Structural Investigation and Energy Transfer of Eu³⁺/Mn⁴⁺ Co-Doped Mg₃Ga₂SnO₈ Phosphors for Multifunctional Applications

Abstract

In recent years, rare earth ion and transition metal ion co-doped fluorescent materials have attracted a lot of attention in the fields of WLEDs and optical temperature sensing. In this study, I successfully prepared the dual-emission Mg₃Ga₂SnO₈:Eu³⁺,Mn⁴⁺ red phosphors and the XRD patterns and refinement results show that the prepared phosphors belong to the Fd-3m space group. The energy transfer process between Eu³⁺ and Mn⁴⁺ was systematically investigated by emission spectra and decay curves of Mg₃Ga₂SnO₈:0.12Eu³⁺,yMn⁴⁺ (0.002 ≤ y ≤ 0.012) phosphors and the maximum value of transfer efficiency can reach 71.2%. Due to the weak thermal quenching effect of Eu³⁺, its emission provides a stable reference for the rapid thermal quenching of the Mn⁴⁺ emission peak, thereby achieving good temperature measurement performance. The relative thermometric sensitivities of the fluorescence intensity ratio and fluorescence lifetime methods reached a maximum value of 2.53% K⁻¹ at 448 K and a maximum value of 3.38% K⁻¹ at 473 K. In addition, the prepared WLEDs utilizing Mg₃Ga₂SnO₈:0.12Eu³⁺ phosphor have a high color rendering index of 82.5 and correlated color temperature of 6170 K. The electroluminescence spectrum of the synthesized red LED device by Mg₃Ga₂SnO₈:0.009Mn⁴⁺ phosphor highly overlaps with the absorption range of the phytochrome P_FR and thus can effectively promote plant growth. Therefore, the Mg₃Ga₂SnO₈:Eu³⁺,Mn⁴⁺ phosphors have good application prospects in WLEDs, temperature sensing, and plant growth illumination.

1. Introduction

Phosphor-converted white LEDs (WLEDs) are widely used in solid-state lighting, liquid crystal displays, and the medical industry because of their high luminous efficiency, long lifetime, and environmental friendliness [1,2]. Currently, the main use of gallium nitride (GaN) or indium gallium nitride (GaInN) is in blue semiconductor chips combined with yellow Y₃Al₅O₁₂:Ce³⁺ phosphor to produce white light [3]. However, it is difficult to meet the conditions for high-quality indoor lighting and backlight display due to insufficient red light [4]. Therefore, the development of stable and efficient red phosphors has important research value. In addition, fluorescence thermometry is a method of measuring temperature by utilizing luminescent properties that are responsive to temperature changes. Fluorescence temperature sensors have a series of advantages over other traditional electrical temperature measurement techniques, such as fast response time, high accuracy, and high spatial and temporal resolution [5,6]. In particular, fluorescence intensity ratio (FIR)- and fluorescence lifetime (FL)-based temperature measurement methods have excellent stability and are less susceptible to emission losses and excitation power fluctuations, thus enabling high-precision and high-resolution temperature measurements [7]. Although fluorescent lighting materials for LEDs and fluorescent temperature sensing materials have been heavily investigated, realizing multifunctional applications with the same material is still a great challenge.

Trivalent rare earth Eu³⁺ ions can be effectively excited by near-ultraviolet (NUV) and blue light due to their special 4f⁶ shell-layer structure, thus displaying strong red emission, which is caused by the ⁵D₀→⁷F_J transitions [8]. Therefore, Eu³⁺ ions are frequently doped into certain matrix materials as activators to emit vivid and enjoyable red light. In contrast, Mn⁴⁺ ions with a 3d³ electronic configuration can exhibit broader excitation and emission bands due to its sensitivity to the coordination environment [9]. The completely different electronic configurations of Eu³⁺ and Mn⁴⁺ result in different temperature dependence of their luminescence intensity [10]. Therefore, Eu³⁺ and Mn⁴⁺ co-doped phosphors are ideal materials for designing a novel optical temperature measurement. In recent years, there have been many reports on Eu³⁺ and Mn⁴⁺ co-doped optical thermometry materials, such as La₂LiSbO₆:Eu³⁺,Mn⁴⁺ [11], Sr₂InTaO₆:Eu³⁺,Mn⁴⁺ [12], Ca₂GdSbO₆:Mn⁴⁺,Eu³⁺ [13], and so on. In addition, the matrix materials are equally important for the photoluminescence and optical thermometry properties of phosphors. Stannate materials are excellent substrates for ions doping due to their excellent stability and unique crystal environment [14]. Mg₃Ga₂SnO₈ (MGS) matrix materials with a cubic structure were first reported by Zhu et al. They constructed MGS substrates based on Mg₂TiO₄ through a co-substitution strategy of [Ga³⁺-Ga³⁺] instead of [Mg²⁺-Ti⁴⁺] and Sn⁴⁺ instead of Ti⁴⁺ [15]. The results show that the MGS matrix material has stable physicochemical properties and low phonon energy of the optical mode [16]. Therefore, the study of Eu³⁺ and Mn⁴⁺ co-doped MGS phosphors for WLEDs and optical temperature measurement has attracted our attention.

