Unconventional S-orbital state of Tb and cooperative Ru(4d)-Tb(4f) spin-ordering in the strongly correlated 4d-4f system Ba₃TbRu₂O₉

The 6H-perovskite Ba₃RRu₂O₉ (R = rare-earth), composed of Ru₂O₉ dimers connected through RO₆ octahedra, exhibits an intriguing variety of magnetic ground states, ranging from non-magnetic to ferromagnetic and antiferromagnetic depending on the specific R ion. In this study, we investigated the compound Ba₃TbRu₂O₉ using magnetic susceptibility measurements and time-of-flight neutron diffraction experiments. Our combined bulk and microscopic analyses revealed that the Tb⁴⁺ (4f⁷) electronic configuration results in an s-like state with an orbital moment (L) of zero and spin-only value (S) of 7/2, and Ru⁴⁺ exhibits S = 1 despite the presence of strong spin-lattice coupling in this compound, representing a sharp contrast to other reported members of this family. Cooperative 4d-4f spin ordering was observed below the Néel temperature (T_N ≈ 9.5 K), indicating strong Ru(4d)-Tb(4f) correlations in the system. Tb moments order antiferromagnetically in the bc-plane, whereas Ru moments align antiferromagnetically along the b-axis. Furthermore, a collinear antiferromagnetic arrangement of spins was observed within the Ru₂O₉ dimers throughout the structure, unlike other reported members of this series (e.g., Ho and Nd).