Metallic state in a lime-alumina compound with nanoporous structure

We report a metallic state in a nanostructured porous crystal 12CaO-7Al₂O₃ by incorporating electrons in the inherent subnanometer-sized cages, in which a three-dimensionally closely packed cage structure acts as an electronic conduction path. High-density electron doping (∼2 x 10²¹ cm^-3), which was achieved by a thermal treatment in Ti metal vapor at ∼1100 °C, induces homogenization of the cage geometry to a symmetric state, resulting in an insulator-metal transition with a sharp enhancement of the electron drift mobility from ∼0.1 to 4 cm² V^-1 s^-1. The results provide an approach for the realization of electroactive functions in materials composed only of environmentally benign elements by utilizing the appropriate nanostructures.