High pressure study of sodium trihydride.

The reactivity between NaH and H has been investigated through a series of high-temperature experiments up to pressures of 78 GPa in diamond anvil cells combined with calculations. Powder X-ray diffraction measurements show that heating NaH in an excess of H to temperatures around 2000 K above 27 GPa yields sodium trihydride (NaH), which adopts an orthorhombic structure (space group ). Raman spectroscopy measurements indicate that NaH hosts quasi-molecular hydrogen () within a NaH lattice, with the stretching mode downshifted compared to pure H (Δ ∼-120 cm at 50 GPa). NaH is stable under room temperature compression to at least 78 GPa, and exhibits remarkable stability, decomposing at pressures below 18 GPa. Contrary to previous experimental and theoretical studies, heating NaH (or NaH) in excess H between 27 and 75 GPa does not promote further hydrogenation to form sodium polyhydrides other than NaH.