Synthesis and Luminescence Properties of Green-to-Red Color-Tunable Upconverting K₂Gd(PO₄)(WO₄):Yb³⁺,Tb³⁺,Eu³⁺ Phosphors

Abstract

Scientists are increasingly interested in new inorganic luminescence materials that could be excited with near-infrared (NIR) radiation. These materials can be used as luminescent thermometers, bio-imaging agents, anti-counterfeiting pigments, etc. In this manuscript, we report the synthesis and investigation of optical properties of two series of K₂Gd(PO₄)(WO₄):20%Tb³⁺ (KGPW): the first, KGPW:20%Tb³⁺ doped with 1–20% Eu³⁺, and the second, KGPW:10%Yb³⁺,20%Tb³⁺ doped with 1–20% Eu³⁺. The phase-pure specimens were prepared using a solid-state synthesis method. Down-shifting and upconversion luminescence studies have been performed using 340 and 980 nm excitation, respectively. For upconversion emission luminescence, Yb³⁺ ions were used as sensitizers in the KGPW phosphors. In these phosphors, Yb³⁺ ions absorb the 980 nm radiation and transfer the energy to Tb³⁺ ions. At his point, Tb³⁺ ions either emit themselves or transfer part or all of their energy to Eu³⁺ ions. It was observed that the emission color of the synthesized phosphors could be successfully tuned from the green to red by varying the Tb/Eu concentration ratio regardless of the 340 or 980 nm excitation. Such color change proves that one luminescent material (KGPW) can provide three colors (i.e., green, orange, and red). Herein, the optical properties, such as reflection, down-shifting excitation and emission spectra, upconversion emission spectra, fluorescence lifetime, thermal quenching, color coordinates, and quantum efficiency, were studied using steady-state and kinetic spectroscopy.

1. Introduction

In modern times, luminescent materials are still engaging due to their range of applications and wide variety of fields to potentially be applied to. Inorganic luminescent materials doped with trivalent lanthanide ions (RE³⁺) are applied in solar cells [1], scintillators [2], light-emitting diodes [3], lasers [4], upconverting materials [5], security pigments [6], nanoscale optical writing [7], nanoscopy [8], etc. When RE³⁺ ions are introduced into suitable host matrices, partly forbidden f-f transitions become weakly allowed, and doped materials show photoluminescence properties. Thus, photoluminescence could be observed in two ways: down-shifting (DS) and upconversion (UC). When materials are excited with UV radiation, DS is observed. UC is a process wherein materials are excited with low-energy photons, commonly from laser excitation, and emit photons with higher-energy. Upconverting materials consist of a host doped with sensitizer and activator ions [9]. The well-known, widely used and reviewed sensitizer/activator pairs are Yb/Ho, Yb/Er, and Yb/Tm. The Yb³⁺ energy level structure is rather simple. It consists of only two energy levels: ²F_7/2 (ground state) and ²F_5/2 (excited state). The energy of the excited level matches well with the f-f energy levels of other RE³⁺ ions. The Yb/Tb sensitizer/activator pair is not widely studied, giving an opportunity to find novel UC phosphors. The excited state of Yb³⁺ (²F_5/2) overlaps with Tb³⁺ excited states (⁵D₂ and ⁵D₄), allowing energy transfer from Yb³⁺ to Tb³⁺.

Up until now, the most efficient UC lattice is still NaYF₄ [10]; this particular host is broadly discussed [11,12,13,14]. This host lattice has already been known for five decades, even though the synthesis of phase-pure NaYF₄ is still challenging. Usually, NaYF₄ synthesis yields two phases (hexagonal β–phase and cubic α–phase), and the synthesis of such nanoparticles requires a large amount of organic solvents [15,16]. For this reason, novel inorganic UC materials have received more attention.

The Yb/Tb/Eu system is the new alternative for the red-emitting Yb/Er sensitizer/activator pair and could possess a longer luminescence lifetime [17]. The upconverting materials possessing Tb³⁺ and Eu³⁺ allow the green/red emission ratio to change in the same matrix. Energy levels of Yb³⁺ do not overlap with Eu³⁺ energy levels. Due to this mismatch in energy levels, energy cannot be transferred from Yb³⁺ to Eu³⁺ directly. On the other hand, Tb³⁺ and Eu³⁺ energy levels overlap perfectly, and energy is transferred by a Yb³⁺ → Tb³⁺ → Eu³⁺ mechanism [18,19,20]. The excited state of Tb³⁺ (⁵D₄) has a similar energy to the ⁵D₀ level of Eu³⁺ ions. In this system, Tb³⁺ plays the role of the bridge between Yb³⁺ and Eu³⁺ ions. Thus, this mechanism allows the achievement of a color-tunable emission by changing the Eu³⁺ concentration in the samples.

