Challenges in the search for magnetic coupling in 3d/4f materials: Syntheses, structures, and magnetic properties of the lanthanide copper heterobimetallic compounds, RE₂Cu(TeO₃) ₂(SO₄)₂

Twelve new lanthanide copper heterobimetallic compounds, RE ₂Cu(TeO₃)₂(SO₄)₂ (RE = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), with two different structural topologies, have been prepared by hydrothermal treatment. Both structure types crystallize in the triclinic space group, P1̄, but the unit cell parameters and structures are quite different. The earlier RE₂Cu(TeO ₃)₂(SO₄)₂ (RE = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm) share a common structural motif consisting of edge-sharing LnO₈ chains and [Cu(TeO₃)₂(SO ₄)₂]^6- units. The later lanthanide version (Yb and Lu) is composed of edge-sharing LnO₇ dimers bridged by similar [Cu(TeO₃)₂(SO₄)₂]^6- units. The change in the structure type can be attributed to the decreasing ionic radii of the lanthanides. The compounds containing RE³⁺ ions with diamagnetic ground states (Y³⁺ and Eu³⁺) exhibit antiferromagnetic ordering at 12.5 K and 15 K, respectively, owing to the magnetic exchange between Cu²⁺ moments. No magnetic phase transition was observed in all the other phases. The lack of magnetic ordering is attributed to the competing magnetic interactions caused by the presence of paramagnetic RE³⁺ ions. The magnetism data suggests that substantial 3d-4f coupling only occurs in the Yb analogue.