High-Temperature Hydration and Conductivity of Mayenite, Ca 12 Al 14 O 33

Mayenite, Ca 12 Al 14 O 33, has been synthesized by a citric acid route, with final sintering at 1200−1350 °C. Phase purity has been characterized by X-ray diffraction and scanning electron microscopy. The hydration has been investigated under oxidizing and reducing conditions using thermogravimetry versus T and p H 2 O, as well as in situ H 2 O/D 2 O isotope exchange. The two data sets are in general correspondence and yield standard entropy and enthalpy changes of hydration (for 1 mol of H 2 O) of −123 ± 10 J/mol K and −240 ± 16 kJ/mol, respectively, in overall agreement with previous literature. The ac conductivity of sintered disks has been measured by the 4-point van der Pauw method versus p O 2 at 900−1200 °C. It reflects mainly n-type electronic conductivity under reducing conditions and ionic (mainly oxide ion) conductivity under oxidizing conditions. The conductivity contributions are modeled and discussed in terms of a defect model involving partial occupancy by oxide ions and hydroxide ions of the structural subnanocages and with electrons as minority defects. A new Kröger−Vink-compatible nomenclature for the partial occupancy, including electronic defects, is introduced and used to model hydration and the unusual p H 2 O dependency of the n-type electronic conductivity.