Ab-initio calculations of titanium solubility in NaAlH4 and Na3AlH6

The possibility of Ti substitution for metal atoms in sodium alanates was investigated by means of the total energy and electronic structure calculations based on the first-principle density functional theory. The enthalpy changes of possible reactions between Ti and NaAlH4 were calculated. The Ti substitution for Al or Na in NaAlH4 was found to be unfavorable, while the formation of TiAl3 was favorable thermodynamically. It is theoretically expected that the Ti substitution for Na and Al in NaAlH4 caused the lattice parameter to decrease, which contradicts the experimental observation that the lattice parameter did not significantly change by adding Ti. These results suggest that the role of titanium added in sodium alanates would not be a “doping” element but a catalyst.