Synergistic luminescent thermometer and indoor plant growth lighting utilizing the Mg₂LaSbO₆:Eu³⁺/Mn⁴⁺ phosphor

Abstract

The development of non-contact temperature sensors has attracted much attention, and high sensitivity is the difficulty to overcome. A new type of red phosphor, Mg₂LaSbO₆:Eu³⁺,Mn⁴⁺ (MLSO:Eu³⁺,Mn⁴⁺), was synthesized by the solid phase reaction method. The effect of different Eu³⁺ concentrations on its luminescence was discussed, and its temperature-sensitive properties were studied. The results indicate that Eu³⁺ and Mn⁴⁺ successfully entered the matrix lattice, causing slight distortion of the crystal structure of the phosphor MLSO:Eu³⁺,Mn⁴⁺. The emission spectrum of the phosphor MLSO:Eu³⁺,Mn⁴⁺ consists of sharp emission peaks of Eu³⁺ (560–640 nm) and broadband emission peaks of Mn⁴⁺ (640–800 nm). The experimental results show that the red light emission (612 nm) intensity of Eu³⁺ and the deep red light emission (695 nm) intensity of Mn⁴⁺ in the phosphor MLSO:Eu³⁺,Mn⁴⁺ are temperature dependent. Therefore, using the Fluorescence Intensity Ratio (FIR) technique, temperature fluorescence testing was conducted on the phosphor MLSO:0.02Eu³⁺,0.002Mn⁴⁺ to achieve visible light temperature measurement. The maximum relative sensitivity (S_r) was calculated to be 3.97% K⁻¹ (@440 K), and the maximum sensitivity (S_a) was 0.17 K⁻¹ (@400 K). Meanwhile, the electroluminescence spectra of LED devices prepared using the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺ were consistent with the absorption spectra of chlorophyll (A and B) and photosensitizers (Pr and Pfr) required for plant growth. It shows that the phosphor MLSO:Eu³⁺,Mn⁴⁺ can be used in the field of optical temperature measurement and indoor plant growth lighting.

---

1. Introduction

In recent years, rapid and accurate temperature measurement has played a crucial role in industries such as medicine and solid-state lighting.^1–4 However, traditional temperature measurement not only requires a long reaction time and has low sensitivity, but also is not adapted to extreme conditions. Non-contact temperature sensors based on optical temperature measurement methods, such as fluorescence lifetime and fluorescence intensity ratio (FIR) technology, have been extensively studied.^5–10 Among them, the optical temperature sensor made of rare-earth ion doped materials and based on the fluorescence intensity ratio (FIR) technology is considered a promising temperature detection tool due to its high sensitivity, high chemical stability and wide applicability in harsh environments and fast-moving objects. The sensitivity of rare earth doped compounds is still not high enough for practical applications. Therefore, it is urgent to find a new kind of high-sensitivity optical temperature sensing material. In order to obtain phosphors with higher sensitivity, it is necessary to find a relatively stable substrate and a matching rare-earth ion. So far, more studies have been conducted on tungstate,^11,12 germanate¹³ and phosphate¹⁴ phosphors, while less research has been conducted on antimonate. In addition, antimonate has high physicochemical stability and luminescence properties, and is an excellent luminescent substrate.¹⁵ Mg₂LaSbO₆ has the unique advantages of antimonate, and no fluorescent material based on Mg₂LaSbO₆ has been reported so far. However, the 4f electrons in the inner layer of Eu³⁺ transition between energy levels to produce a red emission spectrum (∼610 nm), but the emission spectrum of Eu³⁺ is linear because it is shielded by the outer electrons.¹⁶ The ²E_g → ⁴A_2g transition of the transition metal ion Mn⁴⁺ with the 3d³ electron configuration can produce strong red and deep red emission in the range of 600–750 nm, and the phosphors prepared by co-doping of the two can be used for temperature detection, such as Ca₂LaSbO₆:Mn⁴⁺,Eu³⁺,¹⁷ La₂MgTiO₆:Mn⁴⁺,Eu³⁺,¹⁸ Sr₂CaTeO₆:Mn⁴⁺,Eu³⁺,¹⁹etc.

The ²E_g → ⁴A_2g transition of Mn⁴⁺, which is significantly affected by the crystal field, corresponds to strong near-infrared fluorescence radiation.¹⁵ An oxide phosphor doped with Mn⁴⁺ has a long fluorescence lifetime (2–9 ms), and produces strong near-infrared fluorescence radiation (near 695 nm) under ultraviolet and blue light excitation. This band can better match that of plant phytochromes Pr and Pfr, thus promoting flowering and fruit and root growth of plants.²⁰ Eu³⁺ has a good red emission center²¹ and is also a common luminescent ion for the synthesis of red phosphors. Its ⁵D₀ → ⁷F_J (J = 1, 2, 3, 4) transition center is located in the red light region, which can achieve two-site excitation of the plant growth band with the red light ²E_g → ⁴A_2g transition region of Mn⁴⁺ to better promote plant growth.²²

In this work, a novel type of red phosphor, Mg₂LaSbO₆:Eu³⁺,Mn⁴⁺, was successfully prepared by the high-temperature solid phase method, and its crystal structure, microscopic morphology, luminescence properties, thermal stability and application prospects in the field of optical temperature measurement and plant growth illumination were systematically analyzed.

