High-pressure study of NaAlH₄ by Raman spectroscopy up to 17 GPa

Abstract

A high-pressure Raman study was carried out on NaAlH₄ up to 17 GPa using the diamond anvil cell method. In the pressure region 2–5 GPa, several of the original modes split. Although this might be a sign of some structural change, the spectral changes do not allow us to claim the existence of a clear phase transition in this pressure range. The spectra revert to their ambient pressure forms on decreasing pressure below<3.0–1.4 GPa. A phase transition to β-NaAlH₄ was found at 14–16 GPa. This phase transition is also reversible with an unusually strong hysteresis: the β-NaAlH₄ can be followed upon decompression down to 3.9 GPa. Analysis of Raman data shows that this phase transition is compatible with a theoretical prediction of a strong volume collapse.

Keywords:

• Diamond anvils
• Raman spectroscopy
• Phase transitions
• Alanates