Phase stability and structure of spinel-based transition aluminas - art. no. 024102

Using first-principles total energy calculations, we have investigated the structure and phase stability of spinel-based transition aluminas (gamma,de lta,eta), both in the presence and absence of hydrogen. The spinel-based st ructures (formed from dehydration of aluminum hydroxides) necessarily must have vacant cation positions to preserve the Al2O3 stoichiometry, and may h ave residual hydrogen cations in the structure as well. In the absence of h ydrogen, we find the following: (i) Vacancies in octahedral sites are energ etically preferred (or, Al cations prefer tetrahedral positions). (ii) Ther e 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 te ndency vanishes, and "clusters" of near-neighbor vacancies are slightly ene rgetically preferred. (iv) The hydrogen spinel (HAl5O8) proposed in the lit erature as a structural candidate for gamma -alumina, is thermodynamically unstable with respect to decomposition into the anhydrous defect spinel plu s boehmite (gamma -AlOOH). (v) The temperature range for transforming boehm ite into gamma Al2O3 is calculated from first-principles energetics plus me asured thermochemical data of H2O, and is in excellent agreement with the o bserved transformation temperatures. Finally, we comment on the possible im plications of this work on the porous microstructure of the transition alum inas.