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 refine d from room-temperature high-resolution neutron diffraction data. These mat erials 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 (Angs trom)= 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 directi on, layers parallel to the ab plane of tetrameric (GeO4)(4) units, and isol ated CuO6 distorted octahedra which connect these units in the c direction as well as the RO7 chains along b. Magnetic susceptibility measurements bet ween 350 and 1.7 K reveal for Ho and Er compounds the existence of one anom aly appearing at T-1 = 3.3 K in both cases. From low-temperature neutron di ffraction data, three-dimensional (3D) antiferromagnetic (AF) ordering in t hese 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 m agnetic 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 experiment al neutron diffraction data is favorably explained by the ferromagnetic cou pling between all Ho3+ and Cu2+ magnetic moments within ab planes, with a 3 D AF coupling along the c direction. For Er2CuGe4O12, the ferromagnetic arr angement of Er3+ and Cu2+ magnetic moments is observed only in the a direct ion, whereas along the two other axes they show AF coupling.