Dimorphism and magnetic properties in iron rare earth germanates

Polycrystalline samples of iron and lanthanide FeRGe2O7 germanates, R = Dy, Ho, Er, Yb have been prepared. They present the new II-FeRGe2O7 structure type, S.G. P2(1)/m (No. 11) Z = 4. Results of the refinements from neutron diffraction data for Dy and Yb compounds as well as X-ray data (Ho, Er) are given. Lattice dimensions are a/Angstrom = 9.6391(4), 9.635(1), 9.646(2) and 9.6554 (5); b/Angstrom = 8.4743(3), 8.475(1), 8.511(1) and 8.5125(4); c/Angstrom = 6.7113(3), 6.6701(9), 6.655(1) and 6.6804(4); and beta/degrees = 100.538(7), 100.612(7), 100.83(1) and 100.733(3), for R = Dy, Ho, Er and Yb, respectively. Magnetic susceptibility measurements between 350 and 1.7 K reveal the existence of two separated anomalies for each of the four compounds, which appear at T-1 and T-2(T-2 < T-1): 41 and 24 K, 38 and 17 K, 40 and 8 K, 38 and 4 K, respectively. T-1 coincides with the setting up of magnetic ordering in Fe3+ and R3+ sublattices, as determined by neutron diffraction in Tb and Dy compounds, and correspond to the Neel temperatures. chi(T) maxima at T-2 do not correspond to any phase transition but they are caused by the exchange interaction of R3+ magnetic moments with the ordered iron subsystem. The crystal structure and magnetic properties of these FeRGe2O7 compounds are compared with those of I-FeRGe2O7 (R = Pr, Nd and Gd) and other II-(R = Y, Tb) compounds. (C) 1998 Elsevier Science S.A.