Neutron diffraction studies of TbB4 and ErB4

Neutron diffraction data are presented on the rare earth tetraborides TbB4 and ErB4 which crystallize with the tetragonal space group P4/mbm. Positional and thermal parameters were refined by full matrix least-squares analysis from single-crystal measurements to agreement values R of 0.018 and 0.030 respectively. Collinear antiferromagnetic ordering was derived from the low temperature powder data (Tsub(N) = 43 K for TbB4 and Tsub(N) = 13 K for ErB4). The magnetic structures differ in their absolute moment orientations, which are perpendicular to the tetragonal c axis for TbB4 and along the c axis for ErB4. (Auth.)