Structural investigation and Eu3+ luminescence properties of LaF3:Eu3+ nanophosphors

LaF3:Eu3+ nanophosphors were hydrothermally synthesized with different cetyltrimethyl ammonium bromide (CTAB) concentrations (0, 6, 9, and 15 mmol/L) and characterized by X-ray diffraction (XRD) analysis, photoluminescence excitation, emission spectral measurements, and Raman analysis. Among the various samples examined, the sample synthesized with 6 mmol/L of CTAB exhibited the strongest Eu3+ luminescence intensity. Rietveld fitting of the well-defined XRD data indicated that the samples synthesized with 6 mmol/L of CTAB had decreased lattice parameters compared to those of the undoped LaF3, implying an increase in the fraction of Eu3+ ions that replace the La3+ ions in the LaF3 lattice matrix. Raman analysis indicated that the decrease in the lattice parameters of LaF3 (determined from the Rietveld fitting results) was accompanied by a compressive force in the LaF3 nanocrystals. The compressive force is likely a result of the large number of Eu3+ ions (ionic radius, r = 95 pm) replacing La3+ ions (r = 106 pm) in the LaF3 lattice matrix, leading to strong luminescence emissions from the LaF3:Eu3+ nanocrystals. From the chemical point of view, the compressive forces may be explained by the good confinement of the La3+, Eu3+, and F− ions within the CTAB micelle during the LaF3 nanocrystal formation in the hydrothermal reaction.