Enhancing the Luminescence of La3Mg2NbO9:Mn4+ Phosphor through H3BO3 and Charge Compensator Co-Doping for Use in Plant Growth Lamps

Mn4+-doped red-light-emitting phosphors have become a research hotspot that can effectively enhance photosynthesis and promote morphogenesis in plants. Herein, the red phosphor La3Mg2NbO9:Mn4+ was synthesized through the solid-state reaction method. The effects of adding H3BO3 and a charge compensator R+ (R = Li, Na, K) on the crystal structure, morphology, quantum efficiency, and luminous performance of the La3Mg2NbO9:Mn4+ phosphor were systematically analyzed, respectively. The results showed that adding H3BO3 flux and a charge compensator improved the quantum efficiency and luminescence intensity. The emission intensity of the phosphor was enhanced about 5.9 times when Li+ was used as the charge compensator, while it was enhanced about 240% with the addition of H3BO3 flux. Remarkably, it was also found that the addition of H3BO3 flux and a charge compensator simultaneously improved the thermal stability at 423 K from 47.3% to 68.9%. The prototype red LED fabricated using the La3Mg2NbO9:Mn4+,H3BO3,Li+ phosphor exhibited a perfect overlap with the phytochrome absorption band for plant growth. All of these results indicate that the La3Mg2NbO9:Mn4+,H3BO3,Li+ phosphor has great potential for use in agricultural plant lighting.


Introduction
Fluorescent conversion white-light-emitting diodes (WLEDs) are rapidly replacing traditional illuminant sources with their superior intense brightness, longevity, energysaving properties, environmental protection, and simple processes, and are gradually becoming mainstream products in the lighting market [1][2][3][4].Currently, the mainstream WLEDs in the market obtain white light by compounding Y 3 Al 5 O 12 :Ce 3+ yellow phosphor with InGaN blue LED chips [5,6].Nevertheless, it is hard for them to satisfy the basic needs of high-quality indoor lighting and backlighting display due to their shortage of red light.Complementing the red emission by adding red phosphor is an effective way to solve the above problems.At present, there are two main types of commercialized red phosphors for WLEDs: Eu 2+ -activated nitride red phosphors, represented by CaAlSiN 3 :Eu 2+ , and Mn 4+ -activated fluoride red phosphors, represented by K 2 SiF 6 :Mn 4+ [7,8].However, both of these are limited in their widespread application due to the decrease in quantum efficiency caused by the partial overlap of their excessively broad absorption bands with the emission peaks of the yellow phosphor, as well as the high costs of their synthesis processes due to the need for high-temperature and high-pressure conditions [9][10][11].Therefore, finding a high-efficiency red phosphor is a top-priority mission nowadays.
Light plays a pivotal role in plant growth, wherein the essential light requirements in the blue (400-480 nm), red (600-680 nm), and far-red (680-780 nm) regions are responsible for photosynthesis, phototropism, and photomorphogenesis, respectively [12,13].As a transition metal ion, Mn 4+ has plentiful reserves and a low price compared with traditional rare-earth luminescence centers [14].In a suitable crystal structure, Mn 4+ ions with a 3d 3 electron configuration can emit bright light, and the emission spectrum is mainly concentrated in the red/far-red wavelength range of 620-740 nm [15,16].Compared with fluoride phosphors, Mn 4+ -activated oxide phosphors with better chemical stability that are synthesized without the highly corrosive HF have drawn much attention in terms of WLEDs, plant illumination, and infrared illumination [17].To realize a multi-functional Mn 4+ phosphor, the choice of an optimum host material, which immediately decides the external electric fields, luminescent properties, and thermal stability of the synthetic phosphor, is essential.Recently, niobate has aroused the enthusiasm of researchers in the fields of luminescence, photocatalysis, and optical information storage.Jeong Seog Kim et al. (1999) reported the perovskite-type structure La 3 Mg 2 NbO 9 (LMN), which has been widely studied.In this paper, compound LMN was selected as the luminescent host, which can be prepared easily with cheap raw materials including Mg 2 (OH) 2 CO 3 , La 2 O 3 , and Nb 2 O 5 .Compared with traditional aluminates, tellurites, etc., niobates have many advantages, such as their affluent crystalline environment, excellent thermal stability, and environmentally friendly experimental methods [18].To our knowledge, in all of the published literature, relevant developments in the use of the Mn 4+ -doped LMN phosphor have not yet been studied.