There are many reasons why Eu³⁺ upconversion is worth analyzing [21,22]. The main reason is that Yb³⁺/Tb³⁺/Eu³⁺ has not been widely studied yet and that gives us an opportunity to find new information about this system and the upconversion mechanism. Furthermore, the long luminescence lifetime of Eu³⁺ ions could be interesting for bio-imaging processes, using the Yb → Tb → Eu energy transfer mechanism [17]. Thus, the synthesized materials could be used as a red solid-state laser based on upconversion luminescence [23].

Tb³⁺ and Eu³⁺ ions are well known for their high emission efficiency related to the largest energy gap between emitting states and lower lying ⁷F_J (J = 0, 1, 2, 3, 4, 5, 6, and 7) within lanthanides. Therefore, it is very interesting to study this UC system. For a better understanding, excitation and emission lines together with energy transfer are represented in fragments of Tb³⁺, Eu³⁺, and Yb³⁺ in Dieke’s diagram (see Figure 1). Tb³⁺ is directly excited with a 340 nm excitation wavelength, and the ⁷F₆ energy level is excited to ⁵D₂ and then transferred two ways: (1) the energy relaxes to ⁵D₃, ⁵G₆, or ⁵D₄ and emits radiatively; and (2) the energy from ⁵D₂ or ⁵D₄ is transferred to Eu³⁺ and emits from ⁵D₀ to ⁷F_4;2;1 energy levels. For the upconversion optical property investigation, ions should be excited with a laser. For this reason, Yb³⁺ ions were incorporated in the matrix. The Yb³⁺ absorbs the laser radiation via the ²F_7/2 → ²F_5/2 transition and transfers it to Tb³⁺ which emits itself and transfers the energy further to Eu³⁺. The energy from Yb³⁺ could not be transferred directly to Eu³⁺ ions as was confirmed by the emission spectra of the KGPW:10%Yb³⁺,20%Eu³⁺ sample when the sample was exposed to 980 nm laser radiation (please refer to Figure S1 see Supplementary Materials).

The KGPW host was extensively studied in our previous work. We have shown that the KGPW host is suitable for traditional upconversion pairs such as Yb/Er [24], Yb/Ho [25], and Yb/Tm [26]. The obtained upconverting materials showed emission in red, green/red, and blue/deep-red spectral areas, respectively. Herein, we investigate a more difficult system in the same host where three ions are involved in the upconversion process. The main aim of this work was to obtain new inorganic materials with tunable emission from the green-to-red spectral range under 340 and 980 nm excitation. For this reason, the KGPW:20%Tb³⁺ and KGPW:10%Yb³⁺,20%Tb³⁺ doped with 1–20% Eu³⁺ were synthesized using the solid-state reaction method at a fairly low temperature (873 K). The KGPW doped with all three rare-earth elements has not been synthesized before. This novel investigation could be interesting for understanding the Yb-Tb-Eu upconversion mechanism. The morphological and optical properties were investigated. The given data will include XRD, SEM, reflection, excitation, and emission spectra at room temperature and a 77–500 K interval, photoluminescence lifetimes, quantum efficiencies, and CIE 1931 color coordinates. Green (Tb³⁺) and red (Eu³⁺) luminescence can be obtained within the same compound at the same excitation wavelength by exciting Tb³⁺ directly and Eu³⁺ indirectly. The ration of green/red luminescence can be changed by varying the Eu³⁺ concentration. Herein, we will initially discuss the interaction between Tb³⁺ and Eu³⁺ ions when both are introduced into KGPW and KGPW:10%Yb³⁺ host matrices.

