We successfully synthesize ${\mathrm{RuBr}}_3$, which is a candidate material of a Kitaev spin liquid, via a high-pressure synthesis. The material ${\mathrm{RuBr}}_3$ possesses a ${\mathrm{BiI}}_3$-type structure (space group $R\overline{3}$) where ${\mathrm{Ru}}^{3+}$ forms an ideal honeycomb lattice even at room temperature and it does not show a structural transition at low temperatures. ${\mathrm{RuBr}}_3$ has a negative Weiss temperature and it undergoes a zigzag antiferromagnetic transition at $T_{\mathrm{N}}=34$ K, as does $\alpha \text{−}{\mathrm{RuCl}}_3$ $(T_{\mathrm{N}}=7–14\mathrm{K}\mathrm{)}$ which is a promising candidate for a Kitaev spin liquid. Although both compounds of ${\mathrm{RuBr}}_3$ and $\alpha \text{−}{\mathrm{RuCl}}_3$ have a zigzag magnetic order at low temperature, the magnetic order in ${\mathrm{RuBr}}_3$ is more robust than in $\alpha \text{−}{\mathrm{RuCl}}_3$. Our results indicate that the Kitaev and non-Kitaev interactions can be modified in ruthenium trihalides by changing the ligand sites, and will provide another platform for exploring Kitaev spin liquids.

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• Antiferromagnetism
• Electronic structure
• Magnetic susceptibility
• Magnetism
• Spin-orbit coupling
• Kitaev model
• Neutron diffraction
• Nuclear magnetic resonance
• Nuclear quadrupole resonance
• Pressure techniques
• Resistivity measurements
• Specific heat measurements