2. Experiment

2.1. Sample synthesis

The samples Mg₂LaSbO₆:xEu³⁺,0.002Mn⁴⁺ (x = 0.02, 0.04, 0.06, 0.08, 0.10, and 0.12) were prepared using the traditional high-temperature solid-phase method, using La₂O₃ (AR, Aladdin), (MgCO₃)₄·Mg(OH)₂·5H₂O (AR, Aladdin), Sb₂O₅ (99%, Aladdin), MnCO₃ (99.95%, Aladdin) and Eu₂O₃ (99.99%, Aladdin) as synthetic raw materials. The specific details of sample synthesis have been provided in the Sample synthesis section of the ESI.†

2.2. Sample characterization

The crystal structure of the sample was characterized by X-ray powder diffraction (XRD). The crystal structure was refined using GSAS II software, and the cell parameters were calculated. The size and shape of the MLSO:0.06Eu³⁺,0.002Mn⁴⁺ sample particles were observed using scanning electron microscopy (SEM). The morphological characteristics and elemental distribution of the samples were observed using mapping and SEM images. The photoluminescence emission spectrum (PL) and variable temperature fluorescence spectrum were captured using an Edinburgh fluorescence spectrometer (FS5). The specific parameters of the instrument used are presented in detail in the instrument parameters section of the ESI.†

2.3. WLED construction and characterization

We packaged the prepared red phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺ and a UV LED chip (395 nm) to obtain an LED device. Moreover, the photoelectric parameters of the LED device were measured using a HAAS-1200 spectroradiometer (Everfine, China).

3. Results and discussion

3.1. Crystal structure, morphology and elemental composition

As shown in Fig. 1, the XRD patterns of a series of phosphors MLSO:xEu³⁺,0.002Mn⁴⁺ (0.02 ≤ x ≤ 0.12) match the standard card of Ca₂GdNbO₆ (PDF # 89-1438). The diffraction peak positions and shapes of all samples are consistent with the standard card, and there are no impurities. To explore the specific crystal lattice sites occupied by Eu³⁺ and Mn⁴⁺ ions in the phosphor host material (MLSO). As is well known, ions with similar radii and valence states are prone to substitution, which can be measured from the percentage difference in radii between doped ions and substituted ions (D_r), estimated using eqn (1).

Fig. 1 XRD patterns of MLSO:xEu³⁺,0.002Mn⁴⁺ (0.02 ≤ x ≤ 0.12).

In eqn (1), R_s and R_d are the radii of the substituted ion and doped ion, respectively, and CN is the coordination number of the ion. According to the principle of similar valence states, Eu³⁺ occupies the La³⁺ site, while Mn⁴⁺ occupies the Sb⁵⁺ site. The ionic radii (for CN = 6) are as follows: Eu³⁺(0.947 Å), La³⁺(1.032 Å), Mn⁴⁺ (0.530 Å), and Sb⁵⁺ (0.600 Å).²³ The calculation results show that D_r(Eu/La) = 8.98% and D_r(Mn/Sb) = 13.21%, which are far below the critical value of 30%.²⁴ This further proves that Eu³⁺ and Mn⁴⁺ occupy the La³⁺ and Sb⁵⁺ sites, respectively.

As shown in Fig. 1, with the increase of Eu³⁺ doping concentration (x), the diffraction peak of the sample shifts towards higher angles, which can be explained using the Bragg equation.²⁵

2d sinθ = nλ (2)

In formula (2), d represents the interplanar spacing, θ represents the diffraction angle, n is a constant, and λ represents the wavelength of the X-ray. When Eu³⁺ with a smaller radius replaces larger La³⁺ in the matrix lattice, the interplanar spacing (d) decreases, resulting in an increase in the diffraction angle (θ), which once again proves the successful entry of Eu³⁺ into the matrix lattice.

In order to further determine the crystal structure of the synthesized phosphors, the phosphors MLSO:xEu³⁺,0.002Mn⁴⁺ (0.02 ≤ x ≤ 0.12) were refined based on XRD patterns using GSAS II software. The results after refinement are shown in Fig. 2 and Fig. S1–4,† and the relevant refinement parameters are listed in Table 1 and Table S1.† The results of the refinement show that R_wp < 10% and χ² < 2, indicating that the refinement results are reliable.²⁶ It has been demonstrated that doping ions Eu³⁺ and Mn⁴⁺ can occupy the positions of La³⁺ and Sb⁵⁺, respectively, and fuse into the matrix lattice without causing significant changes in the main crystal structure. However, the cell parameters (a, b, c) and cell volume (V) of the phosphors MLSO:xEu³⁺,0.002Mn⁴⁺ decreased linearly with the increase of Eu³⁺ doping concentration (Fig. 3), which fully indicated that Eu³⁺ successfully occupied the La³⁺ site and entered the crystal to form a continuous solid solution.

Fig. 2 XRD patterns based on the Rietveld refinement results for MLSO:0.02Eu³⁺,0.002Mn⁴⁺ and MLSO:0.06Eu³⁺,0.002Mn⁴⁺. The experimental values are shown with black crosses. The calculated Rietveld values and the difference curves are indicated with red and blue lines, respectively. The Bragg reflections are given as magenta short vertical lines.

Fig. 3 The variations of the lattice parameters (a, b, c and V) as a function of the Eu content.