2. Materials and Methods

The samples were synthesized by the conventional solid-state reaction method according to our previous work [3,6]. The stoichiometric amounts of raw materials Gd₂O₃ (99.99% Tailorlux, Münster, Germany), K₂CO₃ (99+% Acros Organics, Geel, Belgium), NH₄H₂PO₄ (99% Reachem, Bratislava, Slovakia), WO₃ (99+% Acros Organics, Geel, Belgium), Eu₂O₃ (99.99% Tailorlux, Münster, Germany), Tb₄O₇ (99.99% Tailorlux, Münster, Germany), and Yb₂O₃ (99.99% Alfa Aesar, Haverhill, MA, USA) were weighed and mixed in an agate mortar using a few milliliters of acetone to ease the homogenization. The starting materials were ground with an agate pestle until all acetone evaporated. Subsequently, the blended materials were placed into the porcelain crucible and annealed three times at 873 K for 10 h in air. The powders were reground after each annealing step. The Eu³⁺ concentration in the KGPW:20%Tb³⁺ and KGPW:10%Yb³⁺,20%Tb³⁺ compounds were 1%, 2.5%, 5%, 10%, and 20%. The phase purity of the synthesized samples was checked using a Rigaku MiniFlexII (Rigaku, Tokyo, Japan) X-ray diffractometer working in a Bragg–Brentano focusing geometry. SEM images were taken using a FE-SEM Hitachi SU-70 (Hitachi, Tokyo, Japan) scanning electron microscope. Reflection, excitation, and emission spectra were recorded using modular a Edinburgh Instruments FLS980 spectrometer (Edinburgh Instruments, Livingston, UK) equipped with a 450 W Xe lamp and 980 nm emitting laser (MDL-III-980-1W) (Changchun New Industries Optoelectronics Tech. Co., Ltd., Changchun, P.R. China) as the excitation sources. The temperature-dependent measurements were performed on the same spectrometer equipped with a cryostat “MicrostatN” (77–500 K temperature range) from Oxford Instruments (Oxford Instruments, Abingdon, UK). The detailed measurement settings are reported in the supporting information file (please refer to Tables S1–S4).

3. Results and Discussion

3.1. Structural Analysis

The XRD patterns of the prepared samples are given in Figure S2. The patterns of the samples match well with the reference pattern of K₂Bi(PO₄)(WO₄) (PDF4+ (ICDD) 04-013-4256). As expected, KGPW adopts the same orthorhombic crystal structure with the Ibca (#73) space group as K₂Bi(PO₄)(WO₄) does. Due to the similar ionic radii, Gd³⁺ (1.053 Å) successfully replaces Bi³⁺ (1.17 Å) in the host matrix [27]. Ionic radii of Yb³⁺ (0.985 Å), Eu³⁺ (1.066 Å), and Tb³⁺ (1.04 Å) are very similar to the Gd³⁺ ionic radius; therefore, the single-phase KGPW compounds formed regardless the concentration of the dopant ions.

To calculate the unit cell parameters of the synthesized compounds, Rietveld refinement of the XRD patterns was performed using FullProf Suite software. K₂Ho(PO₄)(WO₄) was used as a reference (PDF4+ (ICDD) 04-015-9304) [28]. The refinement results for KGPW, KGPW:20%Tb³⁺, KGPW:20%Eu³⁺, and KGPW:10%Yb³⁺,20%Tb³⁺,20%Eu³⁺ samples are shown in Figure 2. The obtained unit cell parameters are given in Table S5. As anticipated, the lattice parameters decreased if samples were doped with Yb³⁺ and Eu³⁺ due to the smaller ionic radii when compared to Gd³⁺.

The morphological features of KGPW powders doped with Yb³⁺, Tb³⁺, and Eu³⁺ were evaluated by taking SEM images, which are shown in Figures S3 and S4 under different magnifications. The powder particles consist of irregularly shaped small crystallites which are heavily agglomerated. No significant difference in crystallite shape and size was observed between the samples doped with different concentrations of lanthanide ions.

