Magnetic versus nonmagnetic polymorphs of RuBr₃ under pressure

Pressure evolution of the crystal structure and magnetism of the honeycomb α-RuBr₃ is studied using high-pressure x-ray diffraction, magnetometry, and density-functional band-structure calculations. Hydrostatic compression transforms antiferromagnetic α-RuBr₃ (R 3) into paramagnetic α'-RuBr₃ (P 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 N\'eel temperature of α-RuBr₃ increases with the slope of 1.8 K/GPa. Pressure tunes α-RuBr₃ 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 α- and α'-RuBr₃ are metastable at ambient pressure, but their transformation into the thermodynamically stable β-polymorph is kinetically hindered at room temperature.