Pressure-induced formation of KBr monohydrate and bromide substitution effects in alkali halide monohydrates

High-pressure behavior of salt hydrates provides insights into ion-water interactions relevant to planetary interiors. Here, potassium bromide monohydrate (KBr·H 2 O) is identified using in situ Raman spectroscopy and synchrotron X-ray diffraction. This monoclinic phase (space group P 2 1 / n ), stable at 2.6 GPa and 298 K, shows lattice parameters closely analogous to those of isostructural KCl·H 2 O, indicating that Br substitution preserves the fundamental packing while slightly distorting the lattice and lengthening O–H···Br hydrogen bonds owing to its larger radius and higher polarizability. Compression of a 3.5 mol% KI aqueous solution produces Raman features identical to those of KCl·H 2 O and KBr·H 2 O, suggesting the possible formation of a structurally analogous KI monohydrate (KI·H 2 O). The observed increase in Raman intensity from KCl·H 2 O to KBr·H 2 O and KI·H 2 O is attributed to increasing halide polarizability. These results advance the understanding of halide hydrate formation under pressure and provide insights into halide-water interactions in brine-rich planetary environments.