In this work, dual-emission MGS:Eu³⁺,Mn⁴⁺ phosphors with tunable red light emission were successfully prepared. The crystal structure, phase purity, and luminescence properties were comprehensively studied. The results of the X-ray diffractometer (XRD) patterns indicate the successful synthesis of the MGS crystal structure. The systematic analysis of the emission spectra and decay curves of MGS:0.12Eu³⁺,yMn⁴⁺ (0.002 ≤ y ≤ 0.012) illustrates the existence of energy transfer between Eu³⁺ and Mn⁴⁺. The maximum efficiency of energy transfer can reach 71.4%. The temperature sensing properties of the MGS:0.12Eu³⁺,0.004Mn⁴⁺ phosphor were explored in detail based on FIR and FL modes, and higher absolute sensitivity (S_a) and relative sensitivity (S_r) were obtained. Moreover, we further completed the package testing of the phosphors for application in LEDs. In conclusion, MGS:Eu³⁺,Mn⁴⁺ phosphors have potential applications in optical temperature measurement and illumination.

2. Results and Discussion

2.1. Crystal Structure

Figure 1a demonstrates the XRD patterns of MGS and MGS:xEu³⁺ (0.04 ≤ x ≤ 0.24) samples. The XRD diffraction peaks of all the samples were sharp and well matched with the MGS standard card (JCPDS 22-1084), indicating that all the synthesized phosphors were well crystallized and in pure phase. Figure 1b also shows the XRD patterns of Mn⁴⁺ and Eu³⁺ co-doped MGS phosphors and the sharp diffraction peaks of the samples indicate that the prepared phosphor has good crystallinity [17]. In addition, the positions of all the diffraction peaks corresponded to the standard cards of MGS, which further confirmed that the MGS:0.12Eu³⁺,yMn⁴⁺ (0.002 ≤ y ≤ 0.012) phosphors were pure phase. The main diffraction peak is slightly shifted to the left for the MGS:xEu³⁺ (0.04 ≤ x ≤ 0.24) samples; however, the main diffraction peak is slightly shifted to the right for the MGS:0.12Eu³⁺,yMn⁴⁺ (0.002 ≤ y ≤ 0.012) samples. Such a result comes mainly from the change in the lattice during ion substitution according to the Bragg equation [18]. Considering the radius similarity and charge balance principle, Eu³⁺ is most likely to take the place of Ga³⁺ and Mn⁴⁺ is most likely to take the place of Sn⁴⁺. The corresponding ionic radii of MGS are as follows: Ga³⁺ (r = 0.86 Å, CN = 6); Sn⁴⁺ (r = 0.62 Å, CN = 6); Eu³⁺ (r = 0.947 Å, CN = 6); and Mn⁴⁺ (r = 0.53 Å, CN = 6) [19]. Obviously, the larger radius Eu³⁺ occupying the Ga³⁺ site causes the expansion of the cell, while the smaller radius Mn⁴⁺ occupying the Sn⁴⁺ site causes the contraction of the cell, and thus the angles are shifted in two different directions in the XRD patterns, respectively. In order to further accurately characterize the lattice positions occupied by the dopant ions, the structure of the samples was further refined.

To further investigate the crystal structure data of MGS, MGS:0.12Eu³⁺ and MGS:0.12Eu³⁺,0.006Mn⁴⁺, the measured XRD result was refined by the EXPGUI version program software of the General Structure Analysis System (GSAS), and the results are shown in Figure 2a–c and

Table 1. The detailed refinement results of MGS, MGS:0.009Mn⁴⁺, MGS:0.12Eu³⁺, and MGS:0.12Eu³⁺,0.006Mn⁴⁺ samples.

Sample	MGS	MGS:0.009Mn4+	MGS:0.12Eu3+	MGS:0.12Eu3+,0.006Mn4+
Space group	Fd-3m	Fd-3m	Fd-3m	Fd-3m
Symmetry	cubic	cubic	cubic	cubic
a/b/c, Å	8.4570	8.4552	8.4585	8.4530
V, Å3	604.85	604.46	605.17	603.99
Z	8	8	8	8
α = β = γ °	90	90	90	90
Rwp	8.5	9.6	8.8	7.5
Rp	6.7	7.5	6.6	5.2
χ2	2.25	2.83	2.45	2.38

