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 NH3BH3, we synthesized previously unknown superhydride BaH12 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-BaH2 and possibly BaH10 and BaH6 as impurities in the samples. Ab initio calculations show that newly discovered semimetallic BaH12 contains H2 and H3 molecular units and detached H12 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.

Wuhao Chen, Dmitrii V. Semenok, Alexander G. Kvashnin, Xiaoli Huang, Ivan A. Kruglov, Michele Galasso, Hao Song, Defang Duan, Alexander F. Goncharov, Vitali B. Prakapenka, Artem R. Oganov, “Synthesis of molecular metallic barium superhydride: pseudocubic BaH12”. Nat Commun 12, 273 (2021). abstract

a Experimental X-ray diffraction pattern from DAC #B1 at 160 GPa and the Le Bail refinement of the pseudocubic Cmc21-BaH12 phase. The experimental data, fitted line, and residues are shown in red, black, and green, respectively. Unidentified reflections are indicated by asterisks. b X-ray diffraction patterns at pressures of 119 to 160 GPa. The inset shows the projection of the Cmc21 structure to the (ac) plane. The hydrogen network is shown by light blue lines. c Convex hulls of the Ba–H system at 100, 150, and 200 GPa calculated with zero-point energy (ZPE) contribution. d Calculated equations of state for different possible crystal modifications of BaH12 (fcc, I4/mmm, and Cmc21) and Ba+nH2. The experimental data are shown by hollow squares.