Localized and Delocalized Electrons in Room-Temperature Stable Electride [Ca24Al28O64]4+(O2-)2-x(e-)2x: Analysis of Optical Reflectance Spectra

Abstract

[Ca24Al28O64]4+(O2-)2-x(e-)2x (0 ≤ x ≤ 2, C12A7:e-) crystal shows a metal−insulator transition at x ∼ 1. Varied concentrations of electrons (Ne) are introduced in the subnanometer-sized crystallographic cages of [Ca24Al28O64]4+(O2-)2 (12CaO·7Al2O3, C12A7) in place of the O2- ions accommodated in the cages. The optical reflectance spectra of C12A7:e- crystals were analyzed using the Drude−Lorentz model to examine the electronic states and carrier transport mechanisms experimentally. The concentrations of localized electrons trapped in the cages were estimated from Lorentz-type optical responses around 0.4 and 2.8 eV, and those of delocalized electrons from a Drude-type response. C12A7:e- with Ne lower than 5 × 1020 cm-3 (x ∼ 0.5) shows polaronic hopping conduction and its reflectance spectrum shows only the Lorentz-type responses. The Drude-type response starts to appear when Ne exceeds 5 × 1020, and coexists with the Lorentz-type responses at Ne between 5 × 1020 and 2 × 1021 cm-3 (x ∼ 2), which indicates that the localized electrons coexist with the delocalized electrons even in the metallic samples. These results support previous theoretical studies proposing that a strong electron−lattice interaction deforms the soft cage structure of C12A7, and forming an insulating state at the low Ne limit, while the cage deformation is reduced at the high Ne limit, leading to the insulator−metal transition.