Magnetic ordering in the rare earth iron germanates HoFeGe₂O₇ and ErFeGe₂O₇

RFeGe2O7 (R = Ho, Er) have been prepared in polycrystalline form, and their crystal structures have been refined from room-temperature high-resolution neutron diffraction data by the Rietveld method. Both materials are isostructural, space group P2(1)/m (no. 11), Z = 4. The most interesting feature of the structure is the existence of flattened chains of RO7 polyhedra along the b axis, which linked in the c direction through pairs of FeO6 octahedra form layers parallel to the be crystal plane. Magnetic susceptibility measurements between 350 and 1.7 K reveal the existence of two anomalies for both compounds, at T-1 and T-2, (T-2 < T-1), 39 and 12 K, 40 and 8 K, for R = Ho and Er, respectively. From low-temperature neutron diffraction data, three-dimensional antiferromagnetic ordering in both compounds is established, with a simultaneous setting up of the order for R3+ and Fe3+ sublattices at T-N = T-1. The propagation vector of the magnetic structure is k = [0, 0, 0]. In each case the magnetic structure consists of a ferromagnetic arrangement of all R3+ and Fe3+ magnetic moments within one ac plane, whereas the corresponding moments in up and down adjacent planes are oppositely aligned, leading to 3D AF coupling along the b direction. Kramers doublets splittings in the region of the I-4(15/2) --> I-4(13/2) optical transitions of the Er compound have been observed in high-resolution optical absorption spectra. From these spectral measurements the temperature found for the magnetic ordering coincides with that T-N determined by neutron diffraction data. Moreover optical data also show that T-2 in chi(m)(T) does not correspond to any phase transition but it is most probably caused by the population changes within the ground Er3+ Kramers doublet split by the exchange interaction with the ordered Fe3+ subsystem.