Synthesis of Weaire-Phelan Barium Polyhydride

By combining pressures up to 50 GPa and temperatures of 1200 K, we synthesize the novel barium hydride, Ba₈H₄₆, stable down to 27 GPa. We use Raman spectroscopy, X-ray diffraction, and first-principles calculations to determine that this compound adopts a highly symmetric $Pm\overline{3}n$ structure with an unusual 5$\frac{3}{4}$:1 hydrogen-to-barium ratio. This singular stoichiometry corresponds to the well-defined type-I clathrate geometry. This clathrate consists of a Weaire-Phelan hydrogen structure with the barium atoms forming a topologically close-packed phase. In particular, the structure is formed by H₂₀ and H₂₄ clathrate cages showing substantially weakened H-H interactions. Density functional theory (DFT) demonstrates that cubic $Pm\overline{3}n$ Ba₈H₄₆ requires dynamical effects to stabilize the H₂₀ and H₂₄ clathrate cages.