Abstract
The role of Ti in the reversible dehydrogenation of
Na
Al
H
4
has been the subject of debate for many years. The authors resolve this controversy by calculating the phase stability diagram of Na-Al-H-Ti and defect formation enthalpy of Ti doped
Na
Al
H
4
from first principles. The calculations show that Ti substitutes Al in
Na
Al
H
4
and forms a defect pair with interstitial H under H-rich and Al-poor conditions. However, the defect pair is not stable under H-poor and Al-rich conditions resulting in the formation of a Ti-Al intermetallic compound. The doped Ti reduces the Al-H bonding via additional Al
2
p
and Ti
3
d
hybridizations. The results give a picture why and how doped Ti acts to enhance hydrogen storage related solid phase transitions.