3.2. Photoluminescence Studies

The reflection spectra of KGPW:10%Yb³⁺ (black line), KGPW:20%Tb³⁺ (green line), and KGPW:20%Eu³⁺ (red line) samples are given in Figure 3a–c, respectively. The reflection spectra were measured in the range from 250 to 800 nm. The sample doped with Eu³⁺ possesses typical Eu³⁺ absorption lines originating from the ⁷F₀ and ⁷F₁ energy levels. The observed Eu³⁺ absorption lines are assigned to ⁷F₀ → ⁵H_J (ca. 318 nm), ⁷F₀ → ⁵D₄ (ca. 360 nm), ⁷F₀ → ⁵L_7,8; ⁵G_J (ca. 374–390 nm), ⁷F₀ → ⁵L₆ (ca. 390–402 nm), ⁷F₀ → ⁵D₃ (ca. 418 nm), ⁷F₀ → ⁵D₂ (ca. 465 nm), ⁷F₀ → ⁵D₁ (ca. 526 nm), ⁷F₁ → ⁵D₁ (ca. 530–539 nm), and ⁷F₁ → ⁵D₀ (ca. 594 nm). The sample doped with Tb³⁺ shows two sets of absorption lines, which are attributed to the Tb^{3+ 7}F₆ → ⁵L₉ (ca. 346–361 nm) and ⁷F₆ → ⁵L₁₀; ⁵D₃ (ca. 361–384 nm) transitions [29]. The body color of the sample doped with Tb³⁺ was brownish; therefore, the reflectance values at longer wavelengths were lower when compared to the sample containing no Tb³⁺. The brownish color is likely caused by traces of Tb⁴⁺ present in the samples due to the incomplete reduction in terbium to the trivalent state. In this case, the charge transfer between Tb³⁺ and Tb⁴⁺ occurs after absorption of photons in the entire visible range (Tb₄O₇ (Tb₂O₃·2TbO₂), used in synthesis, for instance, is dark brown). The broad absorption band observed in all three samples in the range of 250–340 nm could be attributed to the host lattice absorption. The reflection spectra of co-doped samples are depicted in Figure S5. All the co-doped samples show typical sets of absorption lines of the dopant ions. The reflectance values at longer wavelengths decrease to 90–95% if the samples are co-doped with Tb³⁺. The absorption lines are the same as in the singly doped samples and are attributed to Eu³⁺ or Tb³⁺ ions.

The reflection spectrum of undoped KGPW was used to determine the bandgap of the host material. Firstly, the reflection spectrum was converted to a Tauc plot. The subsequent linear approximation of the obtained spectrum showed that the bandgap of KGPW was 3.8 eV.

The excitation (λ_em = 614 nm) spectra of undoped KGPW, KGPW:20%Tb³⁺, KGPW:20%Eu³⁺, and KGPW:20%Tb³⁺,20%Eu³⁺ are depicted in Figure 4. The excitation spectrum (please refer to Figure 4a) of the undoped KGPW host matrix contains three sets of Gd³⁺ excitation lines: ⁸S → ⁶I_J (ca. 270–279 nm), ⁸S → ⁶P_5/2 (ca. 305 nm), and ⁸S → ⁶P_7/2 (ca. 311 nm) [30]. Since Gd³⁺ ions do not emit at 614 nm, there should be no excitation lines at all; therefore, it is likely that this sample contains traces of Eu³⁺ coming either from transport reactions in the furnace or impurities in the starting materials. This assumption is supported by a weak excitation line at ca. 394 nm, which is typical for the ⁷F₀ → ⁵L₆ transition of Eu³⁺. The same weak excitation line was also observed for the KGPW:10%Yb³⁺ sample (please refer to Figure S6b). The excitation spectrum of the KGPW:20%Tb³⁺ (please refer to Figure 4b) sample possesses typical Tb³⁺ excitation lines originating from ⁷F₆ ground state level transitions to ⁵H₇ (ca. 316–323 nm), ⁵D₁ (ca. 324–332 nm), ⁵L_7;8 (ca. 335–346 nm), ⁵D₂; ⁵L₉ (ca. 347–356 nm), ⁵G₅ (ca. 359 nm), ⁵L₁₀ (ca. 365–373 nm), ⁵D₃; ⁵G₆ (ca. 375–384 nm), and ⁵D₄ (ca. 486 nm) excited state levels [29]. The excitation spectrum of the KGPW:20%Eu³⁺ (please refer to Figure 4c) sample possesses the same sets of excitation lines, which were also detected in the reflection spectrum. The excitation spectrum of the KGPW:20%Tb³⁺,20%Eu³⁺ sample shows transitions of all three ions (Gd³⁺, Tb³⁺, and Eu³⁺) (please refer to Figure 4d). As predicted, the most intense transition line is attributed to Eu^{3+ 7}F₀ → ⁵L₆ (ca. 394 nm) because emission was monitored at a 614 nm wavelength. Excitation spectra of KGPW:10%Yb³⁺,20%Tb³⁺,20%Eu³⁺ and KGPW:10%Yb³⁺ (λ_em = 614 nm) are depicted in Figure S6a and Figure S6b, respectively. The excitation spectra of the KGPW:10%Yb³⁺ sample also showed three typical Gd³⁺ excitation lines as well as a vanishingly small Eu³⁺ excitation line at ca. 394 nm, indicating that a small amount of Eu³⁺ ions is present in the sample.

