Blue-emitting Eu2+-activated LaOX (X = Cl, Br, and I) materials: crystal field effect

Novel blue-emitting LaOBr:Eu(2+) and LaOI:Eu(2+) phosphors have been successfully synthesized and compared to LaOCl:Eu(2+). The emission spectra of LaOX:Eu(2+) (X = Cl, Br, and I) show that the peak maxima change somewhat to the red-shift region; 425 nm for LaOCl:Eu(2+), 427 nm for LaOBr:Eu(2+), and 431 nm for LaOI:Eu(2+), which is quite opposite to one based on spectrochemical series (I(-) < Br(-) < Cl(-)). From diffuse reflectance spectra, the band gap energies for LaOCl, LaOBr, and LaOI host lattice are estimated as 5.53 eV (44,594 cm(-1)), 5.35 eV (43,142 cm(-1)), and 4.82 eV (38,868 cm(-1)), respectively, using the Kubelka-Munk function. For LaOX host lattices, the band gap energies are gradually decreased going from Cl to I as the order of energy levels of np orbitals is Cl 3p < Br 4p < I 5p. A quantum wave function calculation from crystal field theory (CFT) indicates the same tendency with experimental data in the LaOX:Eu(2+) (X = Cl, Br, and I) phosphor materials. With considerations of the radial wave function shape, crystral structure differences and electronegativities among phosphor materials, the splitting energies of 5d orbitals are calculaed; ΔECl = 14,597 cm(-1), ΔEBr = 14,864 cm(-1), ΔEI = 15,001 cm(-1) for LaOX:Eu(2+) (X = Cl, Br, and I). It is noteworthy that the crystal field strength decreases when the interatomic distance decreases, which is probably dependent on the ionic radius of halide ions in the series of LaOX:Eu(2+) phosphor materials.