High-pressure chemistry of hydrogen halides featuring Kagome layers and coexisting charge density waves

The high-pressure chemistry and physical properties of hydrogen halides H-X (X = Cl, Br, I) remain largely unexplored. Beyond the textbook example HCl, theoretical predictions suggest that several stoichiometries, such as H2Cl, H3Cl, H5Cl, and H4Cl7, become thermodynamically stable under high pressure. Notably, H4Cl7 has been proposed as a Kagome material, featuring two-dimensional frameworks of corner-shared-triangles of chlorine. Our preliminary experiments on the Cl-rich region of the H-Cl system show that this chemically counterintuitive geometry of Cl Kagome layer indeed can be synthesised at ~60 GPa with significantly more complex structure than predicted. Here, we propose extending the chemistry of binary hydrogen-halogen compounds by systematic investigation of H-Cl, H-Br, and H-I systems in both halogen-rich and H-rich cases by means of single-crystal X-ray diffraction (SCXRD) at ID11.