Abstract: Rare earth (RE) superhydrides exhibit high temperature superconductivity but are difficult to characterize and use in applications due to their high formation and stability pressures, which are typically in excess of 100 GPa. Cryomilling of metal precursors improves hydrogen reactivity and hydrogen uptake for forming such metal hydrides at lower pressures. As an example, an elemental lanthanum precursor was milled at liquid nitrogen temperatures for different time intervals. After exposure to gaseous hydrogen at 380° C. and 100 bar, a systematic enhancement of hydrogen absorption with increasing ball milling time was found for forming the LaHx, x=2-3 phase. Exposing the La precursor to pressures up to 60 GPa with an ammonia borane (BNH6) hydrogen source resulted in a hypervalent LaH4 phase. This LaH4 phase is associated with the suppression of a rhombohedral distortion of the Fm3m cubic structure after cryomilling the precursor.