High pressure study of sodium trihydride

Area of Science:

• High-pressure physics and chemistry
• Materials science
• Solid-state chemistry

Background:

• Investigating the reactivity of sodium hydride (NaH) with hydrogen (H2) is crucial for understanding hydrides under extreme conditions.
• Previous studies on sodium polyhydrides have yielded conflicting experimental and theoretical results.

Purpose of the Study:

• To explore the reactivity of NaH and H2 at high temperatures and pressures.
• To characterize the resulting sodium hydride phases and their stability.
• To resolve discrepancies in previous research regarding sodium polyhydride formation.

Main Methods:

• High-temperature experiments using diamond anvil cells up to 78 GPa.
• First-principles calculations.
• Powder X-ray diffraction for structural analysis.
• Raman spectroscopy to probe hydrogen bonding.

Main Results:

• Sodium trihydride (NaH3) was synthesized above 27 GPa and 2000 K, adopting an orthorhombic Cmcm structure.
• Raman spectroscopy confirmed the presence of quasi-molecular hydrogen (H2δ-) within the NaH3 lattice, with a downshifted stretching mode.
• NaH3 demonstrated significant P-T stability, remaining stable up to 78 GPa at room temperature and decomposing below 18 GPa.
• No further hydrogenation to form sodium polyhydrides beyond NaH3 was observed between 27 and 75 GPa.

Conclusions:

• The synthesis and characterization of NaH3 provide new insights into the sodium-hydrogen phase diagram.
• The study clarifies the limits of NaH hydrogenation under high pressure, refuting claims of higher polyhydride formation.
• NaH3 is a stable compound under specific high-pressure conditions, with potential implications for hydrogen storage research.