Effects of milling, doping and cycling of NaAlH4 studied by vibrational spectroscopy and X-ray diffraction

The effects of milling and doping NaAlH4 with TiCl3, TiF3 and Ti(OBuII)(4), and of cycling doped NaAlH4 have been investigated by infrared (IR) and Raman spectroscopy and X-ray powder diffraction. Milling and doping produce similar effects. Both decrease the crystal domain size (similar to900Angstrom for milled and similar to700Angstrom for doped, as compared to similar to1600Angstrom for unmilled and undoped NaAlH4) and increase anisotropic strain (by a factor >2.5, mainly along c). They also influence structure parameters such as the axial ratio c/a, cell volume and atomic displacement amplitudes. They show IR line shifts by similar to15 cm(-1) to higher frequencies for the Al-H asymmetric stretching mode v(3), and by similar to20 cm(-1) to lower frequencies for one part of the H-Al-H asymmetric bending mode v(4), thus suggesting structural changes in the local environment of the [AIH(4)](-) units. The broad v(3) bands become sharpened which suggests a more homogeneous local environment of the [AIH4]- units, and there appears a new vibration at 7 10 cm(-1). the Raman data show no such effects. Cycling leads to an increase in domain size (1200-1600 Angstrom), IR line shifts similar to doped samples (except for TiF3: downward shift by similar to10cm(-1)) and a general broadening of the v(3) mode that depend on the nature of the dopants. These observations support the idea that some Ti diffusion and substitution into the alanate lattice does occur, in particular during cycling, and that this provides the mechanism through which Ti-doping enhances kinetics during re-crystallisation. (C) 2004 Elsevier B.V. All rights reserved.