Antiferromagnetic Ordering in the Frustrated Rare-Earth Chain Systems M₂Cl₃ (M = Gd, Tb)

The magnetic properties of the two isostructural rare-earth condensed octahedral-cluster chain compounds M2Cl3 (M = Gd, Tb) have been investigated by magnetization, heat capacity, and single-crystal neutron diffraction experiments. Both compounds exhibit antiferromagnetic order below ∼23 and ∼43 K for Gd2Cl3 and Tb2Cl3, respectively. For Gd2Cl3, the large negative Curie–Weiss temperature of ∼−209 K indicates substantial magnetic frustration. The magnetic moments of Gd at the two different crystallographic atom positions are aligned along the chains. Whereas at ∼14.5 K the magnetic moments at the shared edge approach the theoretically expected spin-only value of ∼7 μB, the moments at the geometrically frustrated apical position of the octahedra are significantly reduced. In Tb2Cl3, the magnetic moments lie in the plane perpendicular to the chains and are inclined with respect to each other. The Tb cations in the shared edges have a magnetic moment of 9.3 μB, slightly exceeding the maximum value of 9 μB that is expected for Tb3+ with J = 6 for the Russell–Saunders ground state. Neutron scattering with polarization analysis on a Tb2Cl3 crystal in a magnetic field reveals enhanced polarizability of the Tb moments at the frustrated apical octahedral sites.