Localized and delocalized electrons in room-temperature stable electride [Ca24Al28O64]4+(O2-)2-x(e-)2x:: Analysis of optical reflectance spectra

[Ca24Al28O64](4+)(O2-)(2-x)(e(-))(2x) (O <= x <= 2, C12A7:e(-)) crystal shows a metal-insulator transition at x similar to 1. Varied concentrations of electrons (N-e) are introduced in the subnanometer- sized crystallographic cages of [Ca24Al28O64](4+)(O2-)(2) (12CaO center dot 7Al(2)O(3), 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 N-e lower than 5 x 10(20) cm(-3) (x similar to 0.5) shows polaronic hopping conduction and its reflectance spectrum shows only the Lorentz-type responses. The Drude-type response starts to appear when N-e exceeds 5 x 10(20), and coexists with the Lorentz-type responses at Ne between 5 x 10(20) and 2 x 10(21) cm(-3) (x similar to 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 -l attice interaction deforms the soft cage structure of C12A7, and forming an insulating state at the low N-e limit, while the cage deformation is reduced at the high N-e limit, leading to the insulator-metal transition.