Density functional calculations of Ti-enhancedNaAlH4

Abstract

The crystal structure and thermodynamic stability of a number of proposed models for the inclusion of Ti in NaAlH 4 have been calculated by employing density functional theory in the generalized gradient approximation. It was shown that the least unfavorable location of Ti is close to the surface, replacing Al in the host lattice. Intricate complexation is simulated around the included Ti atom, and the preferred coordination number of H around Ti is eight. The Ti content was varied by the supercell approach, and even at 3 mol % Ti the resulting cell parameters were predicted to be significantly changed from the pure alanate. In addition, the most stable configurations were found to be thermodynamically metastable compared to the pure alanate and Ti standard state phases, and an ordered doped phase with significant bulk Ti content is thus ruled out by this study. It is proposed that Ti most probably works as a catalyst, situated at the surface of the alanate.

The Crystal Structure and Surface Energy of NaAlH4: A Comparison of DFT Methodologies

“…[3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Typically, in these studies some flavor of density functional theory (DFT) 20-22 is used to calculate properties of the bulk solid or of the surface of the solid material. A wide range of DFT methodologies and implementations are currently in regular use for these predictive studies, with little attention being paid to any strengths or weaknesses of particular methods.…”

Prediction of a Ca(BH4)(NH2) quaternary hydrogen storage compound from first-principles calculations

“…14,18,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53 In these calculations, the a priori knowledge of a compound's crystal structure is typically obtained from experiments where the system has already been structurally well characterized. By contrast, for the DFT calculations in this work, the crystal structure information is currently not available.…”

Ca ( Al H 4 ) 2 , CaAlH5, and CaH2+6LiBH4: Calculated dehydrogenation enthalpy, including zero point energy, and the structure of the phonon spectra

“…⌫-centered k-space grids and the plane wave basis set cutoff energies were selected to converge the total energy of each system to better than 1 meV per conventional crystallographic unit cell. Initial crystal structures were taken from Weidenthaler et al 7 for Ca͑AlH 4 ͒ 2 and CaAlH 5 , from Wyckoff 23 for CaH 2 , and from Schmitt et al 24 for CaB 6 .…”

“…Conventional crystallographic unit cells were used in the phonon calculations for Ca͑AlH 4 ͒ 2 and CaAlH 5 . 2ϫ 2 ϫ 2 supercells were used for CaH 2 and CaB 6 .…”

“…The ground state Ca͑AlH 4 ͒ 2 crystal structure of space group Pbca has been determined by density functional theory ͑DFT͒ band-structure calculations, 6 with the simulated x-ray diffraction pattern agreeing well with the powder pattern subsequently measured. 7 The crystal structure of CaAlH 5 has also been predicted using DFT calculations, revealing a ␣Ј-SrAlF 5 -type crystal structure of P2 1 / n symmetry.…”