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

[Ca 24 Al 28 O 64 ] 4+ (O 2- ) 2- x ( e - ) 2 x (0 ≤ x ≤ 2, C12A7: e - ) crystal shows a metal−insulator transition at x ∼ 1. Varied concentrations of electrons ( N e ) are introduced in the subnanometer-sized crystallographic cages of [Ca 24 Al 28 O 64 ] 4+ (O 2- ) 2 (12CaO·7Al 2 O 3, C12A7) in place of the O 2- 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 × 10 20 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 N e exceeds 5 × 10 20, and coexists with the Lorentz-type responses at N e between 5 × 10 20 and 2 × 10 21 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 N e limit, while the cage deformation is reduced at the high N e limit, leading to the insulator−metal transition.