Crystal Structure and Low-Temperature Magnetic Ordering in Rare Earth Iron Germanates RFeGe₂O₇, R = Y, Pr, Dy, Tm, and Yb

Polycrystalline samples of iron and rare earth germanates, RFeGe 2 O 7 (R = Y, Pr, Dy, Tm, and Yb), have been prepared. Depending on the size of R, they present two different monoclinic crystal structures, described either in space group P 2 1 / c (no. 14) for R = Pr or in P 2 1 / m (no. 11) for the smaller rare earths R = Y, Dy, Tm, and Yb. The results of crystal structure refinements from room-temperature high-resolution neutron powder diffraction (NPD) data are given. Magnetization measurements between 350 and 1.6 K reveal the existence of one maximum, at 3 or 38 K for the praseodymium or yttrium germanate, respectively, and two separate anomalies for the remaining materials, which appear at T 1 and T 2 ( T 2 < T 1 ), where T 1 ≈ 40 K and T 2 = 24, 6, and 5 K, for R = Dy, Tm, and Yb, respectively, indicating transitions to antiferromagnetically (AF) ordered states. Judging from low-temperature NPD data, three-dimensional (3D) AF ordering is established for all compounds, with simultaneous ordering of the R 3+ and (or) Fe 3+ (R = Y) sublattices, at 3 K in PrFeGe 2 O 7, 38 K in YFeGe 2 O 7, and at T N = T 1 for the remaining compounds. The ordered magnetic structure of PrFeGe 2 O 7 can be described in terms of the propagation vector k = [ 1 / 2, 1 / 2, 0] and a model that consists of ferromagnetic coupling of the magnetic moments of Fe 3+ and Pr 3+ along the −FeO 5 −PrO 9 −FeO 5 − chains in the c direction, whereas along the two other axes, the moments show AF coupling. For the R = Y, Dy, Tm, and Yb compounds, the propagation vector of the magnetic structures is k = [0, 0, 0]. In each case, the magnetic structure consists of a ferromagnetic arrangement of all R 3+ and (or) Fe 3+ (R = Y) 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.