Magnetic versus nonmagnetic polymorphs of RuBr$_3$ under pressure

Abstract

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$_3$ ($R\bar 3$) into paramagnetic $α'$-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 $α$-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.