The metal oxide known as 12CaO∙7Al2O3 (C12A7) is often used to make aluminous cements. Researchers from RIKEN’s Discovery Research Institute in Wako and the Tokyo Institute of Technology have now found superconductivity in the metallic version of C12A7.

The crystal structure of C12A7 consists of a number of cages formed by calcium, aluminum and oxygen atoms (Fig. 1). Typically, such oxides are electrically insulating and of little interest to scientists. This changed recently, when researchers from the Tokyo Institute of Technology demonstrated that C12A7 can be made metallic by the chemical reduction that replaces negatively charged oxygen atoms with electrons. As a result, the conductivity of C12A7 changes by an impressive thirteen orders of magnitude. Such compounds, where electrons are used to achieve metallic conductivity, are called ‘electrides’. Electrides are commonly used in batteries or fuel cells.

Owing to the open crystal structure of the C12A7 electride, the nature of the metallic state is quite different to normal metals. “The conduction of this material is supported by electrons trapped in the nano-scale cages,” explains Kimitoshi Kono from the RIKEN team. Although the actual conductivity depends on the magnitude of this replacement, the absence of oxygen atoms allows the trapped electrons to extend and reach across the voids. The resultant overlap of electron distribution thereby enables the electrons to travel across the structure.

Superconductivity originates in the coupling of free electrons; so many metals are superconductive at low temperatures. The researchers therefore studied the electronic properties of the C12A7 electride and observed superconductivity at about 0.4 K, which is less than 272°C. Their results have been published in the Journal of the American Chemical Society (1).

As the ‘metallization’ of the material was rather unexpected by scientists, the discovery of superconductivity in this mundane material is surprising to many. “Nobody else expected a cement to become superconductive,” says Kono. Further investigations into the properties of these unusual superconductors are expected to follow from these pioneering findings. The design of the caged structure of the C12A7 electride is also rather flexible, so many similar compounds exist or are theoretically possible. Kono says that “there is no reason why we should not expect that some of these materials might show superconductivity.” Cement might hold further unexpected surprises for physicists and chemists.

Miyakawa, M., Kim, S. W., Hirano, M., Kohama, Y., Kawaji, H., Atake, T., Ikegami, H., Kono, K. & Hosono, H. Superconductivity in an inorganic electride 12CaO∙7Al2O3:e-. Journal of the American Chemical Society 129, 7270–7271 (2007).

Bron: ResearchSEA.