. All refinement fitting parameters—R_p, R_wp, and χ²—converged to low levels, indicating reliable refinement results [20]. Moreover, the cell volume gradually increases from 604.85 ų to 605.17 ų with the addition of Eu³⁺ ions and decreases from 604.85 ų to 603.27 ų with the addition of Mn⁴⁺ ions, which further confirms the successful doping of Eu³⁺ and Mn⁴⁺. Figure 2d displays the crystal structure of MGS. There are two kinds of Mg²⁺ in the crystal structure, one part of Mg(1)²⁺ combines with eight O²⁻ ions in the form of body-centered cube to form [MgO₆] octahedra, and the rest of Mg(2)²⁺ combines with four O²⁻ ions to form [Mg(2)O₄] trihedra. The Mg(1), Ga, and Sn occupy the same position in the lattice and form an octahedral structure with six oxygen atoms. The [Mg(1),Sn,GaO₆] and [Mg(2)O₄] are alternately connected by sharing oxygen atoms and thus form the basic skeleton of the cubic structure.

The SEM image of MGS:0.12Eu³⁺,0.004Mn⁴⁺ is shown in Figure 3a. As can be seen from the figure, the particles of the MGS:0.12Eu³⁺,0.004Mn⁴⁺ phosphor exhibits irregular shapes without obvious agglomerations, and the particle size distribution ranges from 1 to 2 µm. Figure 3b shows all of the coherent elements in the energy dispersive X-ray (EDX) spectra, and the elemental mass ratios are consistent with the chemical formulas, further demonstrating the successful preparation of the MGS:0.12Eu³⁺,0.004Mn⁴⁺ sample. The elemental mapping of the representative particles is also shown in Figure 3b. The elemental mapping profiles of the representative particles showed that the elements Eu, Mn, Sn, Ga, Mg, and O were uniformly distributed on the particles without significant elemental aggregation, further indicating that Eu³⁺ ions and Mn⁴⁺ ions were successfully doped into the MGS matrix.

2.2. Optical Properties

Figure 4a shows the diffuse reflectance spectra in the UV-visible range for the MGS matrix and the MGS:0.12Eu³⁺ and MGS:0.009Mn⁴⁺ samples. A highly reflective region appears in the diffuse reflectance curve of the MGS matrix material (350–800 nm) as well as a strong absorption band (200–350 nm). The MGS:0.009Mn⁴⁺ sample shows two distinct strong absorption bands corresponding to ⁴A₂-⁴T₁ and ⁴A₂-⁴T₂ transitions [21]. However, the MGS:0.12Eu³⁺ sample shows several absorption peaks in the UV region, and they all come from the f-f transitions of Eu³⁺ [22]. In addition, the band gap (E_g) can be calculated according to Kubelka–Munk formula [23]:

$${[h\nu F(R_{\infty })]}^n=A(h\nu -E_g)$$

(1)

$$F(R)={(1-R)}^2/2R$$

(2)

where F(R) represents the absorption coefficient, R denotes the measured diffuse reflection coefficient, hν denotes photon energy, n = 2 and 1/2 correspond to the direct and indirect bandgaps, respectively, and A is a constant. According to previous reports, MGS has a direct bandgap, so n = 2 [24]. As shown in Figure 4b, the optical bandgap of MGS matrix is estimated to be 4.16 eV based on a linear extrapolation of the function [F(R_∞)hv]² = 0. The above results indicate that the MGS matrix has a sufficiently large band gap and is a potential matrix material for the preparation of Eu³⁺ and Mn⁴⁺ co-doped samples.

The excitation and emission spectra of MGS:0.12Eu³⁺ are shown in Figure 5a. The strong absorption of the sample between 200 and 320 nm is from the charge transfer band (CTB) between O²⁻→Eu³⁺ [25]. At 362 nm, 381 nm, 394 nm, 416 nm, 466 nm, and 497 nm excitation centers, the Eu³⁺ ions are sharply excited, and the excitation centers are attributed to the f-f characteristic transitions of the Eu³⁺ ions (⁷F₀→⁵D₄, ⁵L₇, ⁵L₆, ⁵D₃, ⁵D₂ and ⁵D₁) [26]. The ⁷F₀→⁵D₂ transition at 466 nm has the highest intensity, indicating that the MGS:0.12Eu³⁺ sample can be effectively excited by blue light. The characteristic peaks at 582, 594, 613, 659, and 710 nm in the emission spectrum belong to the ⁵D₀→⁷F_J (J = 0, 1, 2, 3 and 4) transitions of Eu³⁺ [27]. Figure 5b shows the excitation and emission spectra of MGS:0.009Mn⁴⁺. The excitation spectrum contains four sets of excitation peaks at 295, 339, 405, and 478 nm after Gaussian split peak fitting. The excitation peak at 295 nm is generated by the CTB from O^2-→Mn⁴⁺, while the excitation peaks at 339, 405, and 478 nm are generated by the ⁴A₂→⁴T₁, ²A₂→²T₂, and ²A₂→⁴T₂ transitions of Mn⁴⁺, respectively [28]. The emission spectrum of MGS:0.009Mn⁴⁺ exhibits broadband emission centered at 674 nm from the ²E→⁴A₂ transition [29]. Comparing the excitation spectrum of MGS:0.009Mn⁴⁺ sample with the emission range of MGS:0.12Eu³⁺ phosphor, the overlap between 500 nm and 550 nm suggests the possibility of energy transfer occurring between the Eu³⁺ ions and the Mn⁴⁺ ions. The excitation and emission spectra of MGS:0.12Eu³⁺,0.004Mn⁴⁺ are shown in Figure 5c. The emission spectra are mainly at 613 and 674 nm, where the 613 nm band is characterized by the emission of Eu³⁺ ions, while the emission at 674 nm is characterized by the emission of Mn⁴⁺ ions. Moreover, the characteristic excitation peaks of Eu³⁺ and Mn⁴⁺ are detected in the same emission peaks. The above results suggest that the energy transfer may occur between Eu³⁺ and Mn⁴⁺.

