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The reactivity between NaH and H 2 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 H 2 to temperatures around 2000 K above 27 GPa yields sodium trihydride (NaH 3 ), which adopts an orthorhombic structure (space group Cmcm ). Raman spectroscopy measurements indicate that NaH 3 hosts quasi-molecular hydrogen ( H 2 δ - ) within a NaH lattice, with the H 2 δ - stretching mode downshifted compared to pure H 2 (Δ ν ∼-120 cm -1 at 50 GPa). NaH 3 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 NaH 3 ) in excess H 2 between 27 and 75 GPa does not promote further hydrogenation to form sodium polyhydrides other than NaH 3 ., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Marqueño, Kuzovnikov, Osmond, Dalladay-Simpson, Hermann, Howie and Peña-Alvarez.)

Using optical spectroscopy, X-ray diffraction, and electrical transport measurements, we have studied the pressure-induced metallization in BaH 2 and Ba 8 H 46 . Our combined measurements suggest a structural phase transition from BaH 2 -II to BaH 2 -III accompanied by band gap closure and transformation to a metallic state at 57 GPa. The metallization is confirmed by resistance measurements as a function of the pressure and temperature. We also confirm that, with further hydrogenation, BaH 2 forms the previously observed Weaire-Phelan Ba 8 H 46 , synthesized at 45 GPa and 1200 K. In this compound, metallization pressure is shifted to 85 GPa. Through a comparison of the properties of these two compounds, a question is raised about the importance of the hydrogen content in the electronic properties of hydride systems.