Abstract
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.