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

The concentration of peroxide (O₂^2-) ions in the cage structures of the mayenite (12CaO·7Al₂O₃ or C ₁₂A₇) crystal lattice has been determined quantitatively by Raman spectroscopy combined with volumetry and electron paramagnetic spin resonance. The formation process of the O₂^2- ion was also investigated using in situ Raman spectroscopy under controlled atmospheres. Although room temperature Raman spectroscopy showed that the equilibrium concentration of O₂^2- ions in the cage structures increased with increased annealing temperatures (especially below 800 °C), no O₂^2- 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 O^2- ions in the cages to form O₂^2- during the cooling process.