Structural Property Analysis
The X-ray powder diffraction (XRD) patterns of LMN:xMn 4+ and the standard cards (PDF#53-0302) are illustrated in Figure 1a.To analyze the effects of H 3 BO 3 and the charge compensator on the crystal structure, the phases of the LMN:Mn 4+ ,yB (0.03 ≤ y ≤ 0.15) and LMN:Mn 4+ ,B,R + (R = Li, Na, K) samples were also tested, as depicted in Figure 1a.All of the characteristic peaks corresponded to the standard cards, indicating that the structural effect of these dopants (Mn 4+ , Li + , Na + , K + , and H 3 BO 3 ) on the matrix was not significant.Additionally, the phosphors with H 3 BO 3 exhibited a higher intensity of XRD patterns, revealing that H 3 BO 3 can improve the crystallinity of the host.The ionic radii for Mn 4+ (CN = 6; CN: coordination number), La 3+ (CN = 8), Mg 2+ (CN = 6), and Nb 5+ (CN = 6) are 0.53 Å, 1.16 Å, 0.72 Å, and 0.64 Å, respectively [19].When introducing Mn 4+ into the LMN matrix, Mn 4+ ions preferentially replace Nb 5+ ions due to the similarity of their ionic radii.In order to obtain further information on the crystal structure of these samples, the LMN, LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,Li + samples were subjected to Rietveld refinement by using the general structure analysis system (GSAS) method, as presented in Figure 1b-e.The low residual factors of R wp , R p , and χ 2 mean the refinement results are credible [20].The detailed refined results for LMN, LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,Li + are shown in Table 1, which shows that these synthesized samples are classified as the monoclinic system and correspond to the P2 1 /n space group.Furthermore, the cell parameters of LMN remained almost unchanged, except for minor shrinkage after the addition of H 3 BO 3 .According to the refinement results, the crystal structure of LMN is shown in Figure 1f.It is observed that only one La cation site with a Wyckoff position is connected to eight oxygen ions to form the LaO 8 octahedron.In addition, Mg and Nb at the same site link with six oxygen ions to form the MgO 6 /NbO 6 polyhedron, which connects with the LaO 8 polyhedron through sharing the same site.
classified as the monoclinic system and correspond to the P21/n space group.Furthermore, the cell parameters of LMN remained almost unchanged, except for minor shrinkage after the addition of H3BO3.According to the refinement results, the crystal structure of LMN is shown in Figure 1f.It is observed that only one La cation site with a Wyckoff position is connected to eight oxygen ions to form the LaO8 octahedron.In addition, Mg and Nb at the same site link with six oxygen ions to form the MgO6/NbO6 polyhedron, which connects with the LaO8 polyhedron through sharing the same site.Shape and morphology can have an effect on the performance of luminescent materials.Thus, the scanning electron microscopy (SEM) images of LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,Li + are shown in Figure 2a-c.As is obvious, the LMN:Mn 4+ phosphor shows an irregular polyhedron morphology and micro-granularity with a particle size of approximately 2-5 µm.The surface morphology shows that the grain shape is significantly more anisotropic and less agglomerated after the introduction of the H 3 BO 3 [21].The melting of H 3 BO 3 can enhance the sliding and rotation of the particles, promoting contact between the particles and particle growth and thus increasing the size of the particles [22].In addition, it is widely known that phosphors with a regular morphology can improve paste properties, improve the filler density, and enhance phosphor luminescence performance [23].Moreover, the elemental distributions of La, Mg, Nb, O, and Mn shown in Figure 2d clearly show that the constituent elementals are uniformly distributed in the LMN:Mn 4+ ,B,Li + powder.
Shape and morphology can have an effect on the performance of luminescent materials.Thus, the scanning electron microscopy (SEM) images of LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,Li + are shown in Figure 2a-c.As is obvious, the LMN:Mn 4+ phosphor shows an irregular polyhedron morphology and micro-granularity with a particle size of approximately 2-5 μm.The surface morphology shows that the grain shape is significantly more anisotropic and less agglomerated after the introduction of the H3BO3 [21].The melting of H3BO3 can enhance the sliding and rotation of the particles, promoting contact between the particles and particle growth and thus increasing the size of the particles [22].In addition, it is widely known that phosphors with a regular morphology can improve paste properties, improve the filler density, and enhance phosphor luminescence performance [23].Moreover, the elemental distributions of La, Mg, Nb, O, and Mn shown in Figure 2d clearly show that the constituent elementals are uniformly distributed in the LMN:Mn 4+ ,B,Li + powder.

