An ab initio study of possible pathways in the thermal decomposition of NaAlH4

Density functional theory (DFT) has been used to study the structural stability of possible intermediate alanate structures, Na5Al3H14 and Na2AlH5, in the thermal decomposition of NaAlH4. Na5Al3H14 crystallizes in the space group P4/mnc with lattice constants , and c/a=1.52. It is shown that both Na5Al3H14 and Na2AlH5 have the right thermodynamics and can fit in as an intermediate state during the thermal decomposition process of NaAlH4. The heat of formation of Na5Al3H14 is -60 kJ/mol H2, which is intermediate between that of NaAlH4 (-51 kJ/mol H2) and Na3AlH6 (-69.7 kJ/mol H2). An alternative decomposition pathway based on Na2AlH5 has also been discussed. Frequency analysis showed that the least energetic Na2AlH5 structure has imaginary frequencies, implying that it is unstable. The presence of soft phonon modes also shows that Na5Al3H14 is mechanically metastable. These results are consistent with the notion that they are the intermediate states that lead to the formation of AlH3. This facilitates the mass transport of aluminum atoms in the decomposition pathway of NaAlH4.