Experimental studies of α-AlD3 and α′-AlD3versus first-principles modelling of the alane isomorphs

Abstract

The thermal phase behaviour of cryomilled α′-AlD₃ and α-AlD₃ was investigated by in situsynchrotron powder X-ray diffraction (SR-PXD), differential scanning calorimetry and first principles atomic modelling. In situ measurements showed that α′-AlD₃ decomposes directly into Al and D₂ at around 80 °C during heating at 1 °C min⁻¹. At higher temperatures the transformation of α′-AlD₃ to α-AlD₃ was observed by DSC measurements at 5 °C min⁻¹, and tentatively by in situSR-PXD at 1 °C min⁻¹. Atomic modelling was carried out to investigate possible structural relationships and transformation pathways between the α- and α′-phase. Group–subgroup relation analyses and direct method lattice dynamics were used to rule out a possible displacive transformation pathway between the α′- and α-phases. The likelihood of a reconstructive transformation was demonstrated by partial transformation of an interface between α′ and α domains during elevated temperature molecular dynamics. Such an α′- to α-phase transformation may be possible when kinetic barriers can be overcome at elevated temperatures or during long time periods. These insights are also relevant to the transformation mechanisms of the β-AlD₃ and γ-AlD₃ isomorphs to the α-phase.