The prediction of novel lithium hydrides with nontraditional stoichiometries at high pressure has been seminal for highlighting a promising line of research on hydrogen-dense materials. Here, we report the evidences of the disproportionation of LiH above 130 GPa to form lithium hydrides containing H2 units. Measurements have been performed using the nonperturbing technique of synchrotron infrared absorption. The observed vibron frequencies match the predictions for LiH2 and LiH6. These polyhydrides remain insulating up to 215 GPa. A disproportionation mechanism based on the diffusion of lithium into the diamond anvil and a stratification of the sample into LiH6/LiH2/LiH layers is proposed. Polyhydrides containing an H2 sublattice do exist and could be ubiquitously stable at high pressure.
Keywords: high pressure; hydride chemistry; hydrogen stoichiometry.