Cage electron-hydroxyl complex state as electron donor in mayenite

It is inferred from the chemical shift of muon spin rotation $(\ensuremath{\mu}\mathrm{SR})$ spectra that muons implanted in pristine (fully oxidized) mayenite, ${[{\mathrm{Ca}}₁₂{\mathrm{Al}}₁₄{\mathrm{O}}₃₂]}²⁺[{\ensuremath{\square}}₅{\mathrm{O}}^{2\ensuremath{-}}]$ (C12A7, with $\ensuremath{\square}$ referring to the vacant cage), are bound to ${\mathrm{O}}^{2\ensuremath{-}}$ at the cage center to form ${\mathrm{OMu}}^{\ensuremath{-}}$ (where Mu represents muonium, a muonic analog of the H atom). However, an isolated negatively charged state $({\mathrm{Mu}}^{\ensuremath{-}}$, an analog of ${\mathrm{H}}^{\ensuremath{-}})$ becomes dominant when the compound approaches the state of electride ${[{\mathrm{Ca}}₁₂{\mathrm{Al}}₁₄{\mathrm{O}}₃₂]}²⁺[{\ensuremath{\square}}₄2{\mathrm{e}}^{\ensuremath{-}}]$ as a result of the reduction process. Moreover, the ${\mathrm{OMu}}^{\ensuremath{-}}$ state in the pristine specimen exhibits depolarization of paramagnetic origin at low temperatures (below $\ensuremath{\sim}30$ K), indicating that ${\mathrm{OMu}}^{\ensuremath{-}}$ accompanies a loosely bound electron in the cage that can be thermally activated. This suggests that interstitial muons (and hence H) forming a ``cage electron-hydroxyl'' complex can serve as electron donors in C12A7.