Coupled Spin Ordering in the Ln₂LiRuO₆ Double Perovskites

Synchrotron X-ray, neutron powder diffraction, and magnetic susceptibility measurements have been used to characterize the Ln 2 LiRuO 6 (Ln = Pr, Nd, Sm, Eu, Gd, and Tb) double perovskites. All compositions have the monoclinic P2 1 /n superstructure with a fully ordered arrangement of Li + and Ru 5+ cations. As the Ln-size decreases, the lattice volume decreases due to the increased tilting of the octahedra, but the Li−O and Ru−O distances remain constant. Antiferromagnetic transitions with Néel temperatures in the 20−35 K range were observed for all compositions, except for Gd 2 LiRuO 6, which does not order down to 5 K. Nd 2 LiRuO 6 shows a field-induced ferrimagnetic transition at ∼4 Tesla. The Ln = Pr, Nd, and Tb materials have type I magnetic structures with coupled spin ordering on the Ln and Ru sublattices. Critical fits reveal that the Ln 3+ sublattice magnetization shows mean field behaviour (β ≈ 0.5), whereas the Ru 5+ spin order is consistent with the three-dimensional Heisenberg model (β = 0.31−0.35).