The emission spectra (λ_ex = 340 nm) of KGPW:20%Tb³⁺,Eu³⁺ and KGPW:10%Yb³⁺,20%Tb³⁺,Eu³⁺ doped with 0% and 20% of Eu³⁺ are depicted in Figure 5 and Figure S7, respectively. Since Yb³⁺ cannot be excited with 340 nm wavelength radiation, the introduction of Yb³⁺ into the crystal lattice has no significant effect on the emission spectra of Tb³⁺ and Eu³⁺. The KGPW:20%Tb³⁺ sample shows typical Tb³⁺ emission lines originating from ⁵D₄ → ⁷F₆ (ca. 480–500 nm), ⁵D₄ → ⁷F₅ (ca. 535–560 nm), ⁵D₄ → ⁷F₄ (ca. 577–592 nm), and ⁵D₄ → ⁷F₃ (ca. 613–627 nm) optical transitions. No emission lines that could be attributed to Eu³⁺ were observed in solely Tb³⁺-doped samples. Completely different spectra were observed when Eu³⁺ ions were included in the samples. The Eu³⁺-concentration-dependent emission spectra of KGPW:20%Tb³⁺ under 340 nm excitation are shown in Figure S8. The integrated intensity of the samples increases with increasing Eu³⁺, although the intensity of Tb³⁺ transitions such as ⁵D₄ → ⁷F₆ and ⁵D₄ → ⁷F₅ drastically decrease with increasing Eu³⁺ concentration. Thus, as it was expected, the emission intensity of the Eu^{3+ 5}D₀ → ⁷F₂ transition coherently increases with increasing Eu³⁺ concentration and reaches the highest intensity in the 20%Eu³⁺-doped sample. A similar tendency was observed with KGPW:10%Yb³⁺,20%Tb³⁺,Eu³⁺ as a function of Eu³⁺ concentration (please refer to Figure S9). However, contrary to the tendency discussed earlier, the integrated intensity of samples increases up to 5% Eu³⁺ and then slightly decreases. As we can see from the emission spectra (λ_ex = 340 nm), the energy from Tb³⁺ is transferred to Eu³⁺; therefore, we see the emission lines attributed to both Tb³⁺ and Eu³⁺ optical transitions. Emission spectra of the samples doped with Eu³⁺ and Tb³⁺ show typical Eu³⁺ emission lines, whereas the Tb³⁺ emission intensity drastically decreases with increasing Eu³⁺ concentration in the material and almost vanishes when Eu³⁺ concentration reaches 20%. Both samples, KGPW:20%Tb³⁺,20%Eu³⁺ and KGPW:10%Yb³⁺,20%Tb³⁺,20%Eu³⁺, show typical Eu³⁺ emission lines in the ranges 583–600 nm (⁵D₀ → ⁷F₁), 605–630 nm (⁵D₀ → ⁷F₂), 645–655 nm (⁵D₀ → ⁷F₃), and 688–708 nm (⁵D₀ → ⁷F₄) [31].

The Eu³⁺ concentration-dependent UC emission spectra of KGPW:10%Yb³⁺,20%Tb³⁺,Eu³⁺ samples were measured under 980 nm wavelength laser excitation. The UC emission spectra of 1% and 20% Eu³⁺-doped specimens are depicted in Figure 6. The inset graph shows normalized integrated emission intensity as a function of Eu³⁺ concentration. The overall UC emission intensity of the samples increases until Eu³⁺ concentration reaches 10%. Increasing the Eu³⁺ concentration further resulted in a decrease in overall UC emission which is likely caused by concentration quenching due to increased probability of cross-relaxation between adjacent Eu³⁺ [32]. Both UC emission spectra of KGPW:10%Yb³⁺,20%Tb³⁺ doped with 1% and 20% Eu³⁺ show typical Tb³⁺ and Eu³⁺ emission lines. The most intense emission line in the sample doped with 20% Tb³⁺ and 1% Eu³⁺ ions is attributed to Tb^{3+ 5}D₄ → ⁷F₅ (λ = 542 nm); for the sample doped with 20% Tb³⁺ and 20% Eu³⁺, as predicted, the most intense emission line is attributed to Eu^{3+ 5}D₀ → ⁷F₂ (λ = 614 nm).

