The crystal structure of $\mathrm{Fe}R{\mathrm{Ge}}_2{\mathrm{O}}_7$ ($R=\mathrm{Y}$, Tb) has been solved ab initio from x-ray powder diffraction data. It is monoclinic, space group ${P2\mathrm{}}_1/m$ (No. 11), $Z=4,$ $a$ (Å)=9.6552(4) and 9.6388(8); $b$ (Å)=8.5197(3) and 8.4789(7), $c$ (Å)=6.6746(3) and 6.7383(5), β (°)=100.761(2) and 100.377(4), and $V\left({\AA }^3\right)=539.39\mathrm{}$ 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 ${\mathrm{FeYGe}}_2{\mathrm{O}}_7$ crystal structure contains three kinds of coordination polyhedra: $R^{3+}$ coordinated to seven oxygens at slightly different lengths forming a capped octahedron, ${\mathrm{FeO}}_6$ distorted octahedra, and four types of ${\mathrm{GeO}}_4$ tetrahedra. Its most interesting feature is the existence of flattened chains of $R{\mathrm{O}}_7$ polyhedra linked in the $c$ direction through pairs of ${\mathrm{FeO}}_6$ octahedra with which they share edges, forming layers running parallel to the $\mathrm{bc}$ 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 ${\mathrm{FeTbGe}}_2{\mathrm{O}}_7,$ three-dimensional antiferromagnetic ordering is established, both Fe and Tb sublattices getting simultaneously ordered at $T_N=42\mathrm{}\mathrm{K}.$ The propagation vector of the magnetic structure is $k=[0,0,0].$ At 1.7 K the magnetic moments $3.91\left(7\right)\mathrm{}{\mu }_B$ $\left({\mathrm{Fe}}^{3+}\right)$ and $7.98\left(6\right)\mathrm{}{\mu }_B$ $\left({\mathrm{Tb}}^{3+}\right)$ lie ferromagnetically coupled in the $\mathrm{ac}$ planes, which contain ${\mathrm{TbO}}_7{-\mathrm{F}\mathrm{e}\mathrm{O}}_6{-\mathrm{T}\mathrm{b}\mathrm{O}}_7$- chains in the $c$ direction, forming relatively small angles with the $c$ axis. The coupling between parallel $\mathrm{ac}$ planes is antiferromagnetic along the $b$ direction. This model leads to a best fit of $R_{\mathrm{mag}}=3.02\%.\mathrm{}$ The thermal evolution of the magnetic moments suggests that below ∼20 K the faster increase of the ${\mathrm{Tb}}^{3+}$ moments is due to the stronger Fe-Tb interactions and crystal field effects. The maximum in $\chi \mathrm{}\left(T\right)\mathrm{}$ at 20 K does not correspond then to any phase transition, but is caused by the exchange interaction with the ordered iron subsystem.

• Received 26 June 1997

DOI:https://doi.org/10.1103/PhysRevB.57.5240