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

Abstract

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=5.010Å and c=11.323Å, while those of the monoclinic Na3AlD6 structure are a=5.402Å, b=5.507Å, c=7.725Å, and β=89.491°. 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 1.631Å, 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) 101]. The calculated GGA formation energies for the decomposition reactions of NaAlD4 and Na3AlD6 are 33.5 and 49.9kJ/mol of H2, respectively. The corresponding LDA values are significantly higher: 44.6 and 70.3kJ/mol of H2. Comparison with the experimental enthalpy data suggests that the GGA results are in a very good agreement with the measured ΔH [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.