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.
Cite
@article{arxiv.1602.06692,
title = {Cage Electron-Hydroxyl Complex State as Electron Donor in Mayenite},
author = {M. Hiraishi and K. M. Kojima and M. Miyazaki and I. Yamauchi and H. Okabe and A. Koda and R. Kadono and S. Matsuishi and H. Hosono},
journal= {arXiv preprint arXiv:1602.06692},
year = {2016}
}