A. Phase purity and crystal structure

To characterize the phase purity and crystallinity of the as-prepared powder samples, the XRD patterns for all samples were examined. In the synthesis process, Ba₂MgMoO₆:Eu³⁺ were synthesized at different temperatures, such as 800, 850, 900, 950, 1000, and 1100 °C, etc., and it is found that the Ba₂MgMoO₆ compound could only be synthesized well at 1000 °C. Figure 1 shows the XRD pattern of the powder sample Ba₂MgMoO₆:5%Eu³⁺, which matches well the standard data of perovskite structure Ba₂MgWO₆ [Joint Committee on Power Diffraction Standards (JCPDS) card no. 22-0541], except for a very weak peak at 27.50°, which might be the diffraction peak of BaMoO₄. Besides, the XRD patterns for other samples are identical to that of Ba₂MgMoO₆:5%Eu³⁺, and no extra impurity diffraction peaks are observed with different doping Eu³⁺ concentrations. This fact suggests that Eu³⁺ ions have been successfully built into the Ba₂MgMoO₆ host lattice and do not destroy the lattice structure of Ba₂MgMoO₆ host.

FIG. 1.

The XRD patterns of Ba₂Mg_0.95MoO₆:0.05Eu³⁺ sample and the standard data of Ba₂MgWO₆ (JCPDS No. 73-2404).

The crystal structure of Ba₂MgMo₆, shown in Fig. 2, was covered to be a cubic system [space group Fm-3m (225)] with lattice constants a = b = c = 8.110 Å, α = β = γ = 90°, V = 533.4 ų, and Z = 4, similar to the structure of (NH₃)₃FeF₆, where Mg²⁺ and Mo⁶⁺ are ordered in alternate {111} cation layers.^{Reference Besse, Wathle and Baud17} There exist three different sites for the cations in Ba₂MgMo₆ host, viz. Ba located at the 8c (T_d) site and Mg and Mo at the 4b (O_h) and 4a (O_h) sites, respectively.^{Reference Besse, Wathle and Baud17} The Eu³⁺ ions would substitute the Mg and Mo sites with an inversion center because the dominant lines (⁵D₀ → ⁷F₁) of the emission spectrum only correspond to an inversion center. Even though the ionic radii for Eu³⁺ (1.07 Å, eight-coordination) have an obvious gap compared with that for Mg²⁺ (0.89 Å, eight-coordination), it is much lager than that for Mo⁶⁺ (0.73 Å) in 6-fold coordination.^{Reference Su18} Thus, Eu³⁺ ions are expected to substitute for Mg²⁺ site ions in Ba₂MgMoO₆ host, and the chemical formula of Eu³⁺-doped Ba₂MgMo₆ can be the expression of Ba₂Mg_1−xMoO₆:xEu³⁺. When Eu³⁺ ions are incorporated into Ba₂MgMoO₆ host, Eu³⁺ ions substitute for Mg²⁺ sites, which might induce the vacancy defect to compensate for the excess positive charge. As a consequence, the crystal lattice for Ba₂MgMoO₆ compound might exhibit slight deformation from cubic symmetry due to the nonequivalent substitution. The compound Ba₂MgMoO₆ is particularly suitable as model for fundamental studies of certain physical properties due to the simplicity of the crystal structure.

FIG. 2.

The crystal structure of Ba₂MgMoO₆ lattice.

B. Diffuse reflectance spectra of Ba₂MgMoO₆:Eu³⁺ phosphor

Figure 3 displays the diffuse reflectance spectrum of undoped Ba₂MgMoO₆ and Ba₂Mg_0.95MoO₆:0.05Eu³⁺ samples in the spectral region from 200 to 800 nm at room temperature. It can be found that both Eu³⁺-free and Eu³⁺-doped samples exhibit the broad absorption band from 200 to 450 nm owing to the charge transfer (CT) transition from oxygen to molybdenum in MoO₆ octahedra. In light of molecular orbital theory,^{Reference Shigeo and William19,Reference Blasse and Grabmaier20} the CT band is due to the electronic transition from the 2p orbital of O²⁻ to the empty d orbital of Mo⁶⁺ in ${\rm{MoO}}_{\rm{6}}^{{\rm{6}} - }$ groups. There is no selection rule that reduces the intensity, so the CT band is broad and large.

FIG. 3.

The diffuse reflection spectra of undoped Ba₂MgMoO₆ and Ba₂Mg_0.95MoO₆:0.05Eu³⁺.

C. Photoluminescence properties of Ba₂MgMoO₆:Eu³⁺ phosphor

Figure 4 shows the excitation spectrum of Ba₂Mg_0.95MoO₆:0.05Eu³⁺ phosphors monitored at 598 nm corresponding to ⁵D₀ → ⁷F₁ transition of Eu³⁺. Surprisingly, the excitation spectrum shows an intense broad band from 250 to 450 nm. This broad band is assigned to the CT from the oxygen ligands to the central molybdenum atom in ${\rm{MoO}}_{\rm{6}}^{{\rm{6}} - }$ group. It is good agreement with the broad absorption band in the diffuse reflectance spectrum. There seems to be nonexistent of the characteristic excitation peaks of Eu³⁺. The reason for this phenomenon is resulting from the fact that the characteristic excitation peaks at about 396 nm must be overlapped by CT band. And the relative intensity of other f–f transitions of Eu³⁺ is much weaker than that of CT band. For this reason, the excitation spectrum comes out as shown in Fig. 4. The full width at half maximum (FWHM) of CT band is approximately 80 nm, which can cover the whole emission wave lengths (395–405 nm) of near UV LED chips (InGaN chip, λ_em = 400 nm).

