Low-temperature magnetic ordering in rare-earth copper germanates R2CuGe4O12, R = Ho, Er

R2CuGe4O12 (R = Y, Ho, Er) have been prepared in polycrystalline form, and crystal structures of both lanthanide-containing compounds have been refined from room-temperature high-resolution neutron diffraction data. These materials are isostructural, with the symmetry of the triclinic space group P<(1) over bar> (No. 2), Z = 1, and unit cell parameters alpha (Angstrom) = 7.1678 (1) and 7.1594 (2); b (Angstrom) = 7.9291 (1) and 7.9205 (2); c (Angstrom)= 4.89409 (8) and 4.9075 (1); alpha (degrees) = 86.874 (1) and 86.926 (2); beta = (degrees) 102.686 (1) and 102.651(2); gamma (degrees) = 113.792 (1) and 113.735 (2), for R = Ho and Er, respectively. The structure can be depicted as formed by chains of RO7 polyhedra running in the alpha direction, layers parallel to the ab plane of tetrameric (GeO4)(4) units, and isolated CuO6 distorted octahedra which connect these units in the c direction as well as the RO7 chains along b. Magnetic susceptibility measurements between 350 and 1.7 K reveal for Ho and Er compounds the existence of one anomaly appearing at T-1 = 3.3 K in both cases. From low-temperature neutron diffraction data, three-dimensional (3D) antiferromagnetic (AF) ordering in these compounds is established, with a simultaneous setting up of the order for R3+ and Cu2+ sublattices at T-N = T-1. The propagation vectors of the magnetic structures are k = [0, 0, 1/2] and [0, 1/2, 1/2] for R = Ho and Er, respectively. For the first compound, the best fit of the 1.6 K experimental neutron diffraction data is favorably explained by the ferromagnetic coupling between all Ho3+ and Cu2+ magnetic moments within ab planes, with a 3D AF coupling along the c direction. For Er2CuGe4O12, the ferromagnetic arrangement of Er3+ and Cu2+ magnetic moments is observed only in the a direction, whereas along the two other axes they show AF coupling.