Synthesis and luminescence characterization of rare earth ions (Tb3+, Sm3+) doped in Sr9La1-x(PO4)7 phosphors

Abstract

The Sr9La(PO4)7 phosphors with different doping concentrations of Tb3+ and Sm3+ were prepared by a solid-state reaction method, respectively. The correlation between crystal structure and photoluminescence (PL) properties of the phosphors were investigated in detail. Sm3+-doped in Sr9La(PO4)7 phosphors was crystallized in a rhombohedral structure. Under the excitation wavelength of 399 nm, Sr9La(PO4)7: xSm3+ exhibited an intense orange-red emission spectrum centered at 556 nm, 595nm, and 670 nm, which can be assigned to radiation transitions from 4G5/2 to 6H5/2, 6H7/2 and 6H9/2 of Sm3+, respectively. The optimal Sm3+ doping content was found to be 3 mol%. The concentration quenching mechanism between Sm3+ ions was attributed to the dipole-dipole interaction and the critical distance of Sm3+ was determined to be 21.917Å. The results indicate that Sr9La(PO4)7: xSm3+ phosphors could be promising candidates as orange-red phosphors for being integrated into white light-emitting diodes. The samples of Sr9La(PO4)7: xTb3+ were synthesized by a solid-state reaction method. From the XRD pattern analysis, it was confirmed that the crystal structure of samples changed from the rhombohedral (space group R3 ̅m) to the monoclinic (space group I2/a) with an increase of the x value. To obtain crystal structure information, the Rietveld refinement and high-resolution transmission electron microscope (HRTEM) of Sr9La(PO4)7: xTb3+ (x= 0, 1) were performed. It was found that Sr9La1-x(PO4)7: xTb3+ phosphors have a broad excitation band at the 200–300 nm region due to the 4f8→4f75d1 transitions of Tb3+ ions. Under the excitation of 376 nm, Tb3+-doped Sr9La(PO4)7 shows green emission with a main peak at 542 nm. As increasing of concentration of Tb3+ ions up to 100 mol%, the PL intensity of Tb3+-doped in Sr9La(PO4)7 was increased, moreover, the decay time of the Sr9La(PO4)7: Tb3+ phosphor abnormally increased from 2204 μs to 4062 μs. The results indicate that Sr9La(PO4)7: Tb3+ could be an excellent candidate as a green-emitting phosphor.