Table 1 Structural parameters for MLSO:0.02Eu³⁺,0.002Mn⁴⁺ and MLSO:0.06Eu³⁺,0.002Mn⁴⁺ phosphors

	MLSO:0.02Eu^3+,0.002Mn^4+	MLSO:0.06Eu^3+,0.002Mn^4+
Crystal system	Monoclinic system	Monoclinic system
Space group	P121/n1(14)	P121/n1(14)
Units, Z	2	2
a (Å)	5.672(0)	5.664(0)
b (Å)	5.852(1)	5.845(7)
c (Å)	8.137(3)	8.132(5)
β (°)	90.22(0)	91.25(4)
V (Å^3)	270.085	269.202
R p (%)	8.21%	10.58%
R wp (%)	7.28%	8.64%
χ ^2	1.23	1.45

---

The schematic diagram of the crystal structure of Mg₂LaSbO₆ (MLSO) is shown in Fig. 4. The MLSO crystal belongs to the monoclinic crystal system with the space group P121/n1. Mg²⁺ and Sb⁴⁺ are, respectively, connected to six O atoms to form [MgO₆] octahedra and [SbO₆] octahedra. The layered stacking of [MgO₆] octahedra and [SbO₆] octahedra and their mutual connection through shared O form the skeleton structure of the MLSO crystal. La³⁺ is filled in the octahedral gaps and connected with six O atoms in the surrounding octahedra to form the [LaO₆] octahedra, thus forming the MLSO crystal. When Eu³⁺ and Mn⁴⁺ successfully occupy the positions of La³⁺ and Sb⁵⁺, respectively, to form [EuO₆] octahedra and [MnO₆] octahedra, a new phosphor, MLSO:Eu³⁺,Mn⁴⁺, is formed.

Fig. 4 Schematic diagram of the crystal structure of Mg₂LaSbO₆.

Fig. 5a shows the microstructure of the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺. The phosphor is composed of micrometer particles of varying sizes and shapes, with particle sizes being roughly within 10 μm and exhibiting certain aggregation phenomena, which is a characteristic of samples synthesized via a high-temperature solid-state reaction (Fig. S5†). The surface of the grain is smooth and embedded with small particles with a layered structure (Fig. S6–8†). Fig. 5b–g show the elemental distribution of MLSO:0.06Eu³⁺,0.002Mn⁴⁺. It can be seen that La, Mg, Sb, O, Eu and Mn elements are uniformly distributed in the sample, once again indicating that Eu³⁺ and Mn⁴⁺ have successfully entered the matrix. The XPS spectra of MLSO:0.06Eu³⁺,0.002Mn⁴⁺ once again confirmed the existence of La, Mg, Sb, O, Eu and Mn (Fig. 6a). In addition, in the high-resolution XPS spectrum, the peak at 1303.6 eV originates from Mg 1s (Fig. 6b), while the peaks at 834.3 eV and 851.0 eV are attributed to the spin–orbit splitting of La 3d_5/2 and La 3d_3/2, respectively. The peaks with binding energies of 837.8 eV and 854.8 eV correspond to the satellite peaks of La 3d_5/2 and La 3d_3/2, respectively (Fig. 6c).²⁷ However, the peak observed at 529.8 eV in the high-resolution XPS spectrum can be deconvoluted into two components at 529.5 eV and 530.9 eV, which are assigned to O 1s and Sb 3d_5/2 spin–orbit peaks, respectively. The peak at 539.2 eV is identified as the Sb 3d_3/2 spin–orbit peak (Fig. 6d).²⁸ The high-resolution XPS map of the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺ (Fig. 6e) confirmed that Eu exists in the matrix MLSO in a trivalent state, and the binding energies of Eu³⁺ 3d_5/2 and Eu³⁺ 3d_3/2 are 1129.3 and 1159.3 eV, respectively. Additionally, a characteristic peak corresponding to the Mn 3s orbital appears at 88.9 eV (Fig. 6f), while Mn exists in a quadrivalent state with a binding energy of 651.6 eV and a weak signal (Fig. S9†). This result is consistent with relevant literature reports.^29,30

Fig. 5 (a) The SEM image of MLSO:0.06Eu³⁺,0.002Mn⁴⁺ and mapping images corresponding to each element in the phosphor: (b) La L-edge, (c) Mg K-edge, (d) Sb L-edge, (e) O K-edge, (f) Eu K-edge, and (g) Mn K-edge.

Fig. 6 (a) XPS spectra of MLSO:0.06Eu³⁺,0.002Mn⁴⁺; high-resolution XPS spectra of MLSO:0.06Eu³⁺,0.002Mn⁴⁺: (b) Mg 1s, (c) La 3d, (d) Sb 3d, O 1s, (e) Eu 3d and (f) Mn 3s.

3.2. Photoluminescence properties of MLSO:Eu³⁺,Mn⁴⁺ phosphors

Fig. 7a shows the photoexcitation and photoemission spectra of the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺. The excitation spectrum under 695 nm monitoring consists of two broad bands and a series of sharp peaks. In the range of 240–460 nm, the broadband at 271 nm belongs to the charge transfer band (CTB) of Eu³⁺–O²⁻, while the broadband at 322 nm is due to the ⁴A_2g → ⁴T_1g,²T_2g electronic transition of Mn⁴⁺.³¹ A series of characteristic peaks related to f–f transitions of Eu³⁺ appear in the range of 350–600 nm, corresponding to the characteristic transitions of ⁷F₀ → ⁵L₆, ⁷F₀ → ⁵D₂, and ⁷F₀ → ⁵D₁ at 392 nm, 463 nm, and 532 nm, respectively.³² The excitation peak intensity is the highest at 392 nm. However, the excitation spectrum monitored at 612 nm did not show the broadband excitation peak of Mn⁴⁺ at 322 nm, which confirms the correctness of the attribution of the excitation peak of the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺.

Fig. 7 (a) Excitation spectra (λ_em = 612 nm, λ_em = 695 nm) and emission spectra (λ_ex = 271 nm, λ_ex = 318 nm, λ_ex = 392 nm) of MLSO:0.06Eu³⁺,0.002Mn⁴⁺; (b) peak-fitting diagram of the emission spectrum (575–635 nm) of the MLSO:0.06Eu³⁺,0.002Mn⁴⁺ phosphor (λ_ex = 392 nm).

