Synthesis, neutron diffraction and photoluminescence properties of a whitlockite structured Ca9MgLi(PO4)7:Pr3+ phosphor

Abstract

The whitlockite structured compound Ca9MgLi(PO4)7 has been investigated as a potential host material for down-conversion phosphors. However, no detailed crystal structure and crystallographic site occupancy have been reported for this compound previously. In this work, Pr3+ activated Ca9MgLi(PO4)7 phosphors were successfully synthesized, and the structure and photoluminescence properties were characterized. Upon neutron and X-ray powder diffraction data, the crystallographic site occupancies of M1-M5 in the whitlockite-type structure were carefully refined by the Rietveld method. The M1, M2 and M3 sites are all found to be 8-fold coordinated and fully occupied with Ca and Pr. The results also revealed that the M4 and M5 sites are co-occupied by Li–Mg and Ca–Mg, respectively. Photoluminescence (PL) study showed that the phosphor has a characteristic excitation band at the visible light region of 430–500 nm. Three peaks observed at 442, 468, and 484 nm in the excitation spectra can be ascribed to the 3H4→3P2, 3H4→3P1, and 3H4→3P0 transition of Pr3+, respectively. The phosphor doped with 0.02 Pr3+ gives the strongest emission peaked at 612 nm, with the CIE chromaticity coordinate of (0.613, 0.366). This reddish-orange emitting phosphor has good thermal stability that it retains 83% of its emission intensity at 200 °C. Energy transfer mechanism of the activators was analyzed to be dipole-dipole multipolar interaction. The present work demonstrates that Ca9MgLi(PO4)7:Pr3+ is a promising phosphor for blue-pumped white light-emitting diodes (WLEDs).