Fascinating properties of a novel CaO-Al2O3 compound

A new player among CaO-Al2O3 compounds

nxCaO.mxAl2O3 (CnAm) chemical compositions of CaO and Al2O3 oxides are known as cements. They are used to bind bricks and stones. It has been recently discovered that 12xCaO.7xAl2O3 (C12A7), due to its unique structure (see below), has unusual properties that make it an important material for electronic and chemical applications.

For example:

  • C12A7 is a good ionic conductor of O2- species. It can be used to extract extremely chemically active O- ions.
  • As grown C12A7 is an insulator. However, it can be converted to a transparent electronic conductor and back to an insulator.
  • C12A7 can be converted to an electride state, i.e. a state where electrons serve as anions. C12A7 electride has an extremely low work function of only 0.6 eV.
  • In the following we describe just a few properties of this novel material. There are many more things to learn and to discover. Get in touch if you would like to participate in this.

Structure of C12A7

The C12A7 lattice is formed by ~7.1021 cage-like structures per cm3 joined together.

The main structural element of C12A7 is the cage. The neighbouring cages are connected via a relatively wide opening. The unit cell consists of two C12A7 molecules which posses a net positive charge of 4e. This charge is balanced by extra-framework species (ions or electrons that are not part of the cage structure). The properties of the material depend on the type and relative concentration of the extra-framework species.There are 7.1021 cages per cm3, which can be considered as anion vacancies. One in three vacancies can be occupied by a negatively charged species, e.g. by an electron. Such a concentration of stable anion vacancies is unique to C12A7. Other materials, e.g. NaCl, would degrade and form droplets of metal (Na in the case of NaCl) at high concentration of anion vacancies.

C12A7 with extra-framework electrons (C12A7:e-)

Extra-framework electrons can be formed by extracting extra-framework O atoms. This can be achieved by heating C12A7 together with metallic Ca at 970K in a sealed container. One observed formation of CaO on the surface and changes in its optical and magnetic properties.

Future applications and new physical phenomena

Future applications

  • Transparent Conducting Oxides (TCO)
  • Transparent transistors
  • Electron emitters
  • Thermoelectric devices
  • Miniature cooling systems

Interesting physics

  • Properties of C12A7 at higher concentration of extra-framework electrons.
  • Properties of other “active” anions.
  • Syntheses of C12A7-like materials.
  • Formation of electron and spin sub-lattices.
  • Surface properties.


More information on C12A7 can be found on the page of David Muñoz.