Cage electron-hydroxyl complex state as electron donor in mayenite

It is inferred from the chemical shift of muon spin rotation (μSR) spectra that muons implanted in pristine (fully oxidized) mayenite, [Ca12Al14O32]2+[□5O2-] (C12A7, with □ referring to the vacant cage), are bound to O2- 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 [Ca12Al14O32]2+[□42e-] as a result of the reduction process. Moreover, the OMu- state in the pristine specimen exhibits depolarization of paramagnetic origin at low temperatures (below ∼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.