Crystal structures and twinning of RuBr${}_3$

The concerted experimental and computational study reports on the polymorphic behavior of RuBr${}_3$. Highly-resolved X-ray powder diffraction measurements of the modification lt-RuBr${}_3$ give direct evidence for peak splitting substantiating an orthorhombic unit cell with additional weak intensities confirming a primitive lattice. The crystal structure of lt-RuBr${}_3$ is determined using the intensities of a "Drilling" individuum. For lt-RuBr${}_3$, findings of density functional theory calculations suggest that the deviation from hexagonal symmetry is mainly driven by electronic correlations. Reinvestigation of the recently reported high-pressure modification hp-RuBr${}_3$ essentially confirms the rhombohedral BiI${}_3$-type crystal structure (space group R$\overline{3}$). Simulations of the atomic arrangement of hp-RuBr${}_3$ indicate that spin-orbit coupling and corrections for the van-der-Waals dispersive forces are mandatory to reach a reasonable agreement with the experimentally determined crystal structure. (© 2023 The Authors. Zeitschrift für anorganische und allgemeine Chemie published by Wiley-VCH GmbH)