The emission spectra of MGS:xEu³⁺ (0.04 ≤ x ≤ 0.24) phosphors with different Eu³⁺ doping concentrations are given in Figure 6a. There is no significant change in the shape and position of the emission peaks with increasing Eu³⁺ doping concentration. Figure 6b shows the variation in luminescence intensity, and the luminescence intensity reaches the highest when the doping concentration of Eu³⁺ is increased to 0.12, and after that, the luminescence intensity of Eu³⁺ gradually decreases due to the concentration quenching. Based on Dexter’s theory, the mechanism of multipolar interactions was determined by the following equation [30]:

$${\displaystyle \frac{I}{x}}=K{\left(1+\beta {\left(x\right)}^{Q/3}\right)}^{-1}$$

(3)

where K and β are substrate-related constants. Q = 6, 8, or 10 correspond to dipole–dipole (d-d), dipole–quadrupole (d-q), and quadrupole–quadrupole (q-q) interactions [31]. The lg(I/x)~lg(x) relationship for MGS:xEu³⁺ phosphors is illustrated in Figure 6c, and the linear fit yields a slope of −1.92, so Q = 5.76, implying that d-d interaction is the main mechanism leading to concentration quenching.

Figure 6d shows the emission spectra of MGS:0.12Eu³⁺,yMn⁴⁺ (0.002 ≤ y ≤ 0.012) samples. Figure 6e shows the variation in normalized emission intensity of MGS:0.12Eu³⁺,yMn⁴⁺ (0.002 ≤ y ≤ 0.012). As the concentration of Mn⁴⁺ increases, the emission intensity of Eu³⁺ at 613 nm shows a monotonically decreasing trend, whereas the emission intensity of Mn⁴⁺ at 673 nm first gradually increases and then decreases. This result comes from the enhancement of energy transfer between Eu³⁺ and Mn⁴⁺ with the increase of Mn⁴⁺ doping concentration [32] and the decrease in strength of Mn⁴⁺ mainly comes from the concentration quenching of Mn⁴⁺. To further investigate the multicolor tunable emission in MGS:0.12Eu³⁺,yMn⁴⁺ (0.002 ≤ y ≤ 0.012) phosphors, the CIE coordinates of MGS:0.12Eu³⁺,yMn⁴⁺ (0.002 ≤ y ≤ 0.012) phosphors were plotted in Figure 6f, from which a color change in the CIE coordinates from red to deep red can be clearly observed. At the same time, the color coordinates of all the samples are at the edge of the coordinate chart, indicating that the samples have very high color purity of light emission, which provides the conditions for the later LED packaging application.

Figure 7a shows the decay curves of MGS:xEu³⁺ (0.04 ≤ x ≤ 0.24) samples. The value of fluorescence lifetime can be obtained by fitting a double exponential equation as follows [33]:

$$I=I_0+A_1\exp (-t/{\tau }_1)+A_2\exp (-t/{\tau }_2)$$

(4)

The average lifetime of a sample can be calculated using the following formula [34]:

$$\tau =({A_1{\tau }_1}^2+{A_2{\tau }_2}^2)/(A_1{\tau }_1+A_2{\tau }_2)$$

(5)

where τ₁ and τ₂ represent the fast and slow parts of the lifetime, I_t denotes the integrated emission intensity at time t, and A₁ and A₂ are constants, respectively. The average lifetime of MGS:xEu³⁺ (0.04 ≤ x ≤ 0.24) decreases monotonically from 2.25 ms to 1.84 ms as the Eu³⁺ doping increases from 0.04 to 0.24. The main reasons for the lifetime attenuation are that the spacing between dopant ions decreases with increasing concentration, leading to the enhanced probability of non-radiative transitions [35]. To further demonstrate the occurrence of energy transfer, the fluorescence decay curves of Eu³⁺ ions in MGS:0.12Eu³⁺,yMn⁴⁺ (0.002 ≤ y ≤ 0.012) are shown in Figure 7b. The average lifetimes τ of MGS:0.12Eu³⁺,yMn⁴⁺ (0.002 ≤ y ≤ 0.012) were calculated from Equation (6) as 1.76, 1.64, 1.55, 1.25, 0.96, and 0.58 ms. Such results provide further evidence that energy transfer from Eu³⁺ ions to Mn⁴⁺ ions occurs in MGS:0.12Eu³⁺,yMn⁴⁺ (0.002 ≤ y ≤ 0.012) samples. The following equation can be used to analyze the energy transfer efficiency [36]:

