Magnetic properties of Ln2Ti2S2O5 compounds and magnetic structure of Tb2Ti2S2O5

A new series of compounds, Ln(2)Ti(2)S(2)O(5) with Ln from Pr to Er, has been recently reported. These compounds crystallize in the tetragonal I4/mmm space group, with a approximate to 3.8 Angstrom and c approximate to 22.9 Angstrom. Their 3D-structure can be schematically described as a 2D-stacking of [Ln(2)S(2)] layers alternating with [Ti2O5] ones, both layers being connected through Ln-O (x4) and Ti-S (x1) bonding. Magnetic measurements using a SQUID magnetometer indicate paramagnetic behavior over a large temperature range, obeying a Curie-Weiss law (at least between 90 and 300 K), for all the studied lanthanides. An antiferromagnetic transition at T-N < 10 K is observed for the Gd- and Tb-derivatives. The Tb2Ti2S2O5 compound was investigated by neutron diffraction studies in the 1.4-15 K temperature range. The presence of new peaks in the diffraction diagrams at low temperature confirms the phase transition from a paramagnetic state to an antiferromagnetic one (T-N approximate to 5-6 K) that has been observed by magnetic measurements. But the main result of this neutron diffraction study has to be related to the phase transition from an incommensurate antiferromagnetic order at temperatures ranging from 4.1 to 2.9 K, with a propagation vector (k) over right arrow (i) (k(x),0,0); k(x) approximate to 0.46, to a commensurate state with a change in the direction of (k) over right arrow (c) (1/2,0,1/2) when the temperature is lowered (T<2.4 K). The origin of this unusual change in direction of the propagation vector is not yet explained. (C) 2002 Elsevier Science B.V. All rights reserved.