Cage Electron-Hydroxyl Complex State as Electron Donor in Mayenite

TL;DR

This study reveals that muons and hydrogen in mayenite form a cage electron-hydroxyl complex that acts as an electron donor, with different charge states depending on the reduction level of the material.

Contribution

It demonstrates the formation of a cage electron-hydroxyl complex in mayenite and its role as an electron donor, providing new insights into the material's electronic properties.

Findings

Muons bind to O$^{2-}$ forming OMu$^-$ in pristine mayenite.

Isolated Mu$^-$ becomes dominant in reduced, electride-like states.

The OMu$^-$ state shows low-temperature depolarization indicating a loosely bound electron.

Abstract

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.