Electronic Properties
The electronic structural characteristics of matrix materials have a significant impact on luminescent materials.In order to gain insight into its electronic properties, the electronic energy band structure of LMN was studied using the Cambridge Sequential Total Energy Package (CASTEP) module.As shown in Figure 3a, the top of the valence band (VB) located at point Γ compares with the bottom of the conduction band (CB) located at point D. This result indicates that the LMN matrix is an indirect bandgap semiconductor and its energy gap (Eg) is 4.237 eV, which indicates that it is sufficient for the energy level of Mn 4+ to be the luminescent material [24].The maps of the partial and total density of states are given in Figure 3b, which can help to understand the band structure component deeply.Compared with other atoms, the p orbital of Nb and O make the most contributions to the CB, which indicates that electronic orbitals within the NbO6 octahedron are rather discrete.Thus, the Mn 4+ ions doped at the Nb 5+ site would also have a high possibility to generate and obtain free carriers through the CB [25].

Electronic Properties
The electronic structural characteristics of matrix materials have a significant impact on luminescent materials.In order to gain insight into its electronic properties, the electronic energy band structure of LMN was studied using the Cambridge Sequential Total Energy Package (CASTEP) module.As shown in Figure 3a, the top of the valence band (VB) located at point Γ compares with the bottom of the conduction band (CB) located at point D. This result indicates that the LMN matrix is an indirect bandgap semiconductor and its energy gap (Eg) is 4.237 eV, which indicates that it is sufficient for the energy level of Mn 4+ to be the luminescent material [24].The maps of the partial and total density of states are given in Figure 3b, which can help to understand the band structure component deeply.Compared with other atoms, the p orbital of Nb and O make the most contributions to the CB, which indicates that electronic orbitals within the NbO 6 octahedron are rather discrete.Thus, the Mn 4+ ions doped at the Nb 5+ site would also have a high possibility to generate and obtain free carriers through the CB [25].
Figure 3c depicts the diffuse reflectance spectra (DRS) of the LMN host, LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,Li + .At 450~800 nm, the LMN presents a high-reflectance plateau, while at 200~450 nm, the reflectance of the LMN decreases sharply because of host absorption.Compared with the host material, the LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,Li + samples show more prominent absorption peaks around 300 nm, which resulted from the O 2− →Mn 4+ charge transfer band (CTB).Additionally, two wide peaks are observed from 350 to 550 nm, which belong to the 4 A 2g → 4 T 1g and 4 A 2g → 4 T 2g transitions of the Mn 4+ ions, respectively [26].Furthermore, LMN:Mn 4+ ,B,Li + shows the most prominent absorption, followed by the LMN:Mn 4+ ,B sample and the LMN:Mn 4+ sample.Based on the DRS of the LMN host and the theory given by Kubelka-Munk, the E g can be estimated with the following equation [27]: where h, ν, and A denote the absorption factor, Planck constant, and constant, respectively.n = 1/2 or 2 indicates that the sample is an indirect or direct bandgap semiconductor [28].
According to the results in Figure 3a, LMN is an indirect bandgap semiconductor, so the value of n is 1/2.The curve of [hν(R ∞ )] 1/2 versus the photon energy of the LMN host is shown in Figure 3d, and the calculated value of E g is 4.30 eV, which is close to the result of the theoretical calculation (4.237 eV).
Molecules 2024, 29, x FOR PEER REVIEW 5 of 15 of the Mn 4+ ions, respectively [26].Furthermore, LMN:Mn 4+ ,B,Li + shows the most prominent absorption, followed by the LMN:Mn 4+ ,B sample and the LMN:Mn 4+ sample.Based on the DRS of the LMN host and the theory given by Kubelka-Munk, the Eg can be estimated with the following equation [27]: where h, ν, and A denote the absorption factor, Planck constant, and constant, respectively.n = 1/2 or 2 indicates that the sample is an indirect or direct bandgap semiconductor [28].According to the results in Figure 3a, LMN is an indirect bandgap semiconductor, so the value of n is 1/2.The curve of [hν(R∞)] 1/2 versus the photon energy of the LMN host is shown in Figure 3d, and the calculated value of Eg is 4.30 eV, which is close to the result of the theoretical calculation (4.237 eV).

