High pressure study of sodium trihydride.

Abstract

Using diamond anvil cells combined with computational methods, the reaction between NaH and excess H₂ was examined at pressures reaching 78 GPa and temperatures near 2000 K. Powder X-ray diffraction revealed that sodium trihydride (NaH₃) forms above 27 GPa, adopting an orthorhombic crystal structure. Raman spectroscopy demonstrated the presence of quasi-molecular H₂ within the NaH lattice, with the H₂ stretching mode red-shifted by approximately 120 cm⁻¹ at 50 GPa relative to pure hydrogen. NaH₃ remained stable under room-temperature compression up to at least 78 GPa but decomposed below 18 GPa. Contrary to earlier experimental and theoretical predictions, no sodium polyhydrides beyond NaH₃ were produced when NaH was heated in excess H₂ between 27 and 75 GPa.

Mechanism

Quasi-molecular H₂ is incorporated within the NaH lattice, stabilizing an orthorhombic NaH₃ phase under high pressure. Below 18 GPa the structure decomposes, and further hydrogenation to higher polyhydrides does not occur in the 27–75 GPa range.