| Abstract
| - We report thermodynamic data for microporous calcium aluminates with a common formula [Ca12Al14O33-δ]2δ+(2δe-), (0 < δ ≤ 1), that represent a new family of electrides with conducting extraframework electrons encaged within the framework. The enthalpies of oxidation to eliminate these electrons and obtain a stoichiometric Ca12Al14O33 compound and formation enthalpies relative to the most stable annealed calcium aluminate, which contains excess oxygen probably as O- and O2- species, are measured by transposed temperature drop calorimetry and drop-solution calorimetry in molten lead borate at 1080 K.
- Microporous calcium aluminates with a common formula [Ca12Al14O33-δ]2δ+(2δe-), (0 < δ ≤ 1),represent a new family of electrides with conducting extraframework electrons encaged within theframework. Their enthalpies of oxidation to eliminate these electrons and obtain a stoichiometricCa12Al14O33 compound, and formation enthalpies relative to the most stable annealed calcium aluminateCa12Al14O33.46 (which contains excess oxygen probably as O- and O2- species), are studied by transposedtemperature drop calorimetry and drop solution calorimetry in molten lead borate at 1080 K. The standardenthalpy of oxidation per O2 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 performula 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 sampleresults in formation of extraframework ions (O2- and O-) with a standard enthalpy at 298 K of −161.9± 11.2 kJ per mol of O2, 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.
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