Optical Luminescence Properties of Samples
The photoluminescence excitation (PLE) spectra of the LMN:Mn 4+ phosphor, which are composed of two broad excitation bands in the range of 250-600 nm, are shown in Figure 4a.The one located at 348 nm compares with the other located at 504 nm.In addition, the PLE spectra can be fitted using the Gaussian distribution method to form four characteristic bands peaking around 504, 417, 367, and 318 nm, which can be attributed to 4 A2g→ 4 T2g, 4 A2g→ 2 T2g, 4 A2g→ 4 T1g transitions of the Mn 4+ ions and the CTB of O 2− →Mn 4+ , respectively [29].As depicted in Figure 4b, the dependence of photoluminescence (PL) spectra for LMN:xMn 4+ (0.15% ≤ x ≤ 0.9%) is excited at 348 nm.The LMN:xMn 4+ phosphors exhibit a deep-red luminescence attributed to the 2 Eg→ 4 A2g spin-forbidden transition of the Mn 4+ ions centered at 708 nm.Furthermore, as the concentration of the Mn 4+ ions increases, the PL spectra show the same peak pattern and position except for different intensities.The inset of Figure 4b demonstrates that the luminous intensity ascends and then

Optical Luminescence Properties of Samples
The photoluminescence excitation (PLE) spectra of the LMN:Mn 4+ phosphor, which are composed of two broad excitation bands in the range of 250-600 nm, are shown in Figure 4a.The one located at 348 nm compares with the other located at 504 nm.In addition, the PLE spectra can be fitted using the Gaussian distribution method to form four characteristic bands peaking around 504, 417, 367, and 318 nm, which can be attributed to 4 A 2g → 4 T 2g , 4 A 2g → 2 T 2g , 4 A 2g → 4 T 1g transitions of the Mn 4+ ions and the CTB of O 2− →Mn 4+ , respectively [29].As depicted in Figure 4b, the dependence of photoluminescence (PL) spectra for LMN:xMn 4+ (0.15% ≤ x ≤ 0.9%) is excited at 348 nm.The LMN:xMn 4+ phosphors exhibit a deep-red luminescence attributed to the 2 E g → 4 A 2g spin-forbidden transition of the Mn 4+ ions centered at 708 nm.Furthermore, as the concentration of the Mn 4+ ions increases, the PL spectra show the same peak pattern and position except for different intensities.The inset of Figure 4b demonstrates that the luminous intensity ascends and then descends with the doping concentration of Mn 4+ ions from 0.15% to 0.9%.The optimal luminescence concentration is 0.6% due to the concentration quenching effect [30].descends with the doping concentration of Mn 4+ ions from 0.15% to 0.9%.The optimal luminescence concentration is 0.6% due to the concentration quenching effect [30].To improve the luminescence properties, the flux of H3BO3 powders with different doping contents were added to the LMN:Mn 4+ sample.As shown in Figure 5a, the PL spectra of LMN:Mn 4+ ,yH3BO3 (y = 0, 3%, 6%, 9%, 12%, and 15%) phosphors were systematically studied.Except for the luminous intensity, no notable changes were detected with the concentration of H3BO3 changing from 3% to 15%.However, compared to the LMN:Mn 4+ sample, the luminescence intensity of all doped H3BO3 phosphors significantly improved.Doping with H3BO3 can reduce the sintering temperature, enhance the crystallinity of the sample, and increase the luminous intensity [31].The variation in luminescence intensity with different concentrations of H3BO3 is shown in the illustration of Figure 5a.Furthermore, the best dopant concentration of H3BO3 was 9%.The LMN:Mn 4+ ,B,R + (R = K, Na, Li) phosphors were successfully synthesized, which can assist researchers with exploring the improvement of luminescent properties.As shown in Figure 5b, it is noted that doping with K + , Na + , and Li + ions enhances the luminous intensity when comparing the PL spectra of LMN:Mn 4+ ,B.In our opinion, the main reason for such a result comes from the charge balance.The ionic radii of K + , Li + , and Na + are similar to that of Nb 5+ , so R + (R = K, Na, Li) will be replaced by Nb 5+ to form [Mn 4+ -R + ] pairs.And these [Mn 4+ -R + ] pairs can interrupt the adverse energy transfer among neighboring Mn 4+ ions to increase the luminescence intensity [32].Accordingly, the luminous intensity of LMN:Mn 4+ ,B can be enhanced through using K + , Na + , and Li + co-doping ions to improve the charge compensation.It is noteworthy that when Li+ ions were doped into LMN:Mn 4+ B, the luminescence intensity of the Li + ions was intensified much more than that of the K + /Na + ions.These outcomes likely stemmed from the fact that [Mn 4+ -Li + ] interrupts the energy transfer between Mn 4+ ions more efficiently than the ionic substitution between K + and Na + .Specifically, the luminescence intensity of the LMN:Mn 4+ sample is considered a regular standard, as shown in Figure 5c, and the luminescent strength of LMN:Mn 4+ ,B,Li + can increase up to 1420%, while the LMN:Mn 4+ ,B, LMN:Mn 