High pressure study of sodium trihydride

Abstract

The reactivity between NaH and H₂ has been investigated through a series of high-temperature experiments up to pressures of 78 GPa in diamond anvil cells combined with first principles calculations. Powder X-ray diffraction measurements show that heating NaH in an excess of H₂ to temperatures around 2000 K above 27 GPa yields sodium trihydride (NaH₃), which adopts an orthorhombic structure (space group Cmcm). Raman spectroscopy measurements indicate that NaH₃ hosts quasi-molecular hydrogen (${\text{H}}_2^{\delta -}$) within a NaH lattice, with the ${\text{H}}_2^{\delta -}$ stretching mode downshifted compared to pure H₂ (Δν ∼-120 cm^-1 at 50 GPa). NaH₃ is stable under room temperature compression to at least 78 GPa, and exhibits remarkable P-T stability, decomposing at pressures below 18 GPa. Contrary to previous experimental and theoretical studies, heating NaH (or NaH₃) in excess H₂ between 27 and 75 GPa does not promote further hydrogenation to form sodium polyhydrides other than NaH₃.

Keywords: Raman; X-ray diffraction; density functional calculations; high pressure; hydrides; sodium.