Formation and quantification of peroxide anions in nanocages of 12CaO·7Al2O3

The concentration of peroxide (O22−) ions in the cage structures of the mayenite (12CaO·7Al2O3 or C12A7) crystal lattice has been determined quantitatively by Raman spectroscopy combined with volumetry and electron paramagnetic spin resonance. The formation process of the O22− ion was also investigated using in situ Raman spectroscopy under controlled atmospheres. Although room temperature Raman spectroscopy showed that the equilibrium concentration of O22− ions in the cage structures increased with increased annealing temperatures (especially below 800 °C), no O22− signal was observed at very high temperatures (1350 °C) during the in situ measurements. We explain this apparent inconsistency by hypothesizing that transient atomic oxygen, possibly originating from the thermal dissociation of atmospheric O2, is formed at interstitial or cage sites within C12A7 at high temperatures and subsequently combines with O2− ions in the cages to form O22− during the cooling process.