the Cu²⁺ cations in RECu₆(OH)₆(AsO₄)₃ · n H₂O (RE = La, Lu; n≈3) exhibit low-dimensional antiferromagnetic properties, which are subject to changes in the Cu–O–Cu bond lengths and angles due to the lanthanide contraction.