Spectroscopic characterization of a multiband complex oxide: Insulating and conducting cement 12CaO · 7Al2O3

Complex oxide 12CaO · 7Al₂O₃ can be represented using the chemical formula [Ca₂₄Al₂₈O₆₄]⁴⁺ · 2O^2- (or C12A7:O^2- for brevity), where the [...]⁴⁺ signifies the lattice framework of cages compensated by extra-framework anions occupying some of the cages. In this form C12A7 is a wide band gap insulator. However, when the extra-framework anions are substituted by electrons forming the electride phase [Ca₂₄Al₂₈O₆₄]⁴⁺ · 4e^- (C12A7:e^-), the material becomes metallic. Understanding modifications of the C12A7 electronic structure accompanying this insulator-metal transition has long been of interest.

According to previous ab initio calculations, the extra-framework oxygen ions produce electronic levels above the top of the framework valence band, while the extra-framework electrons in C12A7:e^- occupy so-called cage conduction band (CCB) and localise in cages (see Figure). However, experimental confirmation of this prediction was lacking.

Detailed spectroscopic measurements, corroborated by theoretical calculations, have identified all major electronic structure features in both stoichiometric C12A7:O^2- and electride C12A7:e^- (see Figure). In particular, these measurements suggest the existence of a narrow conduction band between the main conduction and valence bands common in both conducting and insulating C12A7 and support the theory that extra-framework electrons in oxygen-deficient C12A7 occupy the low-energy states of this narrow band.

This opens up opportunities to further manipulate with the C12A7 electronic structure by selecting extra-framework ions which interact with the electrons occupying the cage conduction band.

For more information see:

• Electron localization and a confined electron gas in nanoporous inorganic electrides, Physical Review Letters 91, 126401 (2003).
• Hopping and optical absorption of electrons in nano-porous crystal 12CaO · 7Al₂O₃, Thin Solid Films 445(2), 161-167 (2003).
• Photo-induced generation of electron anions in H-doped nano-porous oxide 12CaO · 7Al₂O₃: toward an optically controlled formation of electrides, Applied Physics Letters 86(9), 092101(1-3) (2005).
• From insulator to electride: a theoretical model of nanoporous oxide 12CaO · 7Al₂O₃, Journal of the American Chemical Society 129, 942-951 (2007).
• Models of stoichiometric and oxygen-deficient surfaces of subnanoporous 12CaO · 7Al₂O₃, Proceedings of the Royal Society A 467(2131), 2066-2083 (2011).