Abstract

New phosphors Ca₂Mg(BO₃)₂: Ce³⁺ were synthesized by the solid-state reaction method at a high temperature. The phase purity was characterized by powder X-ray diffraction (XRD). The ultraviolet-visible (UV-Vis) optical properties of Ce³⁺ have been investigated, and the lowest 5d levels, the emission, and the Stokes shifts of Ce³⁺ in the host lattice were identified. In addition, its concentration quenching process was also studied. The results show that Ce³⁺ ions enter Ca²⁺ sites with only one emission in a UV-Vis range and that the optimum doping concentration is x = 0.05. The excitation and emission spectra were evaluated to clearly reveal luminescence features.

1. Introduction

Rare earth ion-doped borates were extensively studied because of their excellent luminescent properties and wide applications, such as SrB₄O₇: Eu in UV-emitting medical lamps, GdMgB₅O₁₀:Ce³⁺, Tb³⁺ as a green component of tricolor lamps, and (Y, Gd)BO₃:Eu³⁺ as a red phosphor in plasma display panel devices (PDPs) [1, 2].

Trivalence cerium ions (Ce³⁺) have 4f¹ configurations, and Ce³⁺ activated inorganic luminescent materials can realize the ultraviolet to red emission due to the host-dependent 4f-5d transition luminescence characteristics of Ce³⁺ [3–7], which makes Ce³⁺ activated phosphors significant candidates for pc-LEDs. Because the f-d transitions of Ce³⁺ are partly allowed, they have large absorption cross-sections and appear as intense bands in spectra, and hence, luminescent materials doped with Ce³⁺ absorb the excitation energy efficiently. The most famous is Y₃Al₅O₁₂: Ce³⁺, and it can absorb blue light and converts it with high efficiency into yellow emission. The phosphor is now widely used as the yellow component in GaN-based white-emitting LEDs (light-emitting diodes) [8–12].

Ce³⁺ has the 4f¹ground-state configuration with the first excited states belonging to the 5d configuration. The excitation spectrum of Ce³⁺5d-4f emission provides information on the crystal-field splitting and the centroid energy of the 5d states in a host lattice. Similar crystal-field splitting is expected for all lanthanide ions in the same host lattice. That is, when the 5d level energies of Ce³⁺ are known in a specific host lattice, they can be used to predict the 5d states for the other lanthanide ions in that same lattice. The investigation of the spectroscopic properties of Ce³⁺ in different host lattices is then important not only for possible application but also for basic research.

In 1974, Verstegen first reported the luminescence of Tb³⁺ in X₂Z(BO₃)₂ (X = Ba, Sr; Z = Ca, Mg) [13]. It is reported that Ca₂Mg(BO₃)₂ is tetragonal and that Ba₂Mg(BO₃)₂ is hexagonal and isomorphous with the mineral buetschliite. Ba₂Ca(BO₃)₂ and Sr₂Mg(BO₃)₂ are related to buetschliite, but their diffraction patterns point to a crystallographic system of lower symmetry [14, 15]. As a series of borates with relevant composition and structure, a comparison of Ce³⁺ luminescence properties in Ba₂Mg(BO₃)₂, Ba₂Ca(BO₃)₂, and Sr₂Mg(BO₃)₂ was conducted in our present work [16–20]. Ce³⁺ in isomorphic Ba₂Ca(BO₃)₂ and Sr₂Mg(BO₃)₂ showed similar excitation spectroscopic properties but was different from that in Ba₂Mg(BO₃)₂. Because of the different coordination polyhedrons, Ce³⁺ ions in former two hosts showed larger crystal-field splitting values. Owing to stronger electron-lattice interaction, the Stokes shift was larger for Ce³⁺ in Ba₂Mg(BO₃)₂. To fully present Ce³⁺ luminescence in borates X₂Z(BO₃)₂ (X = Ba, Sr, Ca; Z = Ca, Mg), here, we have reported Ce³⁺ luminescence properties in another borate Ca₂Mg(BO₃)₂.

2. Materials and Methods

The samples were prepared by a solid-state reaction route at a high temperature. The reactants include analytical grade pure CaCO₃ (AR), MgO (AR), and H₃BO₃ (AR, excess 3 mol% to compensate evaporation), and 99.95% pure rare-earth oxide CeO₂ and Na₂CO₃ (AR) were added as charge compensators in all rare-earth doped samples. According to the nominal compositions of compounds, an appropriate amount of starting materials was thoroughly mixed and ground, and subsequently, the mixture was prefired at 500°C for 1 h. After milling for a second time, the samples were calcined at 950°C for 6 h in a CO-reducing atmosphere. After these steps, the temperature was slowly cooled down to room temperature.

