Optical Conductivity of Mayenite: From Insulator to Metal

Mayenite-based oxides [12CaO·7Al2O3] starting from [Ca24Al28O64]4++ 2O2–(insulator) and subsequently annealed so as to obtain [Ca24Al28O64]4++ 4e–(metal) were studied by reflectance and transmission on seven samples with dc conductivities spanning the range 10–10to 1500 Ω–1cm–1. Three of them are essentially insulating. The remaining display an increasing dc conductivity as the electron concentration increases. A set of phonons in the infrared below 0.15 eV and an intense line in the ultraviolet at 6.5 eV account for the optical conductivity of the insulating samples. As the samples become increasingly metallic, we observe an enhancement of spectral weight in the visible range. Simultaneously, the reflectance and the resulting optical conductivity develop a stronger component in the infrared, characteristic of mobile electrons. This electronic response appears to be strongly coupled to the phonons, as shown by their Fano profiles present in the metallic samples. Our results indicate that free carriers promote the formation of polarons and that these two excitations contribute to the dc conducting properties of mayenites. This overall electronic picture is consistent with first principles calculations of the density of states in mayenites. The observed strong electron–phonon coupling may be of interest in view of the superconductivity properties found in the metallic samples.