Vibrational and luminescent properties of LaPO4:Eu3+ with different preparation conditions

Abstract

Monoclinic and hexagonal LaPO4:5 mol.%Eu3+ phosphors were synthesized with merely adjusting pH values of solutions by a hydrothermal method at the same synthesis temperature. The microstructure, morphology and photoluminescence of Eu3+-doped LaPO4, which were influenced by different preparation conditions, were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy and luminescence spectroscopy, respectively. The specimen showed monoclinic structure in strong acid solution, however, it changed into hexagonal structure in weak acid solution and strong alkali solution. The monoclinic LaPO4:Eu3+ showed the longest length to diameter ratio, which was attributed to the preferential growth of [-112]. The monoclinic specimen exhibited a slight red shift in infrared spectra, Raman band positions and charge transfer (CT) compared with hexagonal specimens. Moreover, the calculated results of grain sizes, lattice parameters, full width at half maximum in Raman patterns and emission integral intensity were in good agreement with analysis results. The monoclinic specimen showed the maximal absolute luminescence quantum yield (0.4) and the second lifetime τ2 (0.52 ms), which was accordant with red-orange emission in CIE. The 5D0→7F1 occupied a dominate position in hexagonal specimens, which indicated that more Eu3+ in the hexagonal structure were occupied inversion center of symmetry sites.