Towards quantum spin liquid state Chemical dilution of the magnetic lattice in α-RuCl₃

α-RuCl₃ is a perspective candidate for the elusive quantum-spin-liquid (QSL) state. It is purported to host Kitaev-type interactions, but, nevertheless, it orders long-range antiferromagnetically at low temperatures. Our goal is to suppress the magnetic order and increase the degree of frustration in order to stabilize the QSL state. To attempt that, this experimental work deliberately introduces disorder into α-RuCl₃ by chemical doping in order to magnetically dilute its 2D sublattice of Ru³⁺ (S = ) cations. Non-magnetic centers are created through either reductive cationic substitution or intercalation.
In the first approach, magnetically active Ru³⁺ (S = ) atoms are partially replaced by non-magnetic Rh³⁺ (S = 0) impurities in order to create spin vacancies and dissociate long-range magnetic interactions between the Ru³⁺ atoms. Mixed Ru_1-xRh_xCl₃ crystals (x = 0.02 - 0.6) of various sizes are grown by chemical vapor transport method. We thoroughly evaluate the homogeneity of Rh substitution at the Ru sites using calibrated energy-dispersive X-ray spectroscopy, complemented by powder- and single-crystal X-ray diffraction techniques to accurately analyze its statistical distribution. Our X-ray diffraction studies confirm that all members of the substitution series are isostructural to the parent α-RuCl₃ compound. A magnetic phase diagram of Ru_1-xRh_xCl₃ is constructed based on the bulk DC magnetization and specific heat measurements as a function of Rh content. We observe the targeted gradual decrease and eventually full suppression of the long-range antiferromagnetic order near the critical concentration x_c = 0.2, near which the quantum spin liquid state is anticipated.

In the second approach, bulk α-RuCl₃ crystals are intercalated with K⁺ atoms using electrochemistry. Partial reduction of the magnetically active Ru³⁺ atoms to Ru²⁺ is expected, thus, diluting the magnetic system analogously to the previous approach and suppressing long-range magnetism. Electrochemical intercalation yields a statistically large number of crystals which contain structurally complex phases and phase-segregated regions. Namely, we identify a predominant disordered rhombohedral phase with an increased interlayer spacing (8.2-8.8 Å) and an ordered trigonal KRu₂Cl₆ phase with a tighter stacking (7 Å) existing only in nano- and µm-sized domains. The crystal structure models of both new phases are determined by X-ray diffraction and 3D electron diffraction methods, whereas their average chemical compositions are evaluated by energy-dispersive X-ray spectroscopy. Complementary Raman, IR, and XPS spectroscopy data as well as thermogravimetric analysis combined with the mass spectrometry deliver further information about the elemental composition, oxidation states, phase purity and thermal stability of the intercalated crystals. The QSL state is not achieved in the intercalated K_xRuCl₃ crystals; nevertheless, their magnetic properties are strongly altered by intercalation. In particular, we observe decreasing Néel temperatures at an intermediate K content and a complete suppression of the magnetic order at a higher K content. We give an outlook on how the electrochemical setup can be improved in order to obtain more compositionally and structurally uniform crystals for further research.

Additionally, we perform explorative intercalation of α-RuCl₃ polycrystalline powders by various cations (Li⁺, Na⁺, K⁺, Cs⁺, NH₄⁺) in   organic and aqueous media. Operando X-ray diffraction studies during the intercalation process are done to monitor the phase transformations. We identify new crystalline phases that can be used as a starting point for a new promising research line.