Sr-Doped Superionic Hydrogen Glass: Synthesis and Properties of SrH₂₂

Recently, several research groups announced reaching the point of metallization of hydrogen above 400 GPa. Despite notable progress, detecting superconductivity in compressed hydrogen remains an unsolved problem. Following the mainstream of extensive investigations of compressed metal polyhydrides, here small doping of molecular hydrogen by strontium is demonstrated to lead to a dramatic reduction in the metallization pressure to ≈200 GPa. Studying the high-pressure chemistry of the Sr-H system, the formation of several new phases is observed: C2/m-Sr₃ H₁₃ , pseudocubic SrH₆ , SrH₉ with cubic $$ -Sr sublattice, and pseudo tetragonal superionic P1-SrH₂₂ , the metal hydride with the highest hydrogen content (96 at%) discovered so far. High diffusion coefficients of hydrogen in the latter phase D_H  = 0.2-2.1 × 10^-9 m² s^-1 indicate an amorphous state of the H-sublattice, whereas the strontium sublattice remains solid. Unlike Ca and Y, strontium forms molecular semiconducting polyhydrides, whereas calcium and yttrium polyhydrides are high-T_C superconductors with an atomic H sublattice. The discovered SrH₂₂ , a kind of hydrogen sponge, opens a new class of materials with ultrahigh content of hydrogen.