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 Ca-10(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 Eu ions. An abnormal strong intensity of D-5(0)-F-7(0) transition was observed in bulk and larger Ca10-xEux(PO4)(6)(OH)(2) nanocrystals, but the relative intensities of D-5(0)-F-7(0) transition to D-5(0)-F-7(1,2,3,4) transition of Eu3+ became weaker as the particle sizes decreased. As the particle sizes became smaller, the ratios of the red emission transition (D-5(0)-F-7(2)) to the orange emission transition (D-5(0)-F-7(1)) (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. (C) 2007 Elsevier B.V. All rights reserved.