  First-principles calculations of the mayenite-based oxide,
[Ca12Al14O32]{2+}(2e-), reveal the mechanism responsible for its high
conductivity. A detailed comparison of the electronic and optical properties of
this material with those of the recently discovered novel transparent
conducting oxide, H-doped UV-activated Ca12Al14O33, allowed us to conclude that
the enhanced conductivity in [Ca12Al14O32]{2+}(2e-) is achieved by elimination
of the Coulomb blocade of the charge carriers. This results in a transition
from variable range hopping behavior with a Coulomb gap in H-doped
UV-irradiated Ca12Al14O33 to bulk conductivity in [Ca12Al14O32]{2+}(2e-).
Further, the high degree of the delocalization of the conduction electrons
obtained in [Ca12Al14O32]{2+}(2e-) indicate that it cannot be classified as an
electride, originally suggested.
