  It is inferred from the chemical shift of muon spin rotation ($\mu$SR)
spectra that muons implanted in pristine (fully oxidized) mayenite,
[Ca$_{12}$Al$_{14}$O$_{32}]^{2+}$[$\square_5$O$^{2-}$] (C12A7, with $\square$
referring to the vacant cage), are bound to O$^{2-}$ at the cage center to form
OMu$^-$ (where Mu represents muonium, a muonic analog of the H atom). However,
an isolated negatively charged state (Mu$^-$, an analog of H$^-$) becomes
dominant when the compound approaches the state of electride
[Ca$_{12}$Al$_{14}$O$_{32}]^{2+}$[$\square_4$2e$^{-}$] as a result of the
reduction process. Moreover, the OMu$^-$ state in the pristine specimen
exhibits depolarization of paramagnetic origin at low temperatures (below
$\sim$30~K), indicating that OMu$^-$ 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.
