Size-dependent microstructure and europium site preference influence fluorescent properties of Eu3+-doped Ca10(PO4)6(OH)2 nanocrystal

In this study, Eu3+-doped nanocrystalline Ca10(PO4)6(OH)2 (Ca10−xEux(PO4)6(OH)2) with different particle sizes have been prepared by the thermal decomposition of precursors. Size-dependent microstructure could be observed in nanocrystalline Ca10−xEux(PO4)6(OH)2. The lattices of Ca10−xEux(PO4)6(OH)2 nanocrystals were more distorted in comparison with the bulk, and the smaller the particle size, the more distorted the lattices. Room temperature photoluminescence showed europium site preference was also size-dependent, with the majority of Eu3+ ions occupying Ca(II) sites in the bulk, but more and more Eu3+ ions occupying Ca(I) sites in Ca10−xEux(PO4)6(OH)2 with decreasing particle size. Fluorescent properties of Ca10−xEux(PO4)6(OH)2 were considered to be influenced by both microstructure and site preference of Eu3+ ions. An abnormal strong intensity of 5D0-7F0 transition was observed in bulk and larger Ca10−xEux(PO4)6(OH)2 nanocrystals, but the relative intensities of 5D0-7F0 transition to 5D0-7F1,2,3,4 transition of Eu3+ became weaker as the particle sizes decreased. As the particle sizes became smaller, the ratios of the red emission transition (5D0-7F2) to the orange emission transition (5D0-7F1) (R/O values) first increased by comparing the bulk sample with 96 nm sample, and then decreased by comparing 96 nm sample to 57 nm sample. The quenching concentrations of Ca10−xEux(PO4)6(OH)2 samples increased with decreasing particle size. Possible mechanisms responsible for these phenomena were proposed. Since nanosized Ca10−xEux(PO4)6(OH)2 showed higher fluorescent intensities, higher R/O values and higher quenching concentrations, this material is considered to be a promising phosphor.