The structure of the final products was examined by X-ray powder diffraction (XRD) using Cu Kα radiation on a RIGAKU D/max 2200 vpc X-ray diffractometer with 40 kV, 30 mA. The UV excitation and emission spectra of the phosphors were recorded on an EDINBURGH FLS 920 spectrofluorometer at room temperature, and a 450 W xenon lamp was used as an excitation source.

3. Results and Discussion

3.1. Powder X-Ray Diffraction (XRD)

The measurements on powder X-ray diffraction for all samples were performed to verify the phase purity and to check the crystal structure. The XRD patterns of samples Ca_{2(1 − x)}Ce_xNa_xMg(BO₃)₂ (x = 0, 0.01, 0.03, 0.05, 0.07, and 0.09) are presented in Figure 1. The results show that all Ce³⁺ion-doped samples in the investigated concentration range are also of a single phase, which are also in line with an undoped sample Ca₂Mg(BO₃)₂, and the crystal structure is not significantly changed when the doped ions enter into the host and occupy Ca²⁺ normal sites. It is reported that Ca₂Mg(BO₃)₂ was isomorphous with Na₂Ca(CO₃)₂ and tetragonal with a = 5.14 Å and c = 4.41 Å [13]. According to the effective ionic radii and identical valence, Ce³⁺ will mainly enter the Ca²⁺ sites.

Figure 1 

XRD patterns of Ca_{2(1 − x)}Ce_xNa_xMg(BO₃)₂ (x = 0, 0.01, 0.03, 0.05, 0.07, and 0.09).

3.2. The UV-Vis Luminescence Properties of Ca₂Mg(BO₃)₂: Ce³⁺

A series of phosphors Ca_{2(1 − x)}Ce_xNa_xMg(BO₃)₂ (x = 0.01, 0.03, 0.05, 0.07, 0.09) were measured. As an example, the emission spectra of Ca_{2(1 − x)}Ce_xNa_xMg(BO₃)₂ (x = 0.05) at 340 nm excitation at RT are illustrated in Figure 2. Under 340 nm UV light excitation, a broad emission band peaking at ∼422 nm can be detected, marked as curve a. Usually, Ce³⁺ ions in one specific lattice site will show two emission bands corresponding to the transitions from the lowest 5d excited state to the ²F_5/2 and ²F_7/2spin-orbit split 4f ground states. The energy separation of the two emission bands coincides with the spin-orbit splitting and amounts about 2,000 cm⁻¹ [5]. Often the two bands are resolved much better at lower temperatures than at room temperature due to a decrease in the electron-lattice phonon interaction.

Figure 2 

UV-Vis emission spectra of Ca_1.9Ce_0.05Na_0.05Mg(BO₃)₂ at RT.

To determine the emission peak positions for Ce³⁺, emission spectrum a was fitted with two Gaussian profiles located at about 413 (A) and 457 nm (B). The energy difference is 2331 cm⁻¹, which is slightly larger than the expected value of 2000 cm⁻¹ between ²F_5/2 and ²F_7/2 of Ce³⁺. It may be caused by the weak influence of the nephelauxetic effect in the structure. The full width at half maxima (FWHM) of Ce³⁺ emission is estimated to be ∼91 nm.

Since the energy difference between band A and B approaches with the the ²F_5/2 and ²F_7/2spin-orbit splittings, which are attributed to the emission from Ce³⁺ at one specific site, that is to say, there is only one Ca site in Ca₂Mg(BO₃)₂, similar to Ba₂Mg(BO₃)₂, Ba₂Ca(BO₃)₂, and Sr₂Mg(BO₃)₂ structures. When the Ce³⁺ ion is incorporated in Ca₂Mg(BO₃)₂, it may substitute for Ca²⁺. We have added Na⁺ as a charge compensator during synthesis, so the emission of Ce³⁺ in Ca₂Mg(BO₃)₂: Ce³⁺ will be associated with charge-compensated Ce³⁺ centers. When Ce³⁺ enters one specific type of Ca²⁺ sites, the doped compound will show only one center emission. In addition, we might also take into account that borate samples are partially hygroscopic, and consequently, the effect of UV exposure (due to the Xe lamp) could induce the radiolysis of the adsorbed water molecules, where the hydroxyl groups can be weakly bonded to cations; these OH- ions are released due to the increase in the temperature or illumination. This interesting effect has been observed in the glow emission of many different materials, which may also be one of the inevitable influencing factors on the UV-green emission from insulator solids [21, 22].

