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 (fcc) potassium sublattice with a slight rhombohedral distortion (space group R3̅m). Compression above 78 GPa converts KH9-I to another polymorph, KH9-II, which adopts a primitive simple hexagonal potassium sublattice (space group P6/mmm) 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.