The normalized UC emission spectra (λ_ex = 980 nm) of KGPW:10%Yb³⁺,20%Tb³⁺, KGPW:10%Yb³⁺,20%Tb³⁺,2.5%Eu³⁺, and KGPW:10%Yb³⁺,20%Tb³⁺,20%Eu³⁺ are shown in Figure S10. The spectra were normalized to the most intense Tb³⁺ transition (⁵D₄ → ⁷F₅ ca. 541.5 nm) in order to better evaluate the change between Tb³⁺ and Eu³⁺ emission intensity upon increasing Eu³⁺ concentration. It is evident that Eu³⁺ emission intensity increases with increasing Eu³⁺ concentration. This indicates that Tb³⁺ → Eu³⁺ energy transfer becomes more efficient at higher Eu³⁺ concentrations.

For a further understanding of down-shifting emission, the photoluminescence (PL) decay curves were recorded and analyzed. Samples were excited with 340 nm radiation (⁷F₆ → ⁵L_7;8 transitions of Tb³⁺), and the emission was monitored at 542 nm (⁵D₄ → ⁷F₅ transition of Tb³⁺) and 614 nm (⁵D₄ → ⁷F₃ transition of Tb³⁺ and ⁵D₀ → ⁷F₂ transition of Eu³⁺) (please refer to Figure 7). The effective PL lifetime values were calculated using the following equation [33]:

$${\tau }_{eff}=\frac{{\int }_0^{\infty }I(t)tdt}{{\int }_0^{\infty }I(t)dt}$$

(1)

Here, I(t) is PL intensity at a given time t. The PL decay curves of all synthesized samples as a function of Eu³⁺ concentration are shown in Figure 7. Figure 7a,b show the PL decay curves (λ_ex = 340 nm; λ_em = 542 nm) of KGPW:20%Tb³⁺,Eu³⁺ and KGPW:10%Yb³⁺,20%Tb³⁺,Eu³⁺ specimens as a function of Eu³⁺ concentration. With increasing Eu³⁺ concentration, the PL decay curves become steeper, suggesting that PL lifetime values decrease. The calculated effective PL lifetime values for the given samples are presented in Figure 7c. The effective PL lifetime values for KGPW:20%Tb³⁺,Eu³⁺ samples decrease from 2286 ± 2 μs to 1486 ± 7 μs if Eu³⁺ concentration increases from 0% to 20%. A similar trend was also observed for the KGPW:10%Yb³⁺,20%Tb³⁺,Eu³⁺ samples where the effective PL lifetime values decreased from 2236 ± 2 μs to 1565 ± 6 μs with increasing Eu³⁺ concentration from 0% to 20%, respectively. The effective PL lifetime values for the samples doped with other Eu³⁺ concentrations are tabulated in Table S6. Figure 7d,e show the PL decay curves (λ_ex = 340 nm, λ_em = 614 nm) of KGPW:20%Tb³⁺,Eu³⁺ and KGPW:10%Yb³⁺,20%Tb³⁺,Eu³⁺ samples as a function of Eu³⁺ concentration, respectively. The PL decay curves become steeper with increasing Eu³⁺ concentration. The calculated effective PL lifetime values slightly decrease from 2712 ± 3 μs to 2511 ± 2 μs for KGPW:20%Tb³⁺,Eu³⁺ samples with increasing Eu³⁺ concentration from 1% to 20%. The effective PL lifetime values of KGPW:10%Yb³⁺,20%Tb³⁺,Eu³⁺ samples, in turn, decrease from 2550 ± 3 μs to 2085 ± 4 μs for 1% and 20% Eu³⁺-doped specimens, respectively. The trend of effective PL lifetime values decreasing with increasing Eu³⁺ concentration is depicted in Figure 7f, and the exact calculated PL lifetime values are tabulated in Table S7.

