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

Microporous calcium aluminates with a common formula [Ca12Al14O33-delta]2(delta+)(2 delta e(-)), (0 < 6 <= 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 Ca12Al14O33 compound, and formation enthalpies relative to the most stable annealed calcium aluminate Ca12Al14O33.46 (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 Ca12Al14O33 sample to an oxygen-rich composition. The study supports the idea that the oxidation of the stoichiometric Ca12Al14O33 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.