FIG. 4.

Excitation spectrum of Ba₂Mg_0.95MoO₆:0.05Eu³⁺ phosphor (λ_em = 598 nm).

Figure 5 displays the emission spectrum of Ba₂Mg_0.95MoO₆:0.05Eu³⁺ phosphors under either CT band or 394 nm at room temperature. There exists one dominant emission peak corresponding to the parity-allowed magnetic dipole (MD) transition of ⁵D₀ → ⁷F₁ and the others are very weak as shown in Fig. 5. On the basis of the Judd-Ofeld theory,^{Reference Shigeo and William19} the electronic dipole (ED) transition ⁵D₀ → ⁷F₂ with ∆J = 2 would be dominant when Eu³⁺ ion occupied the lattice site without an inversion center and is hypersensitive while the MD transition ⁵D₀ → ⁷F₁ with ∆J = 1 is insensitive to the crystal field environment. Therefore, the intensity ratio of the transitions ⁵D₀ → ⁷F₁ to ⁵D₀ → ⁷F₂,

$${{{\rm{MD}}} \over {{\rm{ED}}}}{\rm{ = }}{{I{{\rm{(}}^{\rm{5}}}{{\rm{D}}_{\rm{0}}}{ \to ^{\rm{7}}}\,{{\rm{F}}_{\rm{1}}}{\rm{)}}} \over {I{{\rm{(}}^{\rm{5}}}{{\rm{D}}_{\rm{0}}}{ \to ^{\rm{7}}}\,{{\rm{F}}_{\rm{2}}}{\rm{)}}}}\quad ,$$

can be calculated to measure the symmetry of Eu³⁺ site. Obviously, the emission intensity of ⁵D₀ → ⁷F₁ transition is much higher than that of ⁵D₀ → ⁷F₂ transition, the value of MD/ED for Ba₂Mg_0.95MoO₆:0.05Eu³⁺ sample is 8.4. This clearly indicates that Eu³⁺ ions mainly occupy the lattice site of symmetric environment in the Ba₂MgMoO₆ host. The result should come out as no surprise because Mg²⁺ sites for substituted Eu³⁺ have inversion symmetry in this case. It is noted that the most intensive emission at 598 nm is much narrow with a full width at half maximum less than 2 nm.

FIG. 5.

Emission spectrum of Ba₂Mg_0.95MoO₆:0.05Eu³⁺ phosphor (λ_ex = 394 nm).

The effect of the Eu³⁺ doping concentration (x) in Ba₂Mg_1−xMoO₆:xEu³⁺ (x = 0.01, 0.02, 0.03, 0.05, 0.08, 0.15, and 0.20) phosphors on the relative photoluminescence intensity at highest ⁵D₀ → ⁷F₁ transition is shown in Fig. 6. Generally speaking, a low doping concentration gives a weak emission and it is unfavorable to luminescent applications, whereas an overdoping ratio perhaps brings about quenching of the luminescence because the increase in the doping concentration results in the enhancement of nonradiative relaxation between the neighboring Eu³⁺ ions. The luminescence intensity of Ba₂Mg_1−xMoO₆:xEu³⁺ phosphors enhances with the increase of the Eu³⁺ doping ratio and the maximum emission intensity appears at x = 0.05. When the Eu³⁺ doping ratio is higher above 0.05, the luminescence intensity would decrease. Therefore, the optimum doping concentration of Eu³⁺ in Ba₂Mg_1−xMoO₆:xEu³⁺ phosphor is about 0.05. Apparently, a low concentration quenching occurs in Ba₂MgMoO₆:Eu³⁺. Quenching interactions between neighboring Eu³⁺ ion cannot maybe take place due to such a low doping Eu³⁺ concentration. So quenching correlates with the probability of pairing Eu³⁺ ions through intervening oxygens rather than with concentration.^{Reference Van Uitert and Iida21} And it depends upon the overlap of the wave function of the magnetic ions with those of a dumbbell-shaped p-orbital of an intervening oxygen ion. The closer the angle is to 180°, the stronger the interaction. In Ba₂MgMoO₆, the Ba–O–Ba angle is equal to 177.084°, and the Mg–O–Mg angle is about 115.734°or 180°. Thus, interactions between neighboring Eu³⁺ ions through intervening oxygens are involved in quenching. Dexter also pointed out that quenching may occur as a result of energy transfer by means of identical ions to one which acts as a sink due to its presence next to a defect.^{Reference Dexter and Schulman22} For Ba₂MgMoO₆:Eu³⁺, the lattice defects appear due to the nonequivalent substation. Hence, the Eu³⁺ ion next to the defect might lose the energy to the host structure through interaction with the phonon spectrum.

FIG. 6.

The photoluminescence intensity (integrated intensity of ⁵D₀ → ⁷F₁) of Eu³⁺ ions as a function of its doping concentration (x).