In order to understand the UC process of the prepared materials, the PL decay curves under 980 nm laser excitation were measured, monitoring emission at 542 nm (Tb³⁺), 704 nm (Eu³⁺), and 1050 nm (Yb³⁺). The effective UC PL lifetime and rise time values were calculated and tabulated in Tables S8–S10. The UC PL decay curves of the KGPW:10%Yb³⁺,20%Tb³⁺,Eu³⁺ samples as a function of Eu³⁺ concentration for the Tb^{3+ 5}D₄ → ⁷F₅ (λ_em = 542 nm) transition are depicted in Figure 8a. It is evident that the UC PL decay curves of Tb³⁺ become steeper with increasing Eu³⁺ concentration. Figure 8b,c show the UC PL rise time and UC PL lifetime values of the same samples as a function of Eu³⁺ concentration. Both figures indicate that the UC PL rise time and UC PL lifetime values decrease with increasing Eu³⁺ concentration. The UC PL rise time values decrease from 284 ± 7 μs to 113 ± 10 μs if Eu³⁺ concentration increases from 0% to 20%, whereas the UC PL lifetime values decrease from 2055 ±3 μs to 1345 ±15 μs, respectively. The decreasing Tb³⁺ UC PL lifetime values show that Tb³⁺ → Eu³⁺ energy transfer efficiency increases with increasing Eu³⁺ concentration.

Decay curves of Eu^{3+ 5}D₀ → ⁷F₄ transition (λ_ex = 980 nm, λ_em = 704 nm) for the KGPW:10%Yb³⁺20%Tb³⁺ sample as a function of Eu³⁺ concentration were recorded and depicted in Figure 9a. The calculated PL rise time and lifetime values of the samples are shown in Figure 9b and Figure 9c, respectively. The UC PL decay curves become steeper with increasing Eu³⁺ concentration due to the increasing probability of cross-relaxation processes between Eu³⁺ ions. Thus, the PL rise time and lifetime values decrease in samples with higher Eu³⁺ concentration. The UC PL rise time values decrease from 780 ±80 μs to 334 ±18 μs, whereas the UC PL lifetime values decrease from 2414 ± 3 μs to 1765 ± 3 μs.

The Tb³⁺ → Eu³⁺ energy transfer efficiency (η_tr) can be estimated using the given equation [34]:

$${\eta }_{tr}=1-\frac{{\tau }_S}{{\tau }_S{}_0}$$

(2)

Here, τ_S and τ_S0 are PL lifetime values of Tb³⁺ in the presence and absence of Eu³⁺, respectively. The calculated energy transfer efficiency values as a function of Eu³⁺ concentration are depicted in Figure S11. The η_tr values gradually increase with increasing Eu³⁺ concentration. For instance, η_tr values increase from 10.0% to 34.6% if Eu³⁺ concentration increases from 1% to 20% in the KGPW:10%Yb³⁺,20%Tb³⁺,Eu³⁺ samples (λ_ex = 980 nm, λ_em = 542 nm). A similar tendency was also observed for KGPW:20%Tb³⁺,Eu³⁺ and KGPW:10%Yb³⁺,20%Tb³⁺,Eu³⁺ samples (λ_ex = 340 nm, λ_em = 542 nm) where η_tr values increase from 9.4% to 31.5% and from 12.7% to 33.5%, respectively.

To evaluate the PL lifetime values of Yb³⁺, the PL decay curves (λ_ex = 980 nm, λ_em = 1050 nm) for Yb^{3+ 2}F_5/2 → ²F_7/2 transition were recorded (please refer to Figure S12). The PL decay curves become slightly steeper with increasing Eu³⁺ concentration, although PL lifetime values of the samples are almost the same: 1048 ± 2 μs for the sample with 0% Eu³⁺ and 924 ± 1 μs for the sample with 20% Eu³⁺. Whereas the PL lifetime of the KGPW:10%Yb³⁺ sample is 1066 ± 2 μs. This feature indicates that the percentage of Eu³⁺ doping has an insignificant impact on PL lifetime values of Yb³⁺ ions.

The UC PL decay curves (λ_ex = 980 nm, λ_em = 542 nm) and UC emission spectra (λ_ex = 980 nm) of the KGPW:10%Yb³⁺,20%Tb³⁺,5%Eu³⁺ sample as a function of temperature are shown in Figure 10. It is interesting to note that the overall UC emission intensity increases with increasing temperature up to 200 K. The UC emission intensity decreases if the temperature is increased further. It was also observed that the PL decay curves of the same sample change insignificantly with increasing temperature indicating that Tb³⁺ PL lifetime values are relatively stable within a 77–350 K temperature range. Such observation indicates that Tb³⁺ → Eu³⁺ energy transfer does not change in the same temperature range (i.e., 77 to 350 K). Moreover, the intensity ratio between Tb^{3+ 5}D₄ → ⁷F₅ (ca. 550 nm) and Eu^{3+ 5}D₀ → ⁷F₂ (ca. 620 nm) transitions is virtually the same regardless of the temperature.

