Energetics of formation and oxidation of microporous calcium aluminates: A new class of electrides and ionic conductors

Microporous calcium aluminates with a common formula [Ca ₁₂Al ₁₄O _33-δ] ^2δ+(2δ e ^-), (0 < δ≤ 1), represent a new family of electrides with conducting extraframework electrons encaged within the framework. Their enthalpies of oxidation to eliminate these electrons and obtain a stoichiometric Ca ₁₂Al ₁₄O ₃₃ compound, and formation enthalpies relative to the most stable annealed calcium aluminate Ca ₁₂Al ₁₄O _33.46 (which contains excess oxygen probably as O ^- and O ₂ ^- species), are studied by transposed temperature drop calorimetry and drop solution calorimetry in molten lead borate at 1080 K. The standard enthalpy of oxidation per O ₂ consumed is -579.4±9.4 kJ/mol (or -144.8±9.4 kJ per encaged electron). The enthalpies obtained for samples with variable amounts of electrons (and less than 33 oxygens per formula unit) are compared to that for oxidation of a stoichiometric Ca ₁₂Al ₁₄O ₃₃ sample to an oxygenrich composition. The study supports the idea that the oxidation of the stoichiometric Ca ₁₂Al ₁₄O ₃₃ sample results in formation of extraframework ions (O ₂ ^- and O ^-) with a standard enthalpy at 298 K of - 161.9 ± 11.2 kJ per mol of O ₂, which is in good agreement with previous data by Hayashi et al. (J. Phys. Chem. B 2004, 108 (26), 8920) obtained for the same reaction at 898 K by electron paramagnetic resonance. The energetics of oxidation of electron-rich samples are discussed in terms of an electride model and "ionic" bonding of an encaged electron in the framework.