Synthesis and properties of 12CaO · 7Al₂O₃ electride: review of single crystal and thin film growth

Abstract

Over the last decade, experimental studies supported by theoretical calculations have demonstrated that 12CaO · 7Al₂O₃ (C12A7), a typical electrical insulator, could be converted into an electro-active functional material, such as a metallic conductor with a low work function but chemical inertness and superconductivity, at low temperatures. These properties were realised by successful heavy electron doping into positively charged sub-nanometre-sized cages inherent to the crystal structure through removal of oxygen ions accommodated in the cages. The resulting samples, C12A7:e⁻, may be regarded as an electride in which electrons serve as anions. This article reviews the single crystal and thin film growth of both C12A7 and C12A7:e⁻, and describes our approach to these subjects on the basis of understanding the phase diagram of C12A7 and C12A7:e⁻ as well as their molten and glassy states. Finally, the importance of the surface of C12A7:e⁻ is addressed with regard to practical applications.

Keywords:

• 12CaO · 7Al₂O₃
• electride
• single crystal
• thin film

Acknowledgements

We thank Dr Toshio Kamiya, Dr Masahiro Hirano, Dr Katsuro Hayashi, Dr Satoru Matsuishi, Dr Masashi Miyakawa, Dr Yoshitake Toda of Tokyo Institute of Technology, Dr Peter V. Sushko and Dr Alexander L. Shluger of University College London for their collaboration and discussion. This work was supported by the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST), Japan Society for the Promotion of Science and the MEXT Element Strategy Project, Japan.