High-Pressure Synthesis and Characterization of the Novel Potassium Superhydride KH₉

Through high-pressure diamond anvil cell experiments, we report the synthesis of two novel potassium superhydrides (KH₉-I and KH₉-II) and investigate their structural and vibrational properties via synchrotron X-ray powder diffraction and Raman spectroscopy, complemented by density functional theory (DFT) calculations. Above 17 GPa at room temperature, KH-II and H₂ react to form KH₉-I; this reaction can be accelerated with temperature. KH₉-I possesses a face-centered-cubic (<i>fcc</i>) potassium sublattice with a slight rhombohedral distortion (space group <i>R</i>3̅<i>m</i>). Compression above 78 GPa converts KH₉-I to another polymorph, KH₉-II, which adopts a primitive simple hexagonal potassium sublattice (space group <i>P</i>6/<i>mmm</i>) and remains stable up to at least 100 GPa. Both KH₉ polymorphs exhibit ionic character, comprising K⁺ and H^- ions, along with quasi-molecular H₂ units, resulting in rich Raman activity.