$${\eta }_T=1-{\tau }_S/{\tau }_{S0}$$

(6)

where τ_S and τ_S0 are the fluorescence lifetimes of Eu³⁺ at single doping and co-doping, respectively. Figure 7c demonstrates the relationship between the Mn⁴⁺ ion concentration and the energy transfer efficiency. The energy transfer efficiency reaches 71.4% at the Mn⁴⁺ ion doping concentration of 0.012. The multipole–multipole interactions can be used to analyze energy transfer mechanism according to Dexter’s theory [37]:

$${\tau }_{S0}/{\tau }_S\propto C^{n/3}$$

(7)

where C is the total doping concentration of Eu³⁺ and Mn⁴⁺ ions and n is 6, 8, and 10 corresponding to the three types of interactions: d-d, d-q, and q-q interactions, respectively. The best linear fit value of n is 10 and the correlation fitting exponent R² is 0.9948, as shown in Figure 7d. This result suggests that the energy transfer mechanism between Eu³⁺ ions and Mn⁴⁺ ions in the MGS:0.12Eu³⁺,yMn⁴⁺ (0.002 ≤ y ≤ 0.012) samples is mainly a q-q interaction. Figure 7e displays the schematic diagram of Eu³⁺ to Mn⁴⁺ in the MGS matrix. When MGS:0.12Eu³⁺,yMn⁴⁺ (0.002 ≤ y ≤ 0.012) samples were excited by blue light, the ground state electrons of Eu³⁺ ions were lifted to higher excited states. After that, the excited state electrons of Eu³⁺ rapidly relax radiationally to a lower excited state. Then, some of the electrons radiatively return from ⁵D₀ to ⁷F_J (J = 0, 1, 2, 3, 4 and 5), releasing the characteristic emission of Eu³⁺. The other excited state electrons come to the excited state of Mn⁴⁺ through an energy transfer process, and then leap back to the ⁴A₂ state, releasing the characteristic emission of Mn⁴⁺.

Thermal stability is an important parameter for evaluating luminescent performance and commercial availability. The temperature-dependent emission spectra and relative emission contour spectra of the MGS:0.12Eu³⁺ phosphor are displayed in Figure 8a,b and the temperature-dependent emission spectra and relative emission contour spectra of the MGS:0.009Mn⁴⁺ phosphor at different temperatures are displayed in Figure 8c,d. The peak shape and peak position of the MGS:0.12Eu³⁺ and MGS:0.009Mn⁴⁺ phosphors remained essentially unchanged with increasing temperature. Luminous intensity will be reduced accordingly and the temperature increase will intensify the phosphor luminescence center electron non-radiative transition. Figure 8e displays the normalized intensity plots of the characteristic emission peaks of Eu³⁺ (613 nm) in the MGS:0.12Eu³⁺ sample and Mn⁴⁺ (673 nm) in the MGS:0.009Mn⁴⁺ as the function of temperature. The emission intensity of the MGS:0.12Eu³⁺ phosphor at 423 K can still maintain 91.84% of the highest emission intensity at room temperature, which is superior to the majority of the reported Eu³⁺-activated red phosphors, such as: Sr₂LaNbO₆:Eu³⁺ (62.99%@423 K) [38], Ba₂GdSbO₆:0.5Eu³⁺ (79.27%@423 K) [39], and Li₆SrLa₂Nb₂O₁₂:Eu³⁺ (82.25%@423 K) [40]. The result suggests that the synthesized MGS:0.12Eu³⁺ is a promising red phosphor that is expected to be applied in WLEDs for lighting. In comparison, MGS:0.009Mn⁴⁺ phosphor exhibits poor thermal stability. In addition, the activation energy (E_a) can be calculated using the Arrhenius equation [41]:

$$I_{(T)}={\displaystyle \frac{I_0}{1+cexp(-(E_a/kT))}}$$

(8)

where I₀ denotes the luminous intensity of the phosphor at room temperature, I_(T) denotes the luminous intensity of the phosphor at different temperatures, c is a constant, k is Boltzmann’s constant, and T is the temperature of the test environment. Figure 8f shows the linear fit of ln[(I₀/I)-1] versus 1/kT for the MGS:0.12Eu³⁺ and MGS:0.009Mn⁴⁺ samples. According to the slopes of linear fit, the value of E_a for MGS:0.009Mn⁴⁺ is 0.340 eV, while the value of E_a for MGS:0.12Eu³⁺ is 0.382 eV.

