Synthesis of a Room Temperature Stable 12CaO·7Al₂O₃ Electride from the Melt and Its Application as an Electron Field Emitter

A room temperature (RT) stable electride was realized by thermally annealing an insulating 12CaO·7Al 2 O 3 (C12A7) single crystal in a calcium metal vapor. Here we report a simple and direct method for synthesizing polycrystalline C12A7 electride (C12A7:e - ); the solidification of a “melt” in a reducing atmosphere and the crystallization of a “glass” with an oxygen-deficient composition in a vacuum. The carbon-related anion (C 2 2 - ) presumably serves as the template for the formation of the C12A7 phase in the solidification process and may be spontaneously released from the lattice during the cooling process, leaving mobile electrons in the lattice. Also the C 2 2 - ions accommodated in the glass may play a significant role in the formation of C12A7:e - during the crystallization. The polycrystalline C12A7:e - exhibits an electrical conductivity up to 5 S·cm -1 at 300 K, which corresponds to an electron concentration of ∼3 × 10 19 cm -3 and a nearly equal mobility of ∼0.1 cm 2 ·V -1 ·s -1 to that of the single crystalline C12A7:e - . A large current density of ∼12 μA·cm -2 is obtained in the electron field emission from a flat surface of the polycrystalline C12A7:e - with a work function of ∼0.6 eV for an applied electric field of 2.4 × 10 5 V·cm -1 . These simple synthetic methods make it possible to produce efficiently the electride in large volume, which will facilitate the C12A7:e - research, paving a way for various applications including cold electron emission.