Under excitation at 392 nm, the emission spectrum of the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺, as shown in Fig. 7a, is located between 560 and 800 nm and composed of multiple emission peaks. The sharp line peak is caused by the ⁵D_0,1 → ⁷F_J (J = 0, 1, 2, 3, 4) transition of Eu³⁺ excited state ⁵D_0,1 electrons, which can be attributed to ⁵D₀ → ⁷F₁ (587 nm), ⁵D₀ → ⁷F₂ (612 nm), and ⁵D₀ → ⁷F₄ (687 nm, 702 nm), respectively. The emission peak at 612 nm is the strongest, attributed to the ⁵D₀ → ⁷F₂ electric dipole transition of Eu³⁺. However, the broadband emission peak of 640–800 nm comes from the ²E_g → ⁴A_2g electronic transition of Mn⁴⁺. Interestingly, the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺ under 271 nm excitation is dominated by sharp line peak emission of Eu³⁺, while the broadband emission of Mn⁴⁺ dominated the excitation at 322 nm, which once again supports the correctness of excitation peak attribution.

The emission spectrum of the MLSO:Eu³⁺,Mn⁴⁺ phosphor reveals that the dominant emission at 612 nm originates from the electric dipole transition ⁵D₀ → ⁷F₂ of Eu³⁺ (Fig. 8a), indicating that the luminescent center Eu³⁺ primarily occupies a non-centrosymmetric site in the host crystal.³³ To further elucidate the point-group symmetry of Eu³⁺, the emission peaks corresponding to the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions were deconvolved via peak fitting. The fitted results demonstrate that the ⁵D₀ → ⁷F₁ transition splits into three sub-peaks at 582 nm, 588 nm, and 596 nm, while the ⁵D₀ → ⁷F₂ transition splits into five sub-peaks at 608 nm, 611 nm, 614 nm, 617 nm, and 622 nm (Fig. 7b). In monoclinic systems, the low symmetry of the C_s and C₂ sites allows complete lifting of degeneracy for the ⁷F₁ and ⁷F₂ energy levels of Eu³⁺ when it occupies these sites. Consequently, the ⁷F₁ and ⁷F₂ levels split into three and five Stark components, respectively, leading to the observed three-peak splitting for ⁵D₀ → ⁷F₁ and five-peak splitting for ⁵D₀ → ⁷F₂ in the emission spectrum.³⁴ Based on the above analysis, we conclude that in the host MLSO lattice, Eu³⁺ substitutes for La³⁺ sites, forming a coordination environment with adjacent oxygen ligands that adopts C₂ point group symmetry.

Fig. 8 (a) Emission spectra of MLSO:xEu³⁺,0.002Mn⁴⁺ (0.02 ≤ x ≤ 0.012) (λ_ex = 392 nm). (b) Fluorescence intensity of MLSO:xEu³⁺,0.002Mn⁴⁺ phosphors changed with Eu³⁺ concentration (x).

In order to investigate the effect of different Eu³⁺ doping concentrations on the luminescence properties of the phosphor MLSO:Eu³⁺,Mn⁴⁺, we successfully synthesized a series of phosphors MLSO:xEu³⁺,0.002Mn⁴⁺ (0.02 ≤ x ≤ 0.12). As can be seen from Fig. 8a, the shape and position of the luminescence spectrum of the phosphor MLSO:Eu³⁺,Mn⁴⁺ do not change significantly under different Eu³⁺ doping concentrations. It can be intuitively observed from Fig. 8b that the emission intensity of Eu³⁺ in the emission spectrum of the phosphor MLSO:Eu³⁺,Mn⁴⁺ increases with the increase of Eu³⁺ doping concentration, and when the emission intensity is the strongest, the doping concentration is 0.06. After that, the emission intensity decreased because the Eu³⁺ concentration increased, the distance between Eu³⁺ ions became smaller, and energy absorption increased through non-radiative energy transfer, resulting in concentration quenching.^35,36 Non-radiative energy is transferred between Eu³⁺ ions, so the non-radiative energy transfer will gradually dominate as the distance between Eu³⁺ ions shrinks.³⁷ According to Blasse's theory, the type of energy transfer can be judged from the critical distance (R_c) of energy transfer between doped ions. When R_c > 5 Å, the non-radiative energy transfer is mainly caused by electric multipole interaction. When R_c < 5 Å, non-radiative energy transfer is mainly caused by exchange interaction. The calculation formula is as follows:³⁸

In eqn (3), R_c is the critical distance for energy transfer of Eu³⁺ ions, N represents the number of sites available for dopant-activated ions (N = 2), V is the volume of the unit cell (V = 269.202 ų), and x_c is the optimal doping concentration (x_c = 0.06). According to eqn (3), R_c = 16.2444 Å can be calculated, which indicates that the concentration quenching mechanism of the phosphor MLSO:Eu³⁺,Mn⁴⁺ is caused by the electric multipole interaction.

The interaction mechanism between Eu³⁺ can be determined using Dexter's energy transfer theory, eqn (4).

In eqn (4), I is the emission intensity of the phosphor, θ represents the index of electric multipolar characteristics, x is the critical doping concentration of the doped ions, and C is a constant. According to different θ values, electric multipole interactions can be divided into three types: the dipole–dipole (d–d) interaction (θ = 6), the quadrupole–dipole (d–q) interaction (θ = 8), and the quadrupole–quadrupole (q–q) interaction (θ = 10).³⁹ As shown in Fig. 9, the fitting slope is −2.2013, and the calculated θ = 6.6039, which is closest to 6, further indicating that the concentration quenching of Eu³⁺ in the phosphor MLSO:Eu³⁺,Mn⁴⁺ is caused by the dipole–dipole (d–d) interaction.

