Cage Electron-Hydroxyl Complex State as Electron Donor in Mayenite

Abstract

It is inferred from the chemical shift of muon spin rotation ($\hat{I}¼$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.

5 pages, 4 figures