4+ ,B,K + , and LMN:Mn 4+ ,B,Na + samples can increase up to 240%, 420%, and 600%, respectively.The energy level diagram of Mn 4+ in the octahedral coordination situation is further described using a Tanabe-Sugano diagram.This energy level transition process of LMN:Mn 4+ is matched well with the above results, as depicted in Figure 5d.In general, the crystal field strength (Dq) and Racah parameters (B, C) are calculated as follows [33]: To improve the luminescence properties, the flux of H 3 BO 3 powders with different doping contents were added to the LMN:Mn 4+ sample.As shown in Figure 5a, the PL spectra of LMN:Mn 4+ ,yH 3 BO 3 (y = 0, 3%, 6%, 9%, 12%, and 15%) phosphors were systematically studied.Except for the luminous intensity, no notable changes were detected with the concentration of H 3 BO 3 changing from 3% to 15%.However, compared to the LMN:Mn 4+ sample, the luminescence intensity of all doped H 3 BO 3 phosphors significantly improved.Doping with H 3 BO 3 can reduce the sintering temperature, enhance the crystallinity of the sample, and increase the luminous intensity [31].The variation in luminescence intensity with different concentrations of H 3 BO 3 is shown in the illustration of Figure 5a.Furthermore, the best dopant concentration of H 3 BO 3 was 9%.The LMN:Mn 4+ ,B,R + (R = K, Na, Li) phosphors were successfully synthesized, which can assist researchers with exploring the improvement of luminescent properties.As shown in Figure 5b, it is noted that doping with K + , Na + , and Li + ions enhances the luminous intensity when comparing the PL spectra of LMN:Mn 4+ ,B.In our opinion, the main reason for such a result comes from the charge balance.The ionic radii of K + , Li + , and Na + are similar to that of Nb 5+ , so R + (R = K, Na, Li) will be replaced by Nb 5+ to form [Mn 4+ -R + ] pairs.And these [Mn 4+ -R + ] pairs can interrupt the adverse energy transfer among neighboring Mn 4+ ions to increase the luminescence intensity [32].Accordingly, the luminous intensity of LMN:Mn 4+ ,B can be enhanced through using K + , Na + , and Li + co-doping ions to improve the charge compensation.It is noteworthy that when Li+ ions were doped into LMN:Mn 4+ B, the luminescence intensity of the Li + ions was intensified much more than that of the K + /Na + ions.These outcomes likely stemmed from the fact that [Mn 4+ -Li + ] interrupts the energy transfer between Mn 4+ ions more efficiently than the ionic substitution between K + and Na + .Specifically, the luminescence intensity of the LMN:Mn 4+ sample is considered a regular standard, as shown in Figure 5c, and the luminescent strength of LMN:Mn 4+ ,B,Li + can increase up to 1420%, while the LMN:Mn 4+ ,B, LMN:Mn 4+ ,B,K + , and LMN:Mn 4+ ,B,Na + samples can increase up to 240%, 420%, and 600%, respectively.The energy level diagram of Mn 4+ in the octahedral coordination situation is further described using a Tanabe-Sugano diagram.This energy level transition process of LMN:Mn 4+ is matched well with the above results, as depicted in Figure 5d.In general, the crystal field strength (D q ) and Racah parameters (B, C) are calculated as follows [33]: (2) E( ) 3.05 1.8 7.9 According the above equations, the results of Dq, B, and C values for Mn 4+ in the LMN are 1984.1,724.1, and 2911.3 cm −1 .Generally speaking, when Dq/B is greater than 2.1, the dopant ions are generally considered to be in a strong crystal field environment [34].Here, the Dq/B value is calculated as 2.74, indicating that the Mn 4+ ions are located in a strong crystal field.By using the same method, the Dq/B value of LMN:Mn 4+ ,B,Li + is calculated to be 2.83.These results demonstrate that the doping of H3BO3 and Li + into the lattice of the LMN:Mn 4+ phosphor can efficiently enhance the crystal field strength of Mn 4+ .To further investigate the effects of H3BO3 and the charge compensator, decay curves of the LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,R + (R = K, Na, Li) phosphors were obtained by monitoring at 708 nm, as depicted in Figure 6a.The experimental data of the decay curves were fitted using the following two-exponential function [35]: According the above equations, the results of D q , B, and C values for Mn 4+ in the LMN are 1984.1,724.1, and 2911.3 cm −1 .Generally speaking, when D q /B is greater than 2.1, the dopant ions are generally considered to be in a strong crystal field environment [34].Here, the D q /B value is calculated as 2.74, indicating that the Mn 4+ ions are located in a strong crystal field.By using the same method, the D q /B value of LMN:Mn 4+ ,B,Li + is calculated to be 2.83.These results demonstrate that the doping of H 3 BO 3 and Li + into the lattice of the LMN:Mn 4+ phosphor can efficiently enhance the crystal field strength of Mn 4+ .
