Realization of a New Spin-Orbital Quantum State in the 4d Honeycomb System Ag₃LiRh₂O₆

The crystal electric field effect and spin-orbit coupling in 4d/5d honeycomb systems create an isospin-½ (J_eff=½) ground state and anisotropic Kitaev interactions. Notable examples of such systems are the iridate materials and α-RuCl₃ [1, 2, 3]. Based on theoretical predictions, the ground states of such materials can be tuned to other exotic spin-orbital states such as an Ising spin-½ state. However, such a ground state has been considered inaccessible due to the overwhelming influence of spin-orbit coupling in these materials. Here, by using a topochemical reaction, we modified the inter-layer chemical bonds in a honeycomb rhodate, Li₂RhO₃, and synthesized Ag₃LiRh₂O₆. This process induced a trigonal distortion in Ag₃LiRh₂O₆ that led to a transition from the isospin-½ ground state (Kitaev limit) in the parent compound to an Ising spin-½ ground state (Ising limit) in the product. This change in spin orbital state resulted in a dramatic change in magnetism. Whereas Li₂RhO₃ shows a spin-freezing transition at 6 K, Ag₃LiRh₂O₆ reveals a robust long-range antiferromagnetic transition at 94 K. Magnetization, heat capacity, and μSR measurements confirm the robust long-range antiferromagnetic transition at 94 K, and x-ray absorption spectroscopy measurements support our perturbation theory results and confirm that Ag₃LiRh₂O₆ possesses an Ising spin-½ state. This is the first realization of a change of ground state between the Kitaev and Ising limits in a specific honeycomb structure.

[1] Singh, Yogesh, et al. "Relevance of the Heisenberg-Kitaev model for the honeycomb lattice iridates A₂IrO₃." PRL 108, 127203 (2012)

[2] Plumb, K. W., et al. "α−RuCl₃: A spin-orbit assisted Mott insulator on a honeycomb lattice." PRB 90, 041112(R) (2014)

[3] Bahrami, Faranak, et al. "Thermodynamic evidence of proximity to a Kitaev spin liquid in Ag₃LiIr₂O₆." PRL 123, 237203 (2019)