To further explain the process of thermal quenching, bit pattern coordinate diagrams can be used. As shown in Figure 9a, the electrons in the ⁷F_J state are excited to the ⁵D_J, ⁵L₆, or CTB states excited by UV or blue light. After non-radiative relaxation of electrons from the upper excited state to the ⁵D₀ state, most of the electrons will radiatively jump from the ⁵D₀ state to the ground state ⁷F_J and then emit red light. At room temperature, most electrons in the CTB state can overcome the energy potential barrier to reach the ⁵D₀ state under electron–phonon coupling and then radiatively jump to the ground state ⁷F_J. As the temperature increases, electrons in the excited state may overcome the low-energy potential barrier with phonon assistance, reach the intersection with the ground state from the excited state, and then non-radiatively relax to the ground state equilibrium position, resulting in thermal quenching [42]. As shown in Figure 9b, when Mn⁴⁺ is excited by UV light, electrons are pushed from the ground state (⁴A₂) to the first excited level (²E) and higher excited states (⁴T₂ or ⁴T₁). At room temperature, electrons in higher excited states drop to the first excited state (²E) via nonradiative relaxation, and subsequently to the ground state (⁴A₂) with radiative relaxation with a deep red emission. However, with the influence of phonon interaction at high temperatures, electrons in the first excited state (²E) can be thermally generated and released through the crossing point between ⁴T₂ and ⁴A₂ by nonradiative relaxation in the configuration coordinate diagram, leading to the formation of the thermal quenching phenomenon.

2.3. Optical Temperature Sensing

To further explore this phosphor in optical temperature sensing applications, the quantitative relationship between the fluorescence intensity of different emission peaks and the temperature interdependence can be utilized for exploratory studies. Figure 10a,b show the temperature-dependent emission spectra and relative emission contour spectra of the MGS:0.12Eu³⁺,0.004Mn⁴⁺ phosphor at temperatures ranging from 298 K to 473 K. Figure 10c shows the normalized intensity plots of the characteristic emission peaks of Eu³⁺ (613 nm) and Mn⁴⁺ (673 nm) in the MGS:0.12Eu³⁺,0.004Mn⁴⁺ sample at different temperatures. The luminescence at 673 nm of Mn⁴⁺ bursts faster, while the emission intensity at 613 nm of Eu³⁺ bursts slower with the increase in temperature. Since the intensities of the Eu³⁺ and Mn⁴⁺ emission peaks respond differently to temperature changes, they can be used to design non-contact optical thermometers based on the FIR technique. The FIR can be calculated according to the following equation [43]:

$$FIR={\displaystyle \frac{I_{613nm}}{I_{673nm}}}\approx A\exp (-B/T)+C$$

(9)

where A, B, and C are constants and ΔE is the energy gap. Figure 10d shows the relationship between the FIR (I_{613 nm}/I_{673 nm}) value and temperature, which can be expressed by the fitting result as FIR = 4048.8 × exp(−665.5/T) + 0.285. The S_a and S_r can be determined according to the following equations [44]:

$$S_a=\left|{\displaystyle \frac{\partial FIR}{\partial T}}\right|$$

(10)

$$S_r=100\%\left|{\displaystyle \frac{\partial FIR}{FIR\partial T}}\right|$$

(11)

Figure 10e shows the fitted curves of S_a and S_r as a function of temperature calculated by the above equations. The value of S_a shows a decreasing trend with increasing temperature and has a maximum value of 0.041 K⁻¹ at 298 K. The value of S_r shows an increasing and then a decreasing trend and reaches a maximum value of 2.53% K⁻¹ at 448 K. Compared to the optical thermometry phosphors already reported in Table 2, the MGS:0.12Eu³⁺,0.004Mn⁴⁺ phosphor has a wider thermometry range and higher sensitivity. To better analyze the cyclic stability, the change in FIR value with temperature during multiple temperature cycling is shown in Figure 10f, and the FIR value is able to recover to the initial state after three temperature cycles, which indicates that the phosphor has good reversibility and reliability in temperature sensing. In conclusion, the MGS:0.12Eu³⁺,0.004Mn⁴⁺ phosphor has good temperature sensing performance and can be regarded as an optical material with potential value for further research in the field of optical temperature sensing.

