Native Defect Concentrations in NaAlH4 and Na3AlH6

Titanium-doped sodium alanate has been shown to have potentially useful properties for storing hydrogen in fuel cell vehicles. To quantitatively explain the kinetic rates and activation energies for hydrogen release and absorption, it is necessary to calculate the rate of metal diffusion that is required for forming the dehydrogenation products, Na3AlH6 and Al. The bulk defects existing in large concentrations will likely play a dominant role in the transport of metal species. In the first of a series of papers, we use first-principles density functional theory (DFT) calculations to determine the formation free energies and concentrations of native defects in NaAlH4 and Na3AlH6 as functions of temperature, including vibrational and H-2 gas-phase entropy contributions. We find that at low temperatures the largest concentrations of native defects in NaAlH4 are positively charged AlH4 vacancies and negatively charged Na vacancies, which can be thought of as the primary defect types for NaAlH4. At high temperatures (near 100 degrees C), neutral AlH3 vacancies, positively charged AlH4 vacancies, and negatively charged interstitial hydrogen ions and hydrogen vacancies have the largest concentrations at an interface between NaAlH4 and Al. At all temperatures, an interface between NaAlH4 and Na3AlH6 remains predominantly populated by positively charged AlH4 vacancies and negatively charged Na vacancies. In Na3AlH6, the highest defect concentrations belong to negatively charged Na vacancies and positively charged H vacancies throughout the entire temperature range considered. Inclusion of the gas-phase free energy of hydrogen is shown to be crucial for obtaining quantitative estimates of defect free energies and concentrations.