Synthesis of molecular metallic barium superhydride: pseudocubic BaH(12)

Following the discovery of high-temperature superconductivity in the La–H system, we studied the formation of new chemical compounds in the barium-hydrogen system at pressures from 75 to 173 GPa. Using in situ generation of hydrogen from NH(3)BH(3), we synthesized previously unknown superhydride BaH(12) with a pseudocubic (fcc) Ba sublattice in four independent experiments. Density functional theory calculations indicate close agreement between the theoretical and experimental equations of state. In addition, we identified previously known P6/mmm-BaH(2) and possibly BaH(10) and BaH(6) as impurities in the samples. Ab initio calculations show that newly discovered semimetallic BaH(12) contains H(2) and H(3)(–) molecular units and detached H(12) chains which are formed as a result of a Peierls-type distortion of the cubic cage structure. Barium dodecahydride is a unique molecular hydride with metallic conductivity that demonstrates the superconducting transition around 20 K at 140 GPa.