Blue-Emitting Eu²⁺-Activated LaOX (X = Cl, Br, and I) Materials: Crystal Field Effect

Abstract

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.

Dehydration synthesis and crystal structure of terbium oxychloride, TbOCl

“…Rare-earth oxychlorides, REOCl, are promising materials for various applications including use as catalysts, sensors, and phosphors (Podkolzin et al, 2007;Au et al, 1997;Peringer et al, 2009;Marsal et al, 2005,;Kim et al, 2019;Berdowski et al, 1984;Imanaka et al, 2001a,b;Okamoto et al, 2002;Kim et al, 2014). LaOCl is a stable catalyst for converting methane to methyl chloride (Podkolzin et al, 2007) and can be used as a sensor material to detect CO 2 and Cl 2 gases (Marsal et al, 2005;Imanaka et al, 2001b).…”

“…The EuOCl catalyst showed high efficiency in converting ethylene to vinyl chloride (Scharfe et al, 2016). The luminescent properties of REOX (RE = La, Eu; X = F, Cl, Br, I) can be controlled to emit a wide range of visible light from blue to red by changing the crystal symmetries and compositions (Kim et al, 2014(Kim et al, , 2019. As part of our studies in this area, we now describe the dehydration synthesis and structure of the title compound.…”

Efficient Photoluminescence of Isotropic Rare-Earth Oxychloride Nanocrystals from a Solvothermal Route

“…These features were attributed to LaOCl, as observed in previous work. [18][19][20] The slope of absorption front edge was sharper for the undoped sample than for the doped one. An additional broad contribution was detected for the Eu 3+ -doped sample between 280 and 330 nm.…”

“…LaOCl bandgap is 4.17 eV according to density functional theory (DFT) calculation, 18 and it displays several absorption features in the UV range. 19,20 IR and visible light directly excite REs emitters, whereas UV or X-Ray light excitation occur via the absorption from LaOCl followed by energy transfer to the RE emitter. LaOCl then sensitizes RE emission in the UV or X-Ray range.…”

Oxidation State and Local Structure of Chromium Ions in LaOCl

“…Due to the relatively larger ionic radius of La 3+ , other trivalent rare-earth ions can be efficiently accommodated without distortions in cation site symmetry; however, a decrease in the unit cell parameters is expected [31,34,38,43]. The situation is more complicated for activators that have several stable oxidation states; for example, it is possible to stabilize both Eu 3+ [27][28][29][30][31][32][33] and Eu 2+ [33][34][35] ions in LaOCl during the synthesis. The larger Eu 2+ ions expand the lattice and require charge compensation, possibly in the form of Cl − ion vacancies [34].…”