Using first-principles total energy calculations, we have investigated the structure and phase stability of spinel-based transition aluminas(γ, δ, η), both in the presence and absence of hydrogen. The spinel-based structures (formed from dehydration of aluminum hydroxides) necessarily must have vacant cation positions to preserve the Al2O3 stoichiometry, and may have residual hydrogen cations in the structure as well. In the absence of hydrogen, we find the following: (i) Vacancies in octahedral sites are energetically preferred (or, Al cations prefer tetrahedral positions). (ii) There is a strong Al-vacancy ordering tendency, with widely separated vacancies being lower in energy than near-neighboring vacancies. Upon incorporation of hydrogen into the structure: (iii) The strong cation-vacancy ordering tendency vanishes, and “clusters” of near-neighbor vacancies are slightly energetically preferred. (iv) The hydrogen spinel (HAl5O8) proposed in the literature as a structural candidate for γ-alumina, is thermodynamically unstable with respect to decomposition into the anhydrous defect spinel plus boehmite (γAlOOH). (v) The temperature range for transforming boehmite into γ − Al2O3 is calculated from first-principles energetics plus measured thermochemical data of H2O, and is in excellent agreement with the observed transformation temperatures. Finally, we comment on the possible implications of this work on the porous microstructure of the transition aluminas.
|Original language||English (US)|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - Jan 1 2001|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics