Gd₃Li₃Te₂O₁₂:U⁶⁺,Eu³⁺: A Tunable Red Emitting Garnet Showing Efficient U⁶⁺ to Eu³⁺ Energy Transfer at Room Temperature

Abstract

Since the invention of fluorescent light sources, there is strong interest in Eu³⁺ activated phosphors as they are able to provide a high color rendering index (CRI) and luminous efficacy, which will also hold for phosphor converted light emitting diodes. Due to an efficient U⁶⁺ to Eu³⁺ energy transfer in Gd₃Li₃Te₂O₁₂:U⁶⁺,Eu³⁺, this inorganic composition shows red photoluminescence peaking at 611 nm. That means Eu³⁺ photoluminescence can be nicely sensitized via excitation into the U⁶⁺ excitation bands. Therefore, photoluminescence properties, such as temperature dependent emission and emission lifetime measurements, are presented. Charge transfer bands were investigated in detail. Additionally, density functional theory calculations reveal the band structure of the pure, i.e., non-doped host material. Obtained theoretical results were evaluated experimentally by the aid of diffuse UV reflectance spectroscopy.

1. Introduction

In gas discharge light sources, Eu³⁺ activated phosphors (e.g., Y₂O₃:Eu³⁺, (Y,Gd)BO₃:Eu³⁺, and Y(V,P)O₄:Eu³⁺) are widely applied to generate the red fraction of the white lamp spectrum [1,2]. Due to insufficient absorption in the blue spectral range, these phosphors are not applicable in combination with a blue emitting (In,Ga)N light emitting dioded (LED) [3,4,5]. Yet, line emitting Eu³⁺ activated phosphors possess promising properties such as high luminous efficacy, long-term stability, and they are capable of providing a high color-rendering index [6,7]. To improve the absorption cross-section, sensitizers like Bi³⁺ and Tb³⁺ are often used [6,8,9]. Additionally, Blasse and Krol reported about an energy transfer from U⁶⁺ to Eu³⁺ in several compounds. However, this energy transfer only occurs in most compounds at rather low temperatures [10,11]. To further investigate the energy transfer from U⁶⁺ to Eu³⁺, we examined the luminescence properties of Gd₃Li₃Te₂O₁₂:U⁶⁺,Eu³⁺. The luminescence properties of Gd₃Li₃Te₂O₁₂:U⁶⁺, as well as Gd₃Li₃Te₂O₁₂:Eu³⁺, were already described a few decades ago, but a co-doped sample has not been investigated so far [12,13]. The yttrium analogue is also well known when doped with U⁶⁺ and Eu³⁺, respectively [14,15]. The excitation band of the uranate group in the Y³⁺ garnet is located at 325 nm and thus superimposes with the ⁷F₀ → ⁵H_J transitions of Eu³⁺ [14]. In case of the Gd³⁺ garnet, the excitation band is located at 340 nm, which allows one to solely excite the uranate group. This allows for a precise investigation of the energy transfer. Thereby, we strive for a deeper insight into the U⁶⁺ to Eu³⁺ energy transfer. Furthermore, we discuss the usefulness of U⁶⁺ as a sensitizer in the UV-A range for Eu³⁺ activated red-emitting phosphors.

The authors are aware that uranium is a radioactive and toxic element, so commercial use should not be a priority. Depleted uranium, which consists mainly of U²³⁸ isotopes, shows an alpha decay for the most part. With LED encapsulation, the alpha particles would not be emitted into the atmosphere, but are captured by the encapsulation. Consequently, the radiation exposure will be almost zero. Depleted uranium, however, is used as a ballast in space technology. No radiological and chemical toxicities were detected there [16]. Recent calculations demonstrated that the theoretically used amounts of uranium in LEDs would not even exceed the limits of German laws for 10 L of drinking water. Therefore, from our point of view, it is reasonable to investigate U⁶⁺ as a sensitizer for Eu³⁺ phosphors [17].

2. Method of Calculation and Electronic Properties

The Cambridge Serial Total Energy Package (CASTEP) Module of Materials Studio 8.0 (Devoloped by Accelrys) was applied to execute DFT calculations. A non-local sX-LDA functional (local density approximation, abbrv. LDA, with exchange contribution replaced by screeened exchange, abbrv. sX) was applied to the Gd₃Li₃Te₂O₁₂ host structure [18,19,20]. The entire calculation process used a plane-wave basis, linear response functions, norm-conserving pseudo potentials, Pulay density mixing schemes, and fine interpolation methods. If density mixing was not applicable, all bands/ensemble density functional theory (EDFT) was used. For a description of the interaction between the ionic cores and the valence electrons, the norm-conserving pseudopotentials were applied. The following electronic configurations were set in the calculations: Gd: [Xe] 4f⁷5d¹6s²; Te: [Kr] 4d¹⁰5s²5p⁴; Li: [He]2s¹; O: [He] 2s²2p⁴. The Monkhorst-Pack k-points were set to 10 × 10 × 10. The calculations were performed for a cubic cell. The energy convergence parameter was 5 × 10⁻⁶.

The obtained band structure of Gd₃Te₂Li₃O₁₂ is depicted in Figure 1. The following direct band gap was derived from the calculations and determined to be ≈5 eV. This allowed us to classify this compound as a wide band-gap dielectric. Commonly, the usual LDA method underestimates the real band gap. The mean variation goes up to 40% [20,21]. More accurate band gap values are more likely to be expected with the sX-LDA approach. Therefore, we have selected the sX-LDA method for the calculations. An approach using UV reflectance spectroscopy described by Jüstel et al. to derive the optical band gap (Figure 2) served for a Kubelka-Muk transformed Tauc absorption spectrum. These experimental results were compared with the theoretical values [22,23,24,25].

As demonstrated by the absorption spectrum (inset of Figure 2), the optical band gap amounts to ≈5 eV, which is in good accordance with the calculated value obtained from the sX-LDA method. The sX-LDA calculated density of states (DOS) diagrams are depicted in Figure 3. The conduction band mainly consisted of the Te 5s and 5p, as well as the Li 2s states. The valence band (VB) had a width of ≈7 eV and exhibited several narrow sub-bands. The VB was dominated by O 2p states. In addition, the Gd 4f states showed a noteworthy contribution to the VB. In between, the Te 4p states had a major influence on the VB at around ≈6 eV. The wide band gap of the host, taken together with the experimental excitation spectra of U⁶⁺ in Gd₃Te₂Li₃O₁₂, allowed for the conclusion that the energy levels of the U⁶⁺ ion were located in the band gap.

3. Results and Discussion

The recorded XRD patterns as depicted in Figure 4 indicate the formation of cubic Gd₃Li₃Te₂O₁₂ and Eu₃Li₃Te₂O₁₂ in the space group Ia$\overline{3}$d [15,26]. As in a typical garnet structure, Li atoms occupied the tetrahedral sites, and Te and Gd atoms were coordinated in octahedral and square antiprism geometry, respectively. Due to the coinciding ionic radii of Gd³⁺ (105 pm) and Eu³⁺ (106 pm) in eightfold coordination, no changes in the structure could be observed upon substituting Gd³⁺ with Eu³⁺ [27]. The refined cell parameters of Gd₃Li₃Te₂O₁₂ with 1% U⁶⁺ were a = 1.2386 nm and V = 1.9003 nm³, and for Eu₃Li₃Te₂O₁₂ with 1% U⁶⁺, a = 1.2423 nm and V = 1.9176 nm³, which nicely fit with literature data published before [26]. The intermediate samples with different Eu³⁺ content showed a linear behavior of the cell parameters. The critical U⁶⁺ concentration amounted to 1 mol % for the [TeO₆]⁶⁻ site [14].

SEM micrographs of the sample (Gd_0.8Eu_0.2)₃Li₃(Te_0.99U_0.01)₂O₁₂, as shown in Figure 5, reveal a particle size of ≈1 μm, which form bigger agglomerates with a diameter of ≈20 μm. Samples doped with U⁶⁺ had a beige body color under natural light, indicating an absorption in the blue spectral range. Samples in the absence of U⁶⁺ exhibited a white body color.

The body color could be derived from the reflectance spectra in Figure 6. Gd₃Li₃Te₂O₁₂ showed one strong absorption band at 250 nm. This could be assigned to the O²⁻ to Te⁶⁺ charge transfer (CT) in the [TeO₆]⁶⁻ octahedron [13,28]. In addition, at 276 and 313 nm, Gd³⁺ transitions from the ground state ⁸S_7/2 to the ⁶I_J and ⁶P_J levels were observed. Incorporation of Eu³⁺ created an additional band at 260 nm representing the CT from O²⁻ to Eu³⁺. The typical 4f → 4f transitions of Eu³⁺ were also correspondingly present. U⁶⁺ doped samples exhibited several absorption bands in the UV and blue–green spectral range. The precise assignment turned out to be difficult. Bands located in the UV range could tentatively be assigned to parity-allowed CT transitions involving the 6d level of U⁶⁺, whereas parity-forbidden transitions involving the 5f level generated the absorption bands in the visible region [14,29].

The excitation and emission spectra of the single and co-doped samples are plotted in Figure 7. Gd₃Li₃(Te_0.99U_0.01)₂O₁₂ showed parity-forbidden U⁶⁺ emission in the octahedral coordination, peaking at 550 nm. At 3 K, a fine structure of the emission could be observed, revealing a zero-phonon line at 18,498 cm⁻¹ with a high intensity due to the low site symmetry of the Te⁶⁺ site (S₆). Additionally, vibronic modes coupled with the CT transition could be identified. It was possible to assign some internal vibrational modes in the [UO₆]⁶⁻ octahedron. Other than with the UO₂²⁺ emission, the coupling took place with more than one mode [14]. In addition, the ungerade vibrational modes dominated the coupling mechanism [30]. For ν₆ (T_2u), ν₄ (T_1u), and ν₃ (T_1u), we found energy values of 196, 299, and 576 cm⁻¹, respectively. The basic nature of the U⁶⁺ emission process is still not known. However, it was assumed that a charge-transfer transition between the O 2p and U 5f states led to the green emission [31]. In addition, coupling with ungerade vibrational modes indicated a parity-forbidden transition [32]. The emission spectra of the co-doped sample, as well as the Eu³⁺ doped sample, was dominated by the ⁵D₀ → ⁷F₂ transition resulting from a low symmetry of the Gd³⁺ site (D₂). The excitation spectrum of the co-doped sample was evidence for the U⁶⁺ to Eu³⁺ energy transfer. While monitoring the ⁵D₀ → ⁷F₂ transition of Eu³⁺, the parity-allowed CTs of U⁶⁺ were observed. In addition, the Gd³⁺ to U⁶⁺ energy transfer, as described earlier by Smit and Blasse, was present [12]. Additionally, two excitation bands in the visible spectral range around 400 and 470 nm with rather low intensity could be observed, which match well with the diffuse reflectance spectra. In order to reveal the energetic positions of the CT bands, excitation spectra at 3 K were disentangled with the aid of Gaussian peak fitting. Figure 8a demonstrates that U⁶⁺ could be excited via three different U⁶⁺ CT bands, peaking at 30,821, 29,303, and 35,435 cm⁻¹, as well as via the Te⁶⁺ CT band at 38,846 cm⁻¹. The Eu³⁺ emission could be excited through the Te⁶⁺ CT band and the Eu³⁺ CT band located at 37,202 cm⁻¹. The co-doped sample contained too many parameters, thus many possibilities were available to fit with Gaussian components. Hence, here the U⁶⁺ CT band was shifted from 35,435 towards 34,722 cm⁻¹.

Emission spectra of the solid solution according to (Gd_1−xEu_x)₃Li₃(Te_0.99U_0.01)₂O₁₂ with 0 ≤ x ≤ 1 are depicted in Figure 9 and unveil that a Eu³⁺ concentration of 20% yielded the highest emission integral for the emission at 611 nm. With an increasing Eu³⁺ content, the emission integral of U⁶⁺ emission decreased, highlighting the energy transfer. In other words, after the excitation of the uranate group, the absorbed energy was transferred to Eu³⁺. Therefore, the U⁶⁺ emission was hardly visible at a Eu³⁺ concentration higher than 10%. The samples were thus capable of generating green to red light under 338 nm excitation governed by the Eu³⁺ concentration. Color coordinates, as well as the luminous efficacies and the external quantum efficiencies, of each sample are listed in

Table 1. External quantum efficiencies, color coordinates, and luminous efficacy of (Gd_1−xEu_x)₃Li₃(Te_0.99U_0.01)₂O₁₂ with 0 ≤ x ≤ 1 upon 338 nm excitation.

Eu3+ Conc.	EQE	CIE1931 Color Coordinate		Luminous Efficacy
%	(λEx = 338 nm)	x	y	[lm/Wopt]
0	32	0.3812	0.6119	632
1	39	0.4194	0.5748	576
3	39	0.4655	0.5297	511
5	41	0.5031	0.4931	464
10	42	0.5514	0.4459	405
20	38	0.5921	0.4061	357
40	16	0.5933	0.4048	352
70	2	0.5747	0.4224	364
100	<1	0.6079	0.3855	271

. We define the external quantum efficiency as the ratio of absorbed and emitted photons. The luminous efficacy decreased with increasing Eu³⁺ content as the emission was shifted from the green to the red spectral range, where the human eye is much less sensitive. The highest external quantum efficiency was reached at 10% Eu³⁺ and amounted to 42%. The quantum efficiency was thus rather low due to quenching of the U⁶⁺ CT process already at room temperature as well as some re-absorption in the red region as visible in the diffuse reflectance spectra [12]. In addition, due to the synthesis, the uranate group could show some mixed valence states, i.e., U⁵⁺/U⁶⁺. The resulting metal-to-metal charge transfer would decrease the external quantum efficiency as well as explain the slightly brownish body color. The quantum efficiency further decreased with increasing Eu³⁺ content. We thus suggest that the introduction of Eu³⁺ generated traps in the host compound and thus increased the overall defect concentration. Moreover, Eu³⁺ tends to show concentration quenching in some compounds, which further reduces the external quantum efficiency at higher Eu³⁺ concentrations [4,6,9,33]. At first, the traps were capable of inhibiting the quenching mechanism as we will discuss later. However, at higher Eu³⁺ concentrations, the number of defects was too advanced.

Fluorescence lifetime measurements monitoring the U⁶⁺ emission at 550 nm of co-doped (Gd_1−xEu_x)₃Li₃(Te_0.99U_0.01)₂O₁₂ with 0 ≤ x ≤ 0.2 were conducted and plotted in Figure 10. The obtained decay curves with x > 0 could be well-fitted with a bi-exponential fitting function. The bi-exponential curve shape reflected the energy transfer from U⁶⁺ to Eu³⁺. The single-doped sample exhibited only a mono-exponential behavior. Therefore, co-doping with Eu³⁺ led to a second mechanism in which manner the excited U⁶⁺ state could abate. The averaged fluorescence lifetimes τ of the U⁶⁺ emissions are listed in

Table 2. Fluorescence lifetimes τ₁ and τ₂, as well as averaged lifetimes τ, of the U⁶⁺ emission in (Gd_1−xEu_x)₃Li₃(Te_0.99U_0.01)₂O₁₂.

Sample (x)	Fraction 1 (%)	τ1 (μs)	Fraction 2 (%)	τ2 (μs)	τ (μs)
0	100	127.5
1	78	107.8	22	186.8	125.2
3	58	88.8	42	163.7	120.2
5	43	71.0	57	146.1	113.7
10	40	61.6	60	144.0	111.0
20	36	52.4	64	138.7	108.1

. With increasing Eu³⁺ content, the fluorescence lifetimes of the U⁶⁺ emission decrease. As energy transfer processes were often much faster than radiative transitions, this proved the presence of an energy transfer from U⁶⁺ to Eu³⁺ [34].

The decay time of U⁶⁺ at 3 K was found to be ≈250 μs and was shorter than in other compounds, e.g., ordered perovskites (300 μs), indicating a more allowed transition. Generally, the U⁶⁺ decay time strongly depended on the site symmetry, which became shorter when inversion symmetry was no longer present [31,32]. While U⁶⁺ was tentatively occupying the Te⁶⁺ site, it exhibited a local symmetry of S₆, which has no inversion center.

Emission spectra of U⁶⁺ at various temperatures are shown in Figure 11. At higher temperatures, the typical spectral broadening was observed. Already at 150 K, distinct phonon coupled transitions were visible. The determined emission integrals, as well as the emission lifetimes at different temperatures of single-doped Gd₃Li₃(Te_0.99U_0.01)₂O₁₂, are depicted in Figure 12. The decrease of the decay time over increasing temperature was probably due to an increase of the transition probability [33]. Due to orbital mixing, the parity-forbidden character was likely weakened, leading to shorter decay times. The deviation from the sigmoidal shape at higher temperatures was tentatively caused by another excited state, as presumed by Bleijenberg [35,36]. At higher temperatures, another excited level was populated, which exhibited a different decay time. The thermal quenching temperature T_1/2 of the emission intensity depends on the energy difference between the position of the excitation and emission band. With a difference of 11,825 cm⁻¹, the quenching temperature should be between 450 and 500 K, and thus lower than in Y₃Li₃Te₂O₁₂ (T_1/2 = 540 K) due to longer Te–O bond lengths in Gd₃Li₃Te₂O₁₂ [31]. The experimental results indicated a quenching temperature of around 475 K. Blasse also suggested that the process of temperature quenching occurs via three quenching states in the configurational-coordinate diagram since the low energy gap between the ground state and the emitting state would lead to a much lower quenching temperature [31]. Already at low temperatures, quenching sets in, which might be defect related since the external quantum efficiencies were rather low.

Figure 13 shows the temperature dependence of the emission intensity of Eu³⁺ excited by 338 nm radiation as well as emission lifetimes monitored at 611 nm of co-doped (Gd_0.8Eu_0.2)₃Li₃(Te_0.99U_0.01)₂O₁₂. The sample shows the typical quenching behavior with a thermal quenching temperature of about 550 K. The quenching of the U⁶⁺ centered photoluminescence seemed to not affect the energy transfer from U⁶⁺ to Eu³⁺. Thus, the energy transfer was a competitive process for the quenching mechanism. When the temperature-dependent U⁶⁺ emission of the same sample was investigated (Figure 14), it turned out that the quenching mechanism was somewhat hampered. The emission integrals started to decrease faster than the single-doped sample, but changed over to a plateau above 250 K. At higher temperatures, an increase could be observed. The decay measurements could only be fitted with a tri-exponential fit, emphasizing that several energy levels were present. The averaged decay lifetimes showed a similar behavior as the emission integrals. Due to orbital mixing, which weakened the parity regulation, the averaged emission lifetimes exhibited a more rapid decrease. At around 325 K, the lifetimes started to increase again, indicating that another energy level was populated. This phenomenon was more strongly pronounced than in the single-doped sample. The emission intensity, as well as the emission lifetimes, started to quench at around 475 K. Co-doping the sample with Eu³⁺ might have introduced some traps that prevented the reach of the cross-over point, which led to a relaxation into the ground-state level.

4. Experimental Section

A solid solution series of (Gd_1−xEu_x)₃Li₃Te₂O₁₂:U⁶⁺ was prepared using a conventional solid state reaction. The U⁶⁺ concentration was set to 1 at % relative to the Te site to avoid possible concentration quenching [14]. High purity reagents Gd₂O₃ (Treibacher, Althofen, Austria, 99.99%), Li₂CO₃ (Alfa Aesar, Ward Hill, MA, USA, 99%), TeO₂ (Alfa Aesar, 99.99%), Eu₂O₃ (Treibacher, 99.99%), and UO₂(NO₃)₂⋅6H₂O (made of uranium metal, Merck KGaA, Darmstadt, Germany, 99.99%) were weighed in stoichiometric amounts and rigorously mixed with hexane in an agate mortar. The dried blends were transferred to corundum crucibles and fired at 900 °C for 10 h under flowing oxygen. All samples containing U⁶⁺ exhibited a beige body color. The non-doped sample and the sample doped only with Eu³⁺ showed a white body color.

Phase purity of the synthesized samples was controlled using powder X-ray diffraction (XRD). All XRD patterns were recorded on a Panalytical (Almelo, The Netherlands) X’Pert PRO MPD diffractometer working in Bragg-Brentano geometry using Cu Kα radiation.

Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded on an Edinburgh Instruments (Livingston, UK) FLS980 spectrometer equipped with a Xe arc lamp (450 W) and a Peltier cooled (−20 °C) single-photon counting photomultiplier (Hamamatsu (Hamamatsu, Japan), R2658P). The emission spectra were corrected by applying a correction file obtained from a tungsten incandescent lamp certified by the National Physical Laboratory UK.

For time resolved spectroscopy, a micro-second pulsed Xe lamp (Heraeus (Hanau, Germany) µF920H) was used.

Temperature dependent PL spectra measurements from 77 to 500 K were performed using an Oxford Instruments (Abingdon, UK) cryostat MicrostatN2. Liquid nitrogen was used as a cooling agent. The temperature stabilization time was set to 30 s with a tolerance of ±3 K. Measurements below 77 K were performed using an Oxford Instruments Optistat AC-V 12 closed cycle He-cryostat.

Diffuse reflectance spectra (DRS) were recorded on an Edinburgh Instruments FS920 spectrometer equipped with a Xe arc lamp (450 W), a Peltier cooled (−20 °C) single-photon counting photomultiplier (Hamamatsu R928), and a Spectralon (Labsphere, North Sutton, NH, USA) integration sphere. BaSO₄ (99.998%, Sigma Aldrich, St. Louis, MO, USA) was used as a reflectance standard. External quantum efficiencies (eQE) were determined using the approach of Kawamura et al. [37].

Scanning electron microscopy (SEM) micrographs were recorded on a Zeiss (Oberkochen, Germany) EVO MA10 equipped with a secondary electron detector. SEM was operated in high vacuum mode (P = 10⁻⁷ Pa).

5. Conclusions

Solid solutions according to the formula (Gd_1−xEu_x)₃Li₃(Te_0.99U_0.01)₂O₁₂ with x = 0…1 were successfully synthesized via a conventional solid-state reaction. Diffuse UV reflectance spectroscopy, as well as DFT calculations, revealed the band structure of the host. The band gap was determined to be about 5 eV, both from theory and from experiment. Photoluminescence measurements revealed that Gd₃Li₃Te₂O₁₂:U⁶⁺,Eu³⁺ was capable of generating green band and red line emission peaking at 550 and 611 nm, respectively. At low temperatures, vibronic modes coupled to U⁶⁺ emission became visible, allowing the assignment of the energy of some ungerade vibrations. A distinct zero-phonon line was observed at 540.60 nm. Several excitation bands could be assigned to CT transitions from O^2- to Te⁶⁺, U⁶⁺, and Eu³⁺, demonstrating the presence of efficient energy transfer from U⁶⁺ to Eu³⁺ even at high temperatures. Temperature-resolved photoluminescence and emission lifetime measurements confirmed the assumptions of Blasse and Bleijenberg concerning the energy level structure of U⁶⁺. The thermal quenching temperature of the 4f–4f intraconfigurational transitions of Eu³⁺ was found to be at 550 K, which was independent of the U⁶⁺ quenching mechanism. Finally, it turned out that the introduction of Eu³⁺ into the host structure generated traps, which hampered the quenching of U⁶⁺.