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(OBuⁿ)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 (900 Å for milled and 700 Å for doped, as compared to 1600 Å 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 15 cm^?1 to higher frequencies for the Al–H asymmetric stretching mode ν₃, and by 20 cm^?1 to lower frequencies for one part of the H–Al–H asymmetric bending mode ν₄, thus suggesting structural changes in the local environment of the AlH₄]^? units. The broad ν₃ bands become sharpened which suggests a more homogeneous local environment of the AlH₄]^? units, and there appears a new vibration at 710 cm^?1. The Raman data show no such effects. Cycling leads to an increase in domain size (1200–1600 Å), IR line shifts similar to doped samples (except for TiF₃: downward shift by 10 cm^?1) and a general broadening of the ν₃ 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.