Investigation of Interactions between the Doped Rare Earth Ions and Encaged Radicals in C12A7:RE3$+$ Using Optical and EPR Spectroscopy

Doped calcium aluminates (C12A7) (C12A7:Eu3$+$ and C12A7:Mn2$+)$ have been prepared using solid state reaction methods. The Eu3$+$ and Mn2$+$ dopants can both occupy the Ca2$+$ positions in C12A7.The unique cage-like structure of C12A7 allows different anions, such as oxygen, hydrogen to be trapped in cage by modifying the sample preparation or treatment conditions. The effects of these encaged anions/radicals on the local symmetries of Ca2$+$ have been studied using photoluminescence from C12A7 doped with Eu3$+$, which is a sensitive environmental probe. The effects can also be independently observed from the hyperfine structure of electron paramagnetic resonance spectra in C12A7 doped with Mn2$+$. Our results showed: 1) the presence of 5D0 to 7F0 transition implies that Eu3$+$ is at a non-centrosymmetric site in all caged radical centers; 2) this singlet transition is doubled when superoxides are encaged in C12A7, indicating that the sites of calcium (or Eu2$+)$ ions are not identical due to the distortion of the encaged anions; 3) a blue shift of the transition occurred due to nephelauxetic effects in asymmetry sites. We have also observed double sets of EPR signals of sextet hyperfine splitting for Mn2$+$ in C12A7-O but single sets in C12A7-H. We conclude that the local symmetry around Ca2$+$ positions are distorted more when superoxide is encaged in C12A7. Finally, RE3$+$ doped C12A7 samples have been systematically prepared and the interactions between the encaged ions and RE3$+$ emission centers studied using EPR spectroscopy.