Temperature sensing using fluorescence lifetimes is another very promising measurement option with the inherent advantage of calibration-free measurements that are not affected by external factors such as sample size and excitation power fluctuations. Figure 11a displays the decay curves for the MGS:0.12Eu³⁺,0.004Mn⁴⁺ phosphor at different temperatures. The luminescence lifetime of Eu³⁺ decays from 1.889 ms to 0.025 ms with increasing temperature, as shown in Figure 11b, and the variability of the luminescence lifetime can be fitted by an Arrhenius-type equation with the following equation [45]:

$${\displaystyle \frac{1}{{\tau }_{(T)}}}={\displaystyle \frac{1}{{\tau }_0}}(1+D\exp (-\Delta E’/(kT))$$

(12)

where τ_(T) and τ₀ denote the Eu³⁺ luminescence lifetime at test temperature T and room temperature, respectively. As shown in Figure 11c, the relationship between the FL values and temperature can be expressed by the fitting result as 1/τ = 65,8045.6 × exp(−4666.7/T) + 0.478. To assess the feasibility of FL as a model for thermometry parameters, the S_a and S_r can be calculated using the following equations [46]:

$$S_a=\left|{\displaystyle \frac{\partial \tau }{\partial T}}\right|$$

(13)

$$S_r=100\%\left|{\displaystyle \frac{1}{\tau }}{\displaystyle \frac{\partial \tau }{\partial T}}\right|$$

(14)

The S_a value of the MGS:0.12Eu³⁺,0.004Mn⁴⁺ sample decreases with increasing temperature and possesses a maximum value of 0.053 K⁻¹ at 298 K, as shown in Figure 11d. The S_r values of the MGS:0.12Eu³⁺,0.004Mn⁴⁺ sample show an increasing trend, with a maximum S_r of 3.38% K⁻¹ at 473 K, as shown in Figure 11e. The MGS:0.12Eu³⁺,0.004Mn⁴⁺ sample has superior temperature sensitivity properties compared to the temperature parameters of the recently reported co-doped fluorescent materials in Table 2. For optical temperature sensing materials, the reproducibility of FL with temperature is also a prerequisite for the material to be practically applicable. In order to verify the reproducibility of the sample, Figure 11f demonstrates the FL of the MGS:0.12Eu³⁺,0.004Mn⁴⁺ phosphor, with the temperature firstly increasing and then decreasing for three cycles. It can be observed that the phosphor exhibits good reproducibility of temperature measurement and has potential for practical application.

Table 2. S_r-Max of some co-doped phosphors based on FIR or FL mode.

Sample	Temperature Range (K)	Sr-Max (% K−1)	Mode	Ref.
Ca2YZr2Al3O12:Bi3+,Eu3+	297–573	0.664	FIR	[22]
Sr3TaGa3Si2O14:Tb3+,Eu3+	298–498	0.760	FIR	[26]
La2LiSbO6:Eu3+,Mn4+	303–523	0.89	FIR	[11]
Ca2LaNbO6:Eu3+,Mn4+	298–498	1.51	FIR	[47]
BaLaMgNbO6:Dy3+,Mn4+	230–470	1.82	FIR	[48]
MGS:0.12Sm3+,0.004Mn4+	298–473	2.53	FIR	This work
SrGdLiTeO6:Sm3+, Mn4+	298–573	1.30	FL	[8]
Ca2GdSbO6:Mn4+,Eu3+	303–523	1.47	FL	[13]
Ba2GdNbO6:Eu3+, Mn4+	303–483	1.73	FL	[49]
La2MgTiO6:Dy3+,Mn4+	303–503	2.31	FL	[50]
MGS:0.12Sm3+,0.004Mn4+	298–473	3.38	FL	This work

Table 2. S_r-Max of some co-doped phosphors based on FIR or FL mode.

Sample	Temperature Range (K)	Sr-Max (% K−1)	Mode	Ref.
Ca2YZr2Al3O12:Bi3+,Eu3+	297–573	0.664	FIR	[22]
Sr3TaGa3Si2O14:Tb3+,Eu3+	298–498	0.760	FIR	[26]
La2LiSbO6:Eu3+,Mn4+	303–523	0.89	FIR	[11]
Ca2LaNbO6:Eu3+,Mn4+	298–498	1.51	FIR	[47]
BaLaMgNbO6:Dy3+,Mn4+	230–470	1.82	FIR	[48]
MGS:0.12Sm3+,0.004Mn4+	298–473	2.53	FIR	This work
SrGdLiTeO6:Sm3+, Mn4+	298–573	1.30	FL	[8]
Ca2GdSbO6:Mn4+,Eu3+	303–523	1.47	FL	[13]
Ba2GdNbO6:Eu3+, Mn4+	303–483	1.73	FL	[49]
La2MgTiO6:Dy3+,Mn4+	303–503	2.31	FL	[50]
MGS:0.12Sm3+,0.004Mn4+	298–473	3.38	FL	This work

