First-Principles Investigations Of The Pressure-Induced Structrual Transitions In Ternary Alkali Metal Hydrides

Ternary alkali metal hydrides have received considerable attention due to their high content of hydrogen storage. Sodium borohydride (NaBH4) can be used to produce hydrogen by using the method of pyrolysis or hydrolysis. Sodium alanate (NaAlH4) can effectively reduce the temperature of decomposition hydrogen by adding catalysts. So these two kinds of compounds are considered the most promising hydrogen storage material. Recent researches showed that NaBH4 and NaAlH4 are expected to undergo several structural phase transformations displaying novel characteristics. It has important significance for the development and applications of hydrogen storage material.. There are many theoretical and experimental investigations related to phase transition of NaBH4 and NaAlH4 However, the physical mechanism of the phase transition is rarely reported. In this paper, we detailed research on the behavior of NaBH4 and NaAlH4 under high pressure by using ab initio pseudopotential density functional theory. Investigations of structure, electronic properties, and lattice dynamics, are carried out to search the nature of the pressure induced phase transition of NaBH4 and NaAlH4. In order to better understanding the bonding interactions and the charge transfer between atoms, the Mulliken charges and the bond overlap populations are studied. We explore the hydrogen storage performance of NaBH4 and NaAlH4 under high pressure.Calculation results are as follows:(1) We present a study on NaBH4 under high pressure, The calculations show that a structural phase transition takes place at 10.91 GPa. It is found that the phase transition is a first order transition with volumetric collapse. Band-structure calculation reveals that NaBH4 shows an insulator behavior with broadband gap. Pressure induced softening of phonon frequencies is not found during the phase transition. Based on an analysis of the bond length and bond overlap population, B and H form covalent bonds within the BH4 subunits of NaBH4. There is an ionic interaction between Na+and [BH4]- tetrahedron. The activation energy of the tetragonal phase is lower than the orthorhombic phase, so the tetragonal structure is more easy decomposition release hydrogen.(2) We present a study on NaAlH4 under high pressure. The calculations show that a structural phase transition takes place at 15.9 GPa. This transition is identified as first-order in nature with volume collapse. There is no pressure-induced softening behavior from our calculated phonon dispersion curves near the phase transition pressure. The Al and H atom form covalent bonding in the AlH4 subunits of NaAlH4, and there is an ionic interaction between Na+ and [AlH4]- tetrahedron. The monoclinic structure is advantageous to tetragonal structure for hydrogen desorption.