To further investigate the effects of H 3 BO 3 and the charge compensator, decay curves of the LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,R + (R = K, Na, Li) phosphors were obtained by monitoring at 708 nm, as depicted in Figure 6a.The experimental data of the decay curves were fitted using the following two-exponential function [35]: where I (t) and I 0 are the intensities at time t and t = 0, respectively.A 1 and A 2 are constants, and τ 1 and τ 2 are the fast and slow lifetimes, respectively.Based on these fitting parameters, the average lifetime τ can be estimated using the following formula: ters, the average lifetime τ can be estimated using the following formula: Therefore, the average luminescence lifetimes of LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,R + (R = K, Na, Li) are evaluated to be 0.847 ms, 0.863 ms, 0.901 ms, 0.883 ms, and 0.975 ms, as depicted in Table 2. Based on the XRD structural analysis, lattice shrinkage also solidifies the structural rigidity, thereby inhibiting non-radiative transitions [36].The results show that a more efficient fluorescence lifetime can be acquired from adding H3BO3 into LMN:Mn 4+ .Additionally, the lifetimes of LMN:Mn 4+ ,B,R + (R = K, Na, Li) are similar and larger than those of LMN:Mn 4+ ,B, suggesting that the doping of R + (R = K, Na, Li) ions can efficiently decrease the feasibility of non-radiative transitions of Mn 4+ ions.In addition, the internal quantum efficiency (IQE) levels of the LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,Li + phosphors were measured and are shown in Figure 6b and Table 2.The IQE value and absorption coefficient (α abs) can be acquired using the following equation [37]: where LS is the spectral intensity of PL, ES is the spectral intensity of the PLE with the sample, and ER is the spectral intensity of the PLE without it in the integrating sphere.Figure 6b shows that the α abs values of the LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,Li + phosphors were computed to be 44.2%,58.5%, and 67.3%.Meanwhile, the IQE values of the LMN:Mn 4+ phosphors were improved remarkably from 43.3% to 50.2% for LMN:Mn 4+ ,B and 61.7% for LMN:Mn 4+ ,B,Li + .Compared with the LMN:Mn 4+ phosphor, LMN:Mn 4+ ,B and LMN:Mn 4+ ,B,Li + had a higher quantum yield, which was mainly due to the strong absorption of the phosphors to the 348 nm band.In addition, the obtained results are superior to that for several red phosphors including Li2MgTiO4:Mn 4+ (IQE = 32%) [29], Li6SrLa2Sb2O12:Mn 4+ (IQE = 17%) [33], Sr3NaSbO6:Mn 4+ (IQE = 56.2%)[38], Sr3LiSbO6:Mn 4+ (IQE = 52.3%)[39], SrAl3BO7:Mn 4+ (IQE = 32%) [40], and Ba2SrWO6:Mn 4+ Therefore, the average luminescence lifetimes of LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,R + (R = K, Na, Li) are evaluated to be 0.847 ms, 0.863 ms, 0.901 ms, 0.883 ms, and 0.975 ms, as depicted in Table 2. Based on the XRD structural analysis, lattice shrinkage also solidifies the structural rigidity, thereby inhibiting non-radiative transitions [36].The results show that a more efficient fluorescence lifetime can be acquired from adding H 3 BO 3 into LMN:Mn 4+ .Additionally, the lifetimes of LMN:Mn 4+ ,B,R + (R = K, Na, Li) are similar and larger than those of LMN:Mn 4+ ,B, suggesting that the doping of R + (R = K, Na, Li) ions can efficiently decrease the feasibility of non-radiative transitions of Mn 4+ ions.In addition, the internal quantum efficiency (IQE) levels of the LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,Li + phosphors were measured and are shown in Figure 6b and Table 2.The IQE value and absorption coefficient (α abs) can be acquired using the following equation [37]: where L S is the spectral intensity of PL, E S is the spectral intensity of the PLE with the sample, and E R is the spectral intensity of the PLE without it in the integrating sphere.