2.4. Applications in LEDs

The MGS:0.12Eu³⁺ sample and commercial phosphors BaMgAl₁₀O₁₇:Eu²⁺ and (Ba,Sr)₂SiO₄:Eu²⁺ were encapsulated in the 395 nm commercial chip to make a WLED light source, and its electroluminescence spectrum is shown in Figure 12a. The inset is a photo of this WLED after it is illuminated. The correlated color temperature of the prepared WLED device is 6170 K, and the color rendering index reaches 82.5, which indicates that the MGS:0.12Eu³⁺ phosphor has great potential for use in pc-LED backlit displays. The phytochrome has a specific absorption spectrum, so the wavelength at which the emission spectrum of the phosphor is located needs to be within the absorption spectrum of the phytochrome in order to promote plant photosynthesis more effectively [51]. Figure 12b displays the electroluminescence spectrum of the MGS:0.009Mn⁴⁺ sample and absorption spectra of the plant photosensitive pigments P_R and P_FR. Obviously, the strongest peak located at 673 nm is highly overlapped with spectrum of P_FR and is far away from the peak wavelength of the absorption spectrum of the P_R. This suggests that the sample MGS:0.009Mn⁴⁺ can promote the process of energy absorption by the plant photosensitive pigment P_FR, which in turn selectively promotes some specific processes of plant growth.

3. Materials and Methods

3.1. Preparation of Materials

A series of MGS:xEu³⁺ (0.04 ≤ x ≤ 0.24) and MGS:0.12Eu³⁺,yMn⁴⁺ (0.002 ≤ y ≤ 0.012) samples were prepared by adopting a high-temperature solid phase method. Firstly, I weighed the raw materials—(MgCO₃)₄·Mg(OH)₂·5H₂O (99.5%, Aladdin), Ga₂O₃ (99.99%, Aladdin), SnO₂ (99.5%, Aladdin), Eu₂O₃ (99.99%, Aladdin) and MnCO₃ (99.5%, Aladdin)—according to the designed stoichiometric ratios, and ground and stirred them well in an agate mortar. After thorough grinding and homogenization, the mixed powders were transferred to a crucible of alumina, followed by sintering in the muffle furnace at 1450 °C for 6 h, with air as the sintering atmosphere. Finally, the samples were crushed into powder and subjected to subsequent performance testing.

3.2. Characterization of Materials

The XRD data were measured by a Bruker-D8 ADVANCE diffractometer (BRUKER, Germany). The morphology and dimensions of the samples were observed using a field emission scanning electron microscope (SEM, HITACHI, SU8100, Tokyo, Japan). The diffuse reflectance spectra were tested using a spectrophotometer (Pulsar, TU1950, Beijing, China) equipped with an integrating sphere accessory. The excitation and emission spectra of the samples were measured using a FLS1000 fluorescence spectrometer from (Edinburgh, UK). The excitation light source was a xenon lamp, and a variable-temperature accessory was used for variable-temperature testing. For fluorescence lifetime measurement, a microsecond lamp was selected as the light source. The electrochromic performance of packaged light-emitting devices was measured using the Starspec SSP6612 LED photoelectric measuring system (Hangzhou, China).

3.3. Preparation of LEDs

The prepared MGS:0.12Eu³⁺ red phosphor was thoroughly and evenly mixed with the commercial blue phosphor BaMgAl₁₀O₁₇:Eu²⁺ and the commercial green phosphor (Ba, Sr)₂SiO₄:Eu²⁺ according to a mass ratio of 10:2:1. Subsequently, this mixture was mixed with the organic silica gel according to a mass ratio of 1:0.7, and then applied onto a NUV chip with a wavelength of 395 nm and encapsulated to form a white light LED device. A single red LED was fabricated by combining the as-prepared MGS:0.009Mn⁴⁺ phosphor and organic silica gel with a 410 nm InGaN chip.

4. Conclusions

In summary, I successfully synthesized MGS:Eu³⁺,Mn⁴⁺ phosphors with dual emission centers. The XRD, morphology, photoluminescence properties, energy transfer process and temperature-sensitive properties of MGS:Eu³⁺,Mn⁴⁺ samples have been investigated. The results show that the energy transfer of Eu³⁺→Mn⁴⁺ ions exists in MGS:Eu³⁺,Mn⁴⁺ phosphors, and the maximum value of transfer efficiency can reach 71.2%. The CIE chromaticity coordinate plots of the samples show that the effective adjustment of the luminescence color of the double-doped phosphor can be achieved by changing the doping concentration of Mn⁴⁺. Thanks to the weak thermal burst effect of Eu³⁺, its emission provides a stable reference for the fast thermal quenching of the Mn⁴⁺ stokes emission peak, which leads to a good thermometry performance. The relative thermometric sensitivities of the two methods reached a maximum value of 2.53% K⁻¹ at 448 K and a maximum value of 3.38% K⁻¹ at 473 K. The prepared LED lamps have a high color temperature (the relevant color temperature is 6170 K) and a color rendering index R_a of 82.5, indicating that the prepared phosphors have good luminescence performance and can be applied to the field of LED lighting.