Crystal Structures and in-Situ Formation Study of Mayenite Electrides

Mayenite inorganic electrides are antizeolite nanoporous materials with variable electron concentration
[Ca12Al14O32]2+□5-δO1-δ2-e2δ- (0 < δ ≤ 1), where □ stands for empty sites. The oxymayenite crystal structure
contains positively charged cages where loosely bounded oxide anions are located. These oxygens can be removed
to yield electron-loaded materials in which the electrons behave like anions (electrides). Here, a new preparation
method, which allows synthesizing powder mayenite electrides easily, is reported. Accurate structural data for the
white (δ = 0) and green electride (δ ∼ 0.5) are reported from joint Rietveld refinements of neutron and synchrotron
X-ray powder diffraction data and also from single-crystal diffraction. The electride formation at high temperature
under vacuum has been followed in-situ by neutron powder diffraction. The evolution of mayenite crystal structure,
including the changes in the key occupation factor of the intracage oxide anions, is reported. Furthermore, the
stability of mayenite framework in very low oxygen partial pressure conditions is also studied. It has been found
that C12A7 decomposes, at 1373 K in reducing conditions, to give Ca5Al6O14 (C5A3) and Ca3Al2O6 (C3A). The
kinetics of this transformation has also been studied. The fit of the transformed fraction to the classic Avrami−Erofe'ev equation gave an “Avrami exponent”, n = 2, which indicates that nucleation is fast and the two-dimensional
linear growth of the new phases is likely to be the limiting factor.