Fig. 9 Correlation between lg(I/x) and lg x for phosphors MLSO:xEu³⁺,0.002Mn⁴⁺ (0.02 ≤ x ≤ 0.012).

The quantum yield serves as a critical parameter for assessing the luminescence properties of fluorescent materials. Accordingly, the external quantum efficiency (EQE) and internal quantum efficiency (IQE) of the MLSO:0.06Eu³⁺,0.002Mn⁴⁺ phosphor were determined via fluorescence spectroscopic analysis. The results, presented in Fig. 10, reveal that the MLSO:0.06Eu³⁺,0.002Mn⁴⁺ phosphor exhibits high IQE and EQE values of 82.81% and 60.72%, respectively, indicating its superior luminous efficiency.

Fig. 10 Quantitative emission spectra of the reference sample and MLSO:0.06Eu³⁺,0.002Mn⁴⁺ collected using a fluorescence spectrometer equipped with an integrating sphere; the illustration shows the local amplification of the emission spectrum in the range of 560–750 nm.

3.3. Temperature sensing properties of phosphors MLSO:Eu³⁺,Mn⁴⁺

As can be seen from Fig. 11, the peak shapes and positions of the emission peaks of Eu³⁺ and Mn⁴⁺ in the phosphors MLSO:Eu³⁺,Mn⁴⁺ do not change significantly with the increase of temperature (303–503 K). However, the luminescence intensities of Mn⁴⁺ and Eu³⁺ decreased with the increase of temperature, but the luminescence intensity of Mn⁴⁺ decreased more rapidly. As is well known, Eu³⁺ and Mn⁴⁺ have different thermal quenching mechanisms due to their distinct local electronic structures. Therefore, the thermal quenching mechanism diagram of Eu³⁺ and Mn⁴⁺ was drawn (Fig. 12).

Fig. 11 Temperature-dependent PL spectra of MLSO:0.02Eu³⁺,0.002Mn⁴⁺ (a) and MLSO:0.06Eu³⁺,0.002Mn⁴⁺ (b) ranging from 303 to 503 K. A 3D histogram of the temperature-dependent emission intensity of Eu³⁺ (605 to 640 nm) and Mn⁴⁺ (640 to 800 nm) ions of MLSO:0.02Eu³⁺,0.002Mn⁴⁺ (c) and MLSO:0.06Eu³⁺,0.002Mn⁴⁺ (d).

Fig. 12 Schematic diagram of the thermal quenching mechanism of Eu³⁺ (a) and Mn⁴⁺ (b).

As shown in Fig. 12a, the outer electrons of Eu³⁺ are shielded by the fully filled 5S² and 5p⁶ orbitals, so it has almost no effect on the optical transitions within the 4fⁿ configuration. There is no intersection between its ground state and excited state energy levels. In addition, when the external temperature increases, the lattice undergoes severe vibrations. Furthermore, in the absence of radiative transitions, electrons return to their ground state by emitting phonons, known as multi-phonon deexcitation (MPD).⁴⁰ Unlike Eu³⁺, the thermal quenching behavior of Mn⁴⁺ occurs through the parabolic intersection of thermally activated ⁴T_2g and ⁴A_2g (Fig. 12b). The thermal activation energy (E_a) is defined as the energy difference between the lowest point and the intersection point of the ²E_g energy level. In addition, due to the strong coupling effect between Mn⁴⁺ and surrounding ligands, Mn⁴⁺ is highly susceptible to lattice vibration and lattice distortion during external heating.⁴¹ Therefore, Mn⁴⁺ often exhibits stronger thermal quenching behavior than Eu³⁺.

For phosphors MLSO:Eu³⁺,Mn⁴⁺, although the intensity of emission peaks at 612 nm (Eu³⁺: ⁵D₀ → ⁷F₂) and 695 nm (Mn⁴⁺: ²E_g → ⁴A_2g) decreased monotonically, the relative intensity of the two emission peaks was significantly different. The relative variation of the luminescence intensity of the two emission peaks can be used as the fluorescence intensity ratio temperature sensing parameter of the double probe. The fluorescence intensity ratio of Mn⁴⁺–Eu³⁺ dual luminescent centers also shows a gradual decrease with increasing temperature. The variation of FIR (I_Mn/I_Eu) with temperature can be characterized using eqn (5).^42,43

In eqn (5), B₁, B₂, C, D and F are constants, k is the Boltzmann constant (k = 8.629 × 10⁻⁵ eV K⁻¹), and ΔE_Mn, ΔE_Eu, and ΔE_Mn/Eu represent the corresponding activation energies. In the temperature range of 303–503 K, the FIR (I_Mn/I_Eu) values of both the phosphor MLSO:0.02Eu³⁺,0.002Mn⁴⁺ and the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺ show a decreasing trend with increasing temperature, with the FIR (I_Mn/I_Eu) value of the phosphor MLSO:0.02Eu³⁺,0.002Mn⁴⁺ showing a more significant change (Fig. 13a). Absolute sensitivity (S_a) and relative sensitivity (S_r) are important parameters for evaluating fluorescence temperature measurement performance, which can be determined using formulas (6) and (7).^43,44 Therefore, S_a and S_r corresponding to phosphors MLSO:xEu³⁺,0.002Mn⁴⁺ (x = 0.02, 0.06) were calculated and plotted, as shown in Fig. 13b and c. The S_a and S_r of phosphors MLSO:xEu³⁺,0.002Mn⁴⁺ (x = 0.02, 0.06) show a trend of first increasing and then decreasing with increasing temperature. For the phosphor MLSO:0.02Eu³⁺,0.002Mn⁴⁺, the maximum S_a value is 0.17 K⁻¹ (@400 K), while the maximum S_r value is 3.97% K⁻¹ (@440 K). However, for the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺, S_amax = 0.04 K⁻¹ (@362 K) and S_rmax = 2.46% K⁻¹ (@411 K), indicating that the phosphor MLSO:0.02Eu³⁺,0.002Mn⁴⁺ has better sensitivity performance (Fig. S10–S13†). The performance comparison between the phosphor MLSO:0.02Eu³⁺,0.002Mn⁴⁺ and recently reported FIR-based optical thermometric phosphors is presented in Table 2. These results clearly demonstrate that MLSO:0.02Eu³⁺,0.002Mn⁴⁺ possesses high relative sensitivity, indicating its potential application value in the field of optical thermometry.