Thermal Stability Analysis of Phosphors
An important indicator of phosphor commercialization potential is thermal stability.Subsequently, Figure 7a-c show the thermal quenching of LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,Li + and their emission spectra at higher temperatures.In addition, Figure 7d-f display the corresponding contour maps of the thermal evolution of these samples excited at 348 nm from 298 K to 473 K.The relevant emission intensity of these samples exhibits a typical decreasing trend with the increase in temperature.This phenomenon is consistent with the thermal quenching effect found in most oxide phosphor systems doped with Mn 4+ [42].Figure 7g depicts that the luminescence intensity of LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,Li + can maintain 47.3%, 66.7%, and 68.9% of the initial intensity at 423 K.The results show that the doping of H 3 BO 3 and Li + ions significantly improved the thermal stability of the materials, which further demonstrates the effectiveness of the above-described H 3 BO 3 and Li + ion doping in reducing the probability of non-radiative transitions [43].As shown in Table 3, the results from the LMN:Mn 4+ ,B,Li + sample display greater thermal stability compared to some other up-to-date studies of Mn 4+ -doped oxide phosphors, such as Sr 2 LuTaO 6 :Mn 4+ (I 423 K /I 298 K = 25%) [9], SrLa 2 Al 2 O 7 :Mn 4+ (I 423 K /I 298 K = 43%) [34], CaY 0.5 Ta 0.5 O 3 :Mn 4+ (I 423 K /I 298 K = 50%) [44], CaLaLiTeO 6 :Mn 4+ (I 423 K /I 298 K = 63%) [45], Mg 3 Ga 2 SnO 8 :Mn 4+ (I 423 K /I 298 K = 50%) [46], Cs 2 NbOF 5 :Mn 4+ [47], and Li 6 SrLa 2 Sb2O 12 : Mn 4+ [48].In general, the activation energy (E a ) is the distance from the bottom of the 4 T 2 energy level to the intersection of the 4 T 2 and 4 A 2 energy levels [49].Figure 7i illustrates that, in the conformational coordinate scheme, the 4 T 2 and 4 A 2 energy levels intersect the activation energy.As the temperature increases, electrons situated in the 4 T 2 energy level cross the energy barrier of the activation energy to jump to the 4 A 2 energy level and make a non-radiative transition to the ground, resulting in luminescence quenching [50].The E a is calculated using the following Arrhenius equation for further analysis of the thermal quenching effect [51]: In general, the activation energy (Ea) is the distance from the bottom of the 4 T2 ene level to the intersection of the 4 T2 and 4 A2 energy levels [49].Figure 7i illustrates that the conformational coordinate scheme, the 4 T2 and 4 A2 energy levels intersect the acti tion energy.As the temperature increases, electrons situated in the 4 T2 energy level cr the energy barrier of the activation energy to jump to the 4 A2 energy level and make a n radiative transition to the ground, resulting in luminescence quenching [50].The Ea is c culated using the following Arrhenius equation for further analysis of the thermal quen ing effect [51]: ( Using this Arrhenius formula, the activation energies were calculated for each sa ple.The results were 0.149 eV for LMN:Mn 4+ , 0.198 eV for LMN:Mn 4+ ,B, and 0.223 eV LMN:Mn 4+ ,B,Li + , as depicted in Figure 7h.Apparently, the larger the Ea, the better the th mal stability of the phosphors.Using this Arrhenius formula, the activation energies were calculated for each sample.The results were 0.149 eV for LMN:Mn 4+ , 0.198 eV for LMN:Mn 4+ ,B, and 0.223 eV for LMN:Mn 4+ ,B,Li + , as depicted in Figure 7h.Apparently, the larger the E a , the better the thermal stability of the phosphors.

Potential Applications
The color coordinate is an important index for fluorescent materials.Based on the results of emission spectrum data excited at 348 nm, the Commission International deI'Eclairage (CIE) color coordinates for the LMN:Mn 4+ ,B,Li + phosphor were calculated to be (0.7306, 0.2693).Figure 8a shows that CIE chromaticity coordinates are located in the deep-red region and the coordinates are located at the edge of the CIE plot, indicating that this phosphor has a high color purity.The inset of Figure 8a shows the electronic photographs of the non-doped LMN host and the LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,Li + red phosphors exposed to daylight and 365 nm UV light.Compared to the matrix material, the deep-red emission from the other samples under 365 nm UV lamps can be clearly observed with the naked eye, and LMN:Mn 4+ ,B,Li + shows the strongest luminescence intensity.This result indicates that the LMN:Mn 4+ ,B,Li + phosphor as a redlight-emitting phosphor for WLEDs shows good prospects.For evaluating the availability of the LMN:Mn 4+ ,B,Li + phosphor in practical applications, we successfully combined this phosphor with a 410 nm chip to fabricate an LED. Figure 8b depicts the electroluminescence (EL) spectrum of the LED driven at a current of 20 mA.It can be observed that the PL spectrum contains two broadband peaks: a red emission band located in the 600-750 nm range and a blue emission band centered at 410 nm.The inset of Figure 8b also shows that the photo of the LED light driven with a 20 mA current illuminates a bright red emission.In plants, the chlorophylls A and B and the phytochromes P R and P FR are vital for plant growth [52].Figure 8b also shows the absorption spectra of these four major phytochromes.Notably, the PL spectra of the red LED perfectly overlap with the phytochrome absorption band for plant growth.These results confirm that the LMN:Mn 4+ ,B,Li + phosphor can be efficiently used to prepare red LEDs for plants.
