Titanium and native defects in LiBH4and NaAlH4

Abstract

We report combined density functional studies and thermodynamic considerations on Ti-related and native defects in lithium borohydride and sodium alanate. Ti atoms introduced into the bulk of LiBH₄ are thermodynamically unfavorable for all their oxidation states, while high oxidation states of Tiⁿ⁺ cations may become thermodynamically stable in the bulk of NaAlH₄ at certain thermodynamic conditions. Neutral hydrogen vacancies and interstitials or cation vacancies are less stable than their charged counterparts in both compounds. In sodium alanate, the formation of native defects leads to changes of the coordination number of aluminum, while in lithium borohydride BH₄ groups change their mutual orientation but B–H bonds remain intact. The electronic band alignment in LiBH₄ and NaAlH₄ is different.