Fig. 13 Diagram of the relationship between the (a) FIR value, (b) S_a value and (c) S_r value of phosphors MLSO:xEu³⁺,0.002Mn⁴⁺ (x = 0.02, 0.06) and temperature (λ_ex = 392 nm).

Table 2 The temperature-sensing characteristics of the FIR mode in phosphors with diverse matrix materials

Material	Method	Temperature range (K)	Max. Sr (% K^−1) (K)	Ref.
Ca3La2W2O12:Dy^3+/Mn^4+	FIR	298–573	1.95 (523)	45
La2MgSnO6:Eu^3+/Mn^4+	FIR	298–473	1.58 (432)	46
SrGaB2O7:Bi^3+/Eu^3+	FIR	293–573	1.55 (423)	47
YAlO3:Yb^3+/Mn^4+/Ho^3+	FIR	293–563	1.11 (453)	43
MLSO:0.02Eu^3+,0.002Mn^4+	FIR	303–503	3.97 (440)	This work

---

In order to further investigate the luminescence performance of the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺, the thermal stability of the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺ was studied by temperature-dependent photoluminescence spectroscopy. As shown in Fig. 14a, with the increase of environment temperature, the luminescence intensity of the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺ gradually decreased. Notably, when the ambient temperature reaches 383 K, the luminescence intensity of the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺ still retains 51.69% of its room-temperature value. The thermal quenching activation energy (E_a) of the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺ was obtained using eqn (8).⁴⁸

Fig. 14 (a) The inset shows the normalized PL intensity of MLSO:0.06Eu³⁺,0.002Mn⁴⁺ as a function of temperature (λ_ex = 392 nm). (b) The activation energy of MLSO:0.06Eu³⁺,0.002Mn⁴⁺ (λ_ex = 392 nm).

In eqn (8), I_T and I₀ represent the fluorescence intensity at temperature T and room temperature, respectively, where A is a constant and k is the Boltzmann constant. Logarithmic processing was performed on eqn (8) to obtain eqn (9).

A linear relationship was drawn using ln(I₀/I_T − 1) and 1/kT, as shown in Fig. 14b, with a linear slope of −0.3438. The activation energy of MLSO:0.06Eu³⁺,0.002Mn⁴⁺ was calculated as 0.3438 eV, indicating that the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺ has good thermal stability.

3.4. CIE chromaticity coordinates of MLSO:Eu³⁺,Mn⁴⁺ and plant growth illumination

In order to further investigate the luminescence performance of the phosphor MLSO:Eu³⁺,Mn⁴⁺, we tested the color coordinates and color temperature of the phosphor MLSO:Eu³⁺,Mn⁴⁺, and the relevant data are listed in Table 3. As shown in Fig. 15, with the continuous increase of Eu³⁺ doping, the color chart of the phosphor MLSO:Eu³⁺,Mn⁴⁺ does not change significantly, about (0.64, 0.35), all located in the red light region. However, the color temperature significantly increased from 4541 K to 9178 K. This phenomenon is attributed to the gradual enhancement of the intensity ratio (I₆₁₂/I₆₉₅) between the 612 nm and 695 nm emission peaks in the photoluminescence spectrum with increasing Eu³⁺ concentration (Fig. S14†). The phosphor MLSO:Eu³⁺,Mn⁴⁺ emits bright red light under 302 nm and 365 nm light, respectively (Fig. 15).

Fig. 15 CIE chromaticity diagram for MLSO:xEu³⁺,0.002Mn⁴⁺ (0.02 ≤ x ≤ 0.12) phosphors and photos of MLSO:xEu³⁺,0.002Mn⁴⁺ (x = 0.02, x = 0.06) under different lighting conditions.

Table 3 CIE chromaticity coordinates for MLSO:xEu³⁺,0.002Mn⁴⁺ (0.02 ≤ x ≤ 0.012) phosphors

Sample	CIE (x, y)	Color temperature (K)
MLSO:0.02Eu^3+,0.002Mn^4+	(0.6294, 0.3701)	4541 K
MLSO:0.04Eu^3+,0.002Mn^4+	(0.6428, 0.3568)	6991 K
MLSO:0.06Eu^3+,0.002Mn^4+	(0.6437, 0.3559)	7211 K
MLSO:0.08Eu^3+,0.002Mn^4+	(0.6496, 0.3501)	8854 K
MLSO:0.10Eu^3+,0.002Mn^4+	(0.6500, 0.3495)	9025 K
MLSO:0.12Eu^3+,0.002Mn^4+	(0.6506, 0.3491)	9178 K

---

In order to develop and explore the commercial value of the phosphor MLSO:Eu³⁺,Mn⁴⁺, we evenly mixed the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺ with silica gel and coated it on a 395 nm UV chip to make LED devices. At 110 mA direct current, the sample glows red, as shown in the illustration in Fig. 16a. The electroluminescence spectra of LED device beads present two wide spectral bands at 550–800 nm, which are the emission spectra of Eu³⁺ and Mn⁴⁺, and the peak shape is basically consistent with the emission spectra of the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺ (Fig. 7). Considering the perception and demand range of plants for light, the electroluminescence spectrum of LED devices was compared with the absorption spectrum of light required for plant growth. Surprisingly, it was found that the spectrum was consistent with the absorption spectra of chlorophyll (A and B) and photosensitizers (Pr and Pfr), with a high matching degree with chlorophyll (B) and the photosensitizers (Pr and Pfr) (Fig. 16b), fully indicating the potential application value of the LED devices in indoor lighting for plant growth.

