High Pressure Potassium Polyhydrides: A Chemical Perspective

Doping hydrogen by an impurity has emerged as a possible route toward the metallization of hydrogen at experimentally achievable pressures. Evolutionary structure searches coupled with density functional theory calculations have been employed to determine the most stable stoichiometries and structures of potassium polyhydrides, KH_n n > 1, under pressure. Stabilization occurs at pressures as low as 3 GPa, but KH₅, the most stable stoichiometry throughout most of the pressure regime considered, does not become metallic until P > 300 GPa. There are, however, suggestions of metallicity in metastable phases at 100 GPa. Detailed structural, electronic, and chemical analyses of the emergent hydrogenic motifs are provided and related to the rest of the alkali series. The softness of the alkali metal cation is shown to be related to the formation of symmetrical H₃^– molecules in compressed alkali metal polyhydrides.