Electronic structure and Rietveld refinement parameters of Ti-doped sodium alanates

The structure of Ti-doped sodium aluminum deuteride has been determined using Rietveld refinement of X-ray and neutron powder diffraction data and compared to values for undoped NaAlD4 and NaAlH4. The refined lattice parameters for the tetragonal NaAlD4 structure are found to be a 5.010 Angstrom and c = 11.323 Angstrom, while those of the monoclinic Na3AlD6 structure are a = 5.402 Angstrom, b = 5.507 Angstrom, c = 7.725 Angstrom, and beta = 89.491degrees. Results of ab initio calculations of the lattice parameters are within a few percent of the Rietveld-refined values. The generalized gradient approximation (GGA) is found to produce significantly better agreement with the experimental data than the local-density approximation (LDA), which is attributed to a very inhomogeneous distribution of electronic density in the alanates. The Rietveld refinement, LDA, and GGA results for the Al-H bond length in NaAlD4 are 1.626, 1.634, and 1.631 Angstrom, respectively. Refined values of the Al-H bond lengths in Na3AlD6, are in agreement within 2% of earlier work on undoped Na3AlD6 [J. Alloy Comp. 299 (1-2) (2000) 10 1]. The calculated GGA formation energies for the decomposition reactions of NaAlD4 and Na3AlD6 are 33.5 and 49.9 kJ/mol of H-2, respectively. The corresponding LDA values are significantly higher: 44.6 and 70.3 kJ/mol of H-2. Comparison with the experimental enthalpy data suggests that the GGA results are in a very good agreement with the measured DeltaH [Proceedings of the 1999 DOE Hydrogen Program Annual Peer Review, NREL/CP-570-26938; J. Alloy Comp. 302 (1-2) (2000) 36]. Detailed theoretical analyses of the electronic band structure, valence charge density distribution, and lattice bonding properties are given. (C) 2004 Elsevier B.V. All rights reserved.