Fig. 16 (a) LED device prepared by combining an ultraviolet chip (395 nm) with the MLSO:0.06Eu³⁺,0.002Mn⁴⁺ phosphor (110 mA); (b) absorption spectra of chlorophyll (A and B) and phytochromes (Pr and Pfr) and the electroemission spectrum of the LED devices.

4. Conclusion

In summary, a series of novel red phosphors MLSO:Eu³⁺,Mn⁴⁺ have been successfully synthesized through a high-temperature solid-state reaction method. Eu³⁺ and Mn⁴⁺ successfully entered the Mg₂LaSbO₆ matrix lattice, occupying the La³⁺ and Sb⁵⁺ sites, respectively, causing a linear decrease in the lattice parameters (a, b, c) and volume (V) of the phosphors MLSO:Eu³⁺,Mn⁴⁺. The characteristic linear emission (⁵D_0,1 → ⁷F_J) of Eu³⁺ and the characteristic deep red emission (²E_g → ⁴A_2g) of Mn⁴⁺ were observed in the photoluminescence spectrum of phosphors MLSO:Eu³⁺,Mn⁴⁺. Among them, the quenching mechanism of Eu³⁺ concentration in the phosphor MLSO:Eu³⁺,Mn⁴⁺ is the dipole–dipole (d–d) interaction. As the fluorescence intensity of Eu³⁺ and Mn⁴⁺ in the phosphor MLSO:Eu³⁺,Mn⁴⁺ changes at different rates under the influence of temperature, the phosphor MLSO:Eu³⁺,Mn⁴⁺ can be used as a dual probe fluorescence intensity specific temperature sensor. For the phosphor MLSO:0.02Eu³⁺,0.002Mn⁴⁺, the maximum S_a value is 0.17 K⁻¹ (@400 K), while the maximum S_r value is 3.97% K⁻¹ (@440 K). Surprisingly, it was found that the electroluminescence spectra of LED devices prepared using the phosphor MLSO:0.06Eu³⁺,0.002Mn⁴⁺ were consistent with the absorption spectra of chlorophyll (A and B) and photosensitizers (Pr and Pfr) required for plant growth, fully indicating the potential application value of the phosphor MLSO:Eu³⁺,Mn⁴⁺ in indoor lighting for plant growth.

Data availability

All relevant data are within the manuscript and its additional files.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by grants from Anhui Postdoctoral Scientific Research Program Foundation (2024C932), the Anhui Province High-level Talent Young Scholars Project (GDYY2024001), the Key projects of the Department of Education of Anhui Province (KJ2019A0888, KJ2021A1186), and the Road Engineering Materials Research and Innovation Team of Anhui Sanlian University (XJTD2025001).