3.3. Photometric

In order to demonstrate the emission color dependency of the Eu³⁺ concentration of the synthesized phosphors, the chromaticity coordinates in 1931 color space diagrams were calculated and depicted in Figure 11. The color coordinates of the KGPW:10%Yb³⁺,20%Tb³⁺,Eu³⁺ shifted linearly from the green area (sample with 0% Eu³⁺) to the red area (sample with 20% Eu³⁺). The color coordinates of the synthesized samples indicate that the emission color of the sample could be varied by changing the Eu³⁺ concentration. This was also confirmed by taking digital images of the phosphors in daylight and under 365 nm excitation (please refer to Figure S13). The temperature-dependent color coordinates of KGPW:10%Yb³⁺,20%Tb³⁺,5%Eu³⁺ are located in the green area and do not shift to the red area when raising the temperature from 77 to 500 K. The exact calculated Eu³⁺ and temperature-dependent color coordinates are summarized in Table S11 and Table S12, respectively.

3.4. Quantum Yield

In order to evaluate the possible practical value of the synthesized compounds, the external quantum efficiency was calculated. The obtained external quantum efficiencies of KGPW:20%Tb³⁺,Eu³⁺ and KGPW:10%Yb³⁺,20%Tb³⁺,Eu³⁺ samples upon 340 nm excitation are given in Figure S14. The external quantum efficiency was calculated using this formula [35]:

$$QE=\frac{\int I_{em,sample}-\int I_{em,BaS{O}_4}}{\int I_{ref,BaS{O}_4}-\int I_{ref,sample}}\times 100\%=\frac{N_{em}}{N_{abs}}\times 100\%$$

(3)

Here, ${\int }^{}{I}_{em, sample}$ and ${\int }^{}{I}_{em, BaS{O}_4}$ are integrated emission intensities of the phosphor sample and BaSO₄, respectively. ${\int }^{}{I}_{ref, sample}$ and ${\int }^{}{I}_{ref, BaS{O}_4}$ are the integrated reflectance of the phosphor sample and BaSO₄, respectively. $N_{em}$ is the number of emitted photons and $N_{abs}$ is the absorbed photons. A further increase in Eu³⁺ concentration shows a slight decrease in quantum efficiency in KGPW:20%Tb³⁺ samples (the QE varies in the 10–13.5% range). The highest QE value was 13.75% and was obtained for the KGPW:20%Tb³⁺,1%Eu³⁺ sample. However, the KGPW:10%Yb³⁺,20%Tb³⁺,Eu³⁺ samples show lower QE values compared to their KGPW:20%Tb³⁺,Eu³⁺ counterparts. The QE values vary from 5% to 9%, and the highest quantum efficiency of 9.21% was achieved with the KGPW:10%Yb³⁺,20%Tb³⁺,2.5%Eu³⁺ sample. The relatively low QE could be explained due to cross-relaxation processes in the samples.

4. Conclusions

All in all, we have successfully prepared phase-pure KGPW:20%Tb³⁺,Eu³⁺ and KGPW:10%Yb³⁺,20%Tb³⁺,Eu³⁺ where Eu³⁺ concentrations varied from 1% to 20% Eu³⁺. All the prepared samples, depending on the Tb³⁺/Eu³⁺ concentration ratio, emit in a green-to-red spectral range under 340 and 980 nm excitation. Emission spectra of the samples doped with Eu³⁺ and Tb³⁺ under 340 nm excitation show typical Eu³⁺ and Tb³⁺ emission lines, whereas the Tb³⁺ emission intensity drastically decreases with increasing Eu³⁺ concentration. The strongest down conversion emission intensity was obtained for KGPW:20%Tb³⁺,20%Eu³⁺ and KGPW:10%Yb³⁺,20%Tb³⁺,5%Eu³⁺ samples. The UC emission spectra of the KGPW:10%Yb³⁺,20%Tb³⁺,5%Eu³⁺ sample under 980 nm excitation showed an interesting feature, namely, that the overall UC emission intensity increases when increasing the temperature up to 200 K and then slightly decreases. The strongest upconversion emission in the visible range was achieved with KGPW:10%Yb³⁺,20%Tb³⁺,10%Eu³⁺ under 980 nm excitation. The color coordinates of the phosphor samples can be varied from a green-to-red spectral range by adjusting the Eu/Tb ratio.