Magnetic vs. nonmagnetic polymorphs of RuBr_3 under pressure

Pressure evolution of the crystal structure and magnetism of the honeycomb α-RuBr_3 is studied using high-pressure x-ray diffraction, magnetometry, and density-functional band-structure calculations. Hydrostatic compression transforms antiferromagnetic α-RuBr_3 (R3̅) into paramagnetic α'-RuBr_3 (P1̅) 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 α-RuBr_3 increases with the slope of 1.8 K/GPa. Pressure tunes α-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 α- and α'-RuBr_3 are metastable at ambient pressure, but their transformation into the thermodynamically stable β-polymorph is kinetically hindered at room temperature.