References

1. B. T. Huy, A. P. Kumar, T. T. Thuy, N. N. Nghia and Y. Lee, Appl. Spectrosc. Rev., 2020, 57, 1–35.
2. Y. Chen, F. Zhao, K. Qiang, Q. Mao, H. Yang, Y. Zhu, M. Liu and J. Zhong, Adv. Opt. Mater., 2024, 12, 2401688.
3. H. Hou, S. Zhang, H. Wang, T. Lin, B. Zou and R. Zeng, ACS Appl. Mater. Interfaces, 2024, 16, 31332–31340.
4. V. N. Nguyen, Y. H. Lee, T. T. Le-Vu, J. Y. Lin, H. C. Kan and C. C. Hsu, ACS Appl. Nano Mater., 2022, 5, 15172–15182.
5. C. Sun, C. Xu, W. Ren, F. Hu, J. Yuan, Q. Wang, Y. Xie, K. Wang and D. Zhang, J. Alloys Compd., 2025, 1010, 177374.
6. Z. Zhang, J. Tang, Mi. Jin, Y. Li, W. Chen, C. Chen, J. Xiang and C. Guo, Ceram. Int., 2023, 49, 38465–38473.
7. L. Y. Shi, D. Zhao, R. J. Zhang, Q. X. Yao and W. Liu, J. Am. Ceram. Soc., 2022, 105, 7479–7491.
8. M. H. Yu, D. Zhao, R. J. Zhang, Q. X. Yao, L. Jia and Q. Zong, Ceram. Int., 2024, 50, 11766–11775.
9. B. Zhu, L. Wang, Q. Shi, H. Guo, J. Qiao, C. Cui and P. Huang, J. Alloys Compd., 2023, 948, 169717.
10. J. Kong, N. Liu, Z. Wang, R. Zhou and Y. Wang, J. Alloys Compd., 2025, 1010, 178342.
11. X. F. Wang, P. Zhang, X. L. Xiao, R. S. Lei, L. H. Huang, S. Q. Xu, S. L. Zhao and W. X. Lang, J. Mater. Chem. C, 2024, 12, 8977–8986.
12. F. Wang, H. Chen, S. Zhang and H. Jin, J. Am. Ceram. Soc., 2025, 108, e20200.
13. R. Sun, X. Wei, H. Yu, P. Y. Chen, H. Y. Ni, J. H. Li, J. B. Zhou and Q. H. Zhang, Dalton Trans., 2023, 52, 2825–2832.
14. W. G. Ran, Z. C. Zhang, X. L. Ma, G. S. Sun and T. J. Yan, J. Lumin., 2023, 255, 119562.
15. F. Wang, H. Chen, S. Zhang and H. Jin, Dalton Trans., 2024, 53, 12465–12476.
16. F. Wang, H. H. Chen, S. W. Zhang and H. Q. Jin, J. Alloys Compd., 2023, 942, 168888.
17. H. Fan, Z. Lu, Y. Meng, P. Chen, L. Zhou, J. Zhao and X. He, Opt. Laser Technol., 2022, 148, 107804.
18. J. Long, Y. Xu, K. Cheng, X. Liu, W. Huang and C. Deng, Opt. Mater., 2023, 141, 113967.
19. Y. X. Bi, D. Zhao, R. J. Zhang, L. Jia and Q. Zong, Microchem. J., 2024, 205, 111375.
20. W. Xia, L. Liang, Q. Mao, Y. Ding, M. Liu and J. Zhong, Mater. Des., 2024, 241, 112936.
21. F. Wang and H. Chen, Inorg. Chem. Commun., 2024, 159, 111720.
22. L. Fu, Y. Yang, Y. Zhang, X. Ren, Y. Zhu, J. Zhu, Y. Wu, J. Wang and X. Feng, J. Lumin., 2021, 237, 118165.
23. R. D. Shannon, Acta Crystallogr., Sect. A, 1976, 32, 751–767.
24. P. Du, L. H. Luo, H. K. Park and J. S. Yu, Chem. Eng. J., 2016, 306, 840–848.
25. F. Wang, H. Chen, S. Zhang and H. Jin, Ceram. Int., 2023, 49, 1487–1495.
26. P. Zhang, X. F. Wang, S. L. Zhao, X. L. Wang, R. S. Lei, L. H. Huang and S. Q. Xu, J. Phys. Chem. C, 2023, 127, 12770–12778.
27. T. X. Nguyen, Y. C. Liao, C. C. Lin, Y. H. Su and J. M. Ting, Adv. Funct. Mater., 2021, 2101632, 1–10.
28. Y. Pan, X. Xu, Y. Zhong, L. Ge, Y. Chen, J.-P. M. Veder, D. Guan, R. O'Hayre, M. Li, G. Wang, H. Wang, W. Zhou and Z. Shao, Nat. Commun., 2020, 11, 2002.
29. C. Yang, Q. You, M. Liu, X. Zhou, X. Lu, H. Wang, R. Hu, W. Liu and X. Jiang, Ceram. Int., 2025, 51, 16969–16977.
30. K. Qiang, Y. Ye, Q. Mao, F. Chen, L. Chu, M. Liu and J. Zhong, Mater. Des., 2024, 241, 112906.
31. G. Li, Y. Xue, Q. Mao, L. Pei, H. He, M. Liu, L. Chu and J. Zhong, Dalton Trans., 2022, 51, 4685–4694.
32. F. Wang, H. Chen, S. Zhang and H. Jin, J. Alloys Compd., 2023, 969, 172394.
33. F. Wang and H. Chen, Inorg. Chem. Commun., 2024, 159, 111720.
34. C. A. Morrison and R. P. Leavitt, Handbook on the physics and chemistry of rare earths, Elsevier, Amsterdam, 1982, p. 146.
35. A. Fu, Q. Pang, H. Yang and L. Zhou, Opt. Mater., 2017, 70, 144–152.
36. Z. Y. Mao, Y. C. Zhu, L. Gan, Y. Zeng, F. F. Xu, Y. Wang, H. Tian, J. Li and D. J. Wang, J. Lumin., 2013, 134, 148–153.
37. H. Li, L. Li, W. Zhao, X. Zhou and Y. Hu, Mater. Today Chem., 2023, 32, 101661.
38. H. K. Liu, K. Nie, Y. Y. Zhang, L. F. Mei, D. V. Deyneko and X. X. Ma, J. Rare Earths, 2023, 41, 1288.
39. H. Chen and F. Wang, Inorg. Chem. Commun., 2023, 150, 110470.
40. X. Shi, Y. Chen, G. Li, K. Qiang, Q. Mao, L. Pei, M. Liu and J. Zhong, Ceram. Int., 2023, 49, 20839–20848.
41. T. Senden, R. J. A. Dijk-Moes and A. Meijerink, Light: Sci. Appl., 2018, 7, 8.
42. X. Shi, M. Zhang, X. Lu, Q. Mao, L. Pei, H. Yu, J. Zhang, M. Liu and J. Zhong, Mater. Today Chem., 2023, 27, 101264.
43. D. Chen, W. Xu, S. Yuan, X. Li and J. Zhong, J. Mater. Chem. C, 2017, 5, 9619–9628.
44. P. Nayak, S. S. Nanda, S. Pattnaik, V. K. Rai, R. K. Sharma and S. Dash, Opt. Laser Technol., 2023, 159, 108990.
45. X. T. Lv, D. Zhao, R. Zhang and Q. X. Yao, Ceram. Int., 2024, 50, 53281–53287.
46. R. Ma, K. Cheng, B. Li, C. Yang, B. Cao, X. Gong, C. Deng and W. Huang, Ceram. Int., 2024, 50, 39811–39822.
47. X. Liu, S. Shi, K. Yang, L. Chen, D. Deng and S. Xu, J. Alloys Compd., 2021, 879, 160247.
48. Z. G. Xia, S. H. Miao, M. S. Molokeev, M. Y. Chen and Q. L. Liu, J. Mater. Chem. C, 2016, 4, 1336–1344.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5dt00651a

‡ These authors contributed equally to this work and should be considered co-first authors.

---