Magnetic ordering of Fe and Tb in the ab initio determined FeRGe2O7 structure (R = Y, Tb)

The crystal structure of FeRGe2O7 (R=Y, Tb) has been solved ab initio from x-ray powder diffraction data. It is monoclinic, space group P2(1)/m (No. 11), Z=4, a (Angstrom)=9.6552(4) and 9.6388(8); b (Angstrom)=8.5197(3) and 8.4789(7), c (Angstrom)=6.6746(3) and 6.7383(5), beta (degrees)=100.761(2) and 100.377(4), and V(Angstrom(3))=539.39 and 541.69, for R=Y and Tb, respectively. Precise oxygen positions were determined for the Tb compound from a room temperature neutron diffraction profile, refined by the Rietveld method to an R-f=3.99% using 58 parameters. The FeYGe2O7 crystal structure contains three kinds of coordination polyhedra: R3+ coordinated to seven oxygens at slightly different lengths forming a capped octahedron, FeO6 distorted octahedra, and four types of GeO4 tetrahedra. Its most interesting feature is the existence of flattened chains of RO7 polyhedra linked in the c direction through pairs of FeO6 octahedra with which they share edges, forming layers running parallel to the be crystal plane. Magnetization measurements between 350 and 1.7 K show one peak at 38 K for R=Y and two maxima at 42 and 20 K for the Tb compound, which could indicate transitions to antiferromagnetically ordered states. From low-temperature neutron diffraction data on FeTbGe2O7, three-dimensional antiferromagnetic ordering is established, both Fe and Tb sublattices getting simultaneously ordered at T-N=42 K. The propagation vector of the magnetic structure is k=[0,0,0]. At 1.7 K the magnetic moments 3.91(7)mu(B) (Fe3+) and 7.98(6)mu(B) (Tb3+) lie ferromagnetically coupled in the ac planes, which contain TbO7-FeO6-TbO7- chains in the c direction, forming relatively small angles with the c axis. The coupling between parallel ac planes is antiferromagnetic along the b direction. This model leads to a best fit of R-mag=3.02%. The thermal evolution of the magnetic moments suggests that below similar to 20 K the faster increase of the Tb3+ moments is due to the stronger Fe-Tb interactions and crystal field effects. The maximum in (X)(T) at 20 K does not correspond then to any phase transition, but is caused by the exchange interaction with the ordered iron subsystem.