Honeycomb Layered Ruthenium(III) Trihalide Polymorphs Stabilized by High-Pressure Synthesis

Magnetic latices with strong bond-dependent competing spin anisotropy, yielding frustrated spin configurations on a single site, described by the Kitaev model, can give birth to interesting magnetic behavior and uncommon state of quantum materials. The layered honeycomb lattice material <i>α</i>-RuCl₃ has emerged as a prime candidate for displaying the Kitaev quantum spin liquid state and as such has attracted much research interest. Here we describe the iodine- and bromine-based version of RuCl₃, whose layered honeycomb structured polymorphs are synthesized at moderately high pressures and are stable under ambient conditions. Preliminary characterization reveals that layered α-RuI₃ is a metallic conductor, with the absence of long-range magnetic order down to 0.35 K and an unusually large <i>T</i>-linear contribution to the heat capacity. However, α-RuBr₃ is an insulator with a broad antiferromagnetic ordering transition at around 35 K. The layered honeycomb α-Ru(Br_1-xI_x)₃ solid solution with varying <i>x</i> was then prepared. The variation with changing I to Br ratio was thus revealed, including an insulator-to-metal transition between RuBr_0.75I_2.25 and RuBr_0.5I_2.5, and a dramatic change in their magnetic properties. By introducing the disorder of halogen atoms, the frustration and randomness increased, triggering short-range magnetic correlations and what may be spin-glass-like behavior in the materials.<br/>We propose that this system, with a high-pressure-stabilized layered honeycomb lattice and strong spin-orbit coupling, provides a new route to fabricate and study quantum materials. The solid solution may further provide a new venue for studying quantum spin phenomena with disorder and strong spin-orbit coupling for both magnetic frustrated insulators and metallic systems, and thus may open a door to further understanding and modifying the interaction of spin and orbital degrees of freedom of Kitaev systems.