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

Polycrystalline samples of La- and Lu-agardite with the composition RECu${}_6$(OH)${}_6$(AsO${}_4$)${}_3$ · n H${}_2$O (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 RECu${}_6$(OH)${}_6$(AsO${}_4$)${}_3$ · n H${}_2$O (RE = La, Lu; n ≈ 3) are isostructural with the mineral mixite and crystallize with a hexagonal structure which contains ribbons of edge-sharing [CuO${}_5$] square-pyramids extending along the hexagonal axis. They are interconnected via (AsO${}_4$)${}^{3-}$ groups to form hexagonal tubes of about 10 Å inner diameter. Such zeolite-like tubes host water molecules, which can be reversibly removed at moderate temperatures (T ≈ 100°C). Like in mixite and YCu${}_6$(OH)${}_6$(AsO${}_4$)${}_3$ · 3 H${}_2$O, the Cu${}^{2+}$ cations in RECu${}_6$(OH)${}_6$(AsO${}_4$)${}_3$ · n H${}_2$O (RE = La, Lu; n ≈ 3) exhibit lowdimensional antiferromagnetic properties, which are subject to changes in the Cu-O-Cu bond lengths and angles due to the lanthanide contraction.