Structure and dynamics of Ti-Al-H compounds in Ti-doped NaAlH4

Structure and stability of pristine and modified NaAlH4 are first investigated using density functional theory (DFT) with plane-wave basis and PW91 functional. Vacancy and Ti-dopant effects in the sodium alanate bulk, (001) surface, and on-top (001) surface are then studied considering Na and interstitial lattice sites. Calculated substitution energies of Ti-doped (001) NaAlH4 surfaces have shown almost equal probability of substitution at both lattice and interstitial sites. Ti-Al-H complexes are formed depending on the accessible AlH4 groups around the Ti dopant. TiAl2H7 and TiAl2H2 complexes are found after geometry optimising doped-NaAlH4 surface models. Their stability and dynamics over time at 423 and 448K are investigated using periodic density functional molecular dynamics (DFT-MD) simulations. Results have shown increased association of Al and H with the complexes as time evolves. DFT-MD simulations show evolution from TiAl2H7 to TiAl5H7 as time and temperature increase in case of Ti dopant at Na surface site (TiSNa), and evolution from TiAl2H2 to TiAl3H6 at 423K and TiAl3H7 at 448K in case of Ti dopant on-top of the interstitial surface site (Ti -> T-I) with time.