Hydrides of the Alkali Metals and Alkaline Earth Metals Under Pressure

First-principles calculations coupled with evolutionary algorithms have been employed to predict the structures of hydrides of the alkali metals and alkaline earth metals under pressure. A plethora of novel phases with MH_n, n> 1 and M=Li, Na, K, Rb, Cs, and MH_n, n > 2 and M=Mg, Ca, Sr, Ba have been predicted to be stable at pressures that can be achieved in a diamond anvil cell. A number of structural motifs, including H^δ’₂, H^’, H^’₃, one-dimensional hydrogenic chains or three-dimensional sodalite-like lattices of hydrogen atoms, are found in the stable phases. The electronic structure and mechanism of metalization under pressure turn out to depend on the hydrogenic motifs that are present. Phases with 1D or 3D hydrogenic lattices are predicted to be the best candidates for high-temperature superconductivity, with those containing H^δ’₂ molecules following suit. Structures with H^’ or H^’₃, are not predicted to be good superconductors. Chemical trends regarding the pressure required for stabilization of the polyhydrides are discussed. Experimental studies that have successfully synthesized the polyhydrides of lithium and sodium are described. Lithium subhydrides, Li_mH with, are m > 1 also predicted to be stable between 50–100 GPa.