The UV excitation spectra of Ca_1.9Ce_0.05Na_0.05Mg(BO₃)₂ under different monitoring wavelengths are displayed in Figure 3, marked as curves a, b, c, and d, respectively. Curves a, b, c, and d are similar in the range of 250–450 nm. Three broad bands labeled A (∼274 nm), B (∼309 nm), and C (∼340 nm) are clearly observed, and we consider that these bands are mainly associated with the crystal-field split 5d states of Ce³⁺ in the host lattice. As we all know, no extra line or band caused by the interactions of f-f or f-d electrons occurred in Ce³⁺ excitation spectrum, which directly exhibits the information on the crystal-field splitting of the 5d states. The 5d¹ electronic configuration can be split into 2–5 components by the crystal field, such as bands A, B, and C in Figure 3. Here, we can estimate the lowest f-d transition(5d¹) absorption of Ce³⁺ locating at ∼340 nm (band C: ∼29 412 cm⁻¹). Though the spin-orbit splitting doublet emission was not well resolved at RT, we can estimate the Stokes shift (from 340 to 413 nm) to be ∼5199 cm⁻¹. Because the Stokes shift is fairly small, obvious spectral overlap is observed between excitation and emission spectra, which directly shows that the energy transfer between neighbor Ce³⁺ is efficient.

Figure 3 

UV excitation spectra of Ca_1.9Ce_0.05Na_0.05Mg(BO₃)₂ at RT.

Figure 4 shows the emission spectra of Ca_1.9Ce_0.05Na_0.05Mg(BO₃)₂ under 254, 309, 340, and 370 nm excitation wavelengths. In Figure 3, we can see these bands were ascribed to the f-d transition absorption of Ce³⁺ in Ca₂Mg(BO₃)₂. It can be observed that these emission spectra are almost similar in shape, except for slightly red shifts at 309 and 370 nm excitation. This means that, no matter which wavelength is excited, only the emission from one lattice site can be observed, which is consistent with the above discussion.

Figure 4 

The emission spectra of Ca_1.9Ce_0.05Na_0.05Mg(BO₃)₂ under different excitation wavelengths at RT.

Figure 5 presents the emission spectra of Ca_{2(1 − x)}Ce_xNa_xMg(BO₃)₂ (x = 0.01, 0.03, 0.05, 0.07, 0.09) samples at 340 nm excitation. All the emission spectra are very similar in shape, the emission intensity first increases and then decreases regularly, and the optimal emission intensity occurs at x ≈ 0.01–0.09. The decline of the emission intensity may be ascribed to the concentration quenching effect. In Figure 6, the concentration quenching curve of Ce³⁺ emission at 340 nm excitation is plotted. It can be found that the emission intensity of Ce³⁺ first increases with an increase in Ce³⁺ concentrations (x), reaching a maximum around x = 0.05, and then decreases with an increasing concentration due to concentration quenching.

Figure 5 

The emission spectra of Ca_{2(1 − x)}Ce_xNa_xMg(BO₃)₂ (x = 0.01, 0.03, 0.05, 0.07, and 0.09) samples with λ_ex = 340 nm.

Figure 6 

The emission intensities of Ce³⁺ as a function of its doping concentration (x) in Ca_{2(1 − x)}Ce_xNa_xMg(BO₃)₂ samples at RT.

Based on the optimal Ce³⁺ doping concentration, the crucial energy transfer distance (R_c) among Ce³⁺ ions in the Ca₂Mg(BO₃)₂ host can be calculated using the following equation (23):where V is the cell volume, N is the number of cations which were substituted by Ce³⁺ ions in the unit cell, and x_c is the optimal doping concentration. In present case, V = 320.5 ų, N = 4 (for Ca sites), and x_c = 0.05. Therefore, the crucial energy transfer distance can be estimated as 8 Å. The determined R_c value is slightly larger than 5 Å, indicating that multipolar interactions are dominant for the concentration quenching of Ce³⁺.

4. Conclusions

The spectroscopic properties of phosphors Ca₂Mg(BO₃)₂: Ce³⁺ in UV-Vis ranges were investigated. The lowest 5d state of Ce³⁺ in Ca₂Mg(BO₃)₂ was observed at 340 nm, and the Stokes shift was estimated to be ∼5199 cm⁻¹. A wider broad band from 350 to 600 nm emission peaking at 413 and 457 nm was obtained. These information gives us a good idea that Ce³⁺ is a good sensitizer to the other doping ions in Ca₂Mg(BO₃)₂, such as Cr³⁺, Eu³⁺, Tb³⁺, Tm³⁺, and Nd³⁺. It should be noted it could broaden the emission wavelength range of a phosphor by codoping of Ce³⁺ and other doped ions in the Ca₂Mg(BO₃)₂ host lattice; then, potential multifunctional applications will be realized.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by the Guangdong Basic and Applied Basic Research Foundation (Nos. 2021A1515012358 and 2020A1515011403), the Innovation Team Program of Higher Education of Guangdong, China (2017KCXTD023), the Hanshan Normal University Start-Up Fund for Professor Scientific Research (QD202203), and the University-Enterprise Collaborative Innovation Center for Big Health Industry (2022 Hybribio Special Project, 0002/b22088).