Phase stability and structure of spinel-based transition aluminas

Abstract

Using first-principles total energy calculations, we have investigated the structure and phase stability of spinel-based transition aluminas $(\ensuremath{\gamma},\ensuremath{\delta},\ensuremath{\eta}),$ 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 ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ 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 $({\mathrm{HAl}}_{5}{\mathrm{O}}_{8})$ proposed in the literature as a structural candidate for $\ensuremath{\gamma}$-alumina, is thermodynamically unstable with respect to decomposition into the anhydrous defect spinel plus boehmite $(\ensuremath{\gamma}$-AlOOH). (v) The temperature range for transforming boehmite into $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ is calculated from first-principles energetics plus measured thermochemical data of ${\mathrm{H}}_{2}\mathrm{O},$ 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.