Ce–O Covalence in Silicate Oxyapatites and Its Influence on Luminescence Dynamics

Cerium substituting gadolinium in Ca2Gd8(SiO4)6O2 occupies two intrinsic sites of distinct coordination. The coexistence of an ionic bonding at a 4F site and an ionic–covalent mixed bonding at a 6H site in the same crystalline compound provides an ideal system for comparative studies of ion–ligand interactions. Experimentally, the spectroscopic properties and photoluminescence dynamics of this white-phosphor are investigated. An anomalous thermal quenching of the photoluminescence of Ce3+ at the 6H site is analyzed. Theoretically, ab initio calculations are conducted to reveal the distinctive properties of the Ce–O coordination at the two Ce3+ sites. The calculated eigenstates of Ce3+ at the 6H site suggest a weak Ce–O covalent bond formed between Ce3+ and one of the coordinated oxygen ions not bonded with Si4+. The electronic energy levels and frequencies of local vibrational modes are correlated with specific Ce–O pairs to provide a comparative understanding of the site-resolved experimental results. On...