Pressure-Temperature Phase Relations in Complex Hydrides

Bertil Sundqvist

doi:10.4028/www.scientific.net/ssp.150.175

Abstract

Interest in hydrogen as a future energy carrier in mobile applications has led to a strong increase in research on the structural properties of complex alkali metal and alkaline earth hydrides, with the aim to find structural phases with higher hydrogen densities. This contribution reviews recent work on the structural properties and phase diagrams of these complex hydrides under elevated pressures, an area where rapid progress has been made over the last few years. The materials discussed in greatest detail are LiAlH4, NaAlH4, Li3AlH6, Na3AlH6, LiBH4, NaBH4, and KBH4. All of these have been studied under high pressure by various methods such as X-ray or neutron scattering, Raman spectroscopy, differential thermal analysis or thermal conductivity measurements in order to find information on their structural phase diagrams. Based mainly on experimental studies, preliminary or partial phase diagrams are also given for six of these materials. In addition to this information, data are provided also on experimental results for a number of other complex hydrides, and theoretical predictions of new phases and structures under high pressures are reviewed for several materials not yet studied experimentally under high pressure.

Highlights

Over the last few years it has become increasingly clear that oil products must be replaced as energy carriers in mobile applications in a not very distant future
The results clearly showed that at room temperature, the sample remained in its original α-LiAlH4 monoclinic P21/c structure at all pressures
The structural diversity among the complex hydrides is quite surprising even at atmospheric pressure, considering that most structures are based on very simple building blocks, i.e. metal ions and small hydride complexes such as BH4

Summary

Introduction

Over the last few years it has become increasingly clear that oil products must be replaced as energy carriers in mobile applications in a not very distant future. After the prediction by Vajeeston et al [3,15] that at 293 K, LiAlH4 should transform from its monoclinic P21/c α phase into first the tetragonal I41/a α-NaAlH4 structure at 2.6 GPa, corresponding to an increase in the density by about 17%, into an orthorhombic Pnma structure at 33.8 GPa, Raman studies under pressure clearly showed strong anomalies in the spectra obtained in the range 2.5 – 3 GPa [16,17,18,19].

Results

Conclusion