Magnetic versus nonmagnetic polymorphs of RuBr${}_3$ under pressure

Pressure evolution of the crystal structure and magnetism of the honeycomb $\alpha$-RuBr${}_3$ is studied using high-pressure x-ray diffraction, magnetometry, and density-functional band-structure calculations. Hydrostatic compression transforms antiferromagnetic $\alpha$-RuBr${}_3$ ($R\bar{3}$) into paramagnetic ${\alpha }^{\mathrm{\prime }}$-RuBr${}_3$ ($P\bar{1}$) where short Ru-Ru bonds cause magnetism collapse above 1.3 GPa at 0 K and 2.5 GPa at 295 K. Below this critical pressure, the Néel temperature of $\alpha$-RuBr${}_3$ increases with the slope of 1.8 K/GPa. Pressure tunes $\alpha$-RuBr${}_3$ away from the Kitaev limit, whereas increased third-neighbor in-plane coupling and interlayer coupling lead to a further stabilization of the collinear zigzag state. Both $\alpha$- and ${\alpha }^{\mathrm{\prime }}$-RuBr${}_3$ are metastable at ambient pressure, but their transformation into the thermodynamically stable $\beta$-polymorph is kinetically hindered at room temperature.