deep-red region and the coordinates are located at the edge of the CIE plot, indicatin this phosphor has a high color purity.The inset of Figure 8a shows the electronic p graphs of the non-doped LMN host and the LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ red phosphors exposed to daylight and 365 nm UV light.Compared to the matrix rial, the deep-red emission from the other samples under 365 nm UV lamps can be c observed with the naked eye, and LMN:Mn 4+ ,B,Li + shows the strongest luminescen tensity.This result indicates that the LMN:Mn 4+ ,B,Li + phosphor as a red-light-em phosphor for WLEDs shows good prospects.For evaluating the availability o LMN:Mn 4+ ,B,Li + phosphor in practical applications, we successfully combined this phor with a 410 nm chip to fabricate an LED. Figure 8b depicts the electrolumines (EL) spectrum of the LED driven at a current of 20 mA.It can be observed that th spectrum contains two broadband peaks: a red emission band located in the 600-75 range and a blue emission band centered at 410 nm.The inset of Figure 8b also show the photo of the LED light driven with a 20 mA current illuminates a bright red emi In plants, the chlorophylls A and B and the phytochromes PR and PFR are vital for growth [52].Figure 8b also shows the absorption spectra of these four major p chromes.Notably, the PL spectra of the red LED perfectly overlap with the phytoch absorption band for plant growth.These results confirm that the LMN:Mn 4+ ,B,Li + phor can be efficiently used to prepare red LEDs for plants.

Preparation of Materials and LEDs
The LMN:xMn 4+ , LMN:Mn 4+ ,yB, and LMN:Mn 4+ ,B,R + (R = Li, Na, K) phosphors were obtained through the solid-state method.La 2 O 3 (99.99%),Mg 2 (OH) 2 CO 3 (A.R. (Analytical Reagent)), Nb 2 O 5 (A.R.), MnCO 3 (A.R.), H 3 BO 3 (A.R.), Li 2 CO 3 (A.R.), Na 2 CO 3 (A.R.), and K 2 CO 3 (A.R.) were the starting materials, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.(Shanghai, China).The samples were weighed according to the molar ratio of La 2 O 3 :Mg 2 (OH) 2 CO 3 :Nb 2 O 5 = 3:0.8:1.In addition, H 3 BO 3 was employed as the cosolvent source and Li 2 CO 3 , Na 2 CO 3 , and K 2 CO 3 were employed as the charge compensators added into the LMN:xMn 4+ phosphors.The raw materials were mixed and ground thoroughly for 30 min using an agate mortar.After completely grinding the materials, the obtained mixed powder was transferred to an alumina crucible and heated to 450 • C at a rate of 10 • C/min in an air atmosphere.Then, the evenly mixed materials were moved from the crucible to calcine at 1350 • C for 8 h in a muffle furnace.Eventually, these harvested products could be obtained to make the following characterizations.A red pc-LED device for planting was fabricated using the as-prepared LMN:Mn 4+ ,B,Li + phosphors and a 410 nm blue LED chip.Following a typical fabrication process, the phosphors were uniformly mixed with silicone resin A and B (A:B = 1:1) in agate mortar, and the resulting mixture was coated onto LED chips.The packaged device was cured at 75 • C for 6 h to form pc-LED devices.

Figure 3 .
Figure 3. (a) Electronic band structure (The red lines represent the band) and (b) partial density of states of LMN.(c) DRS of LMN host, and LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,Li + phosphors.(d) The experimental energy gap of the LMN host and the dashed line is the tangent of the curve.

Figure 3 .
Figure 3. (a) Electronic band structure (The red lines represent the band) and (b) partial density of states of LMN.(c) DRS of LMN host, and LMN:Mn 4+ , LMN:Mn 4+ ,B, and LMN:Mn 4+ ,B,Li + phosphors.(d) The experimental energy gap of the LMN host and the dashed line is the tangent of the curve.

Figure 4 .
Figure 4. (a) The PLE spectra of LMN:Mn 4+ .(b) The PL spectra of LMN:xMn 4+ .The inset shows the dependence of the PL intensity on the Mn 4+ concentration level for the LMN:xMn 4+ phosphors.

Figure 4 .
Figure 4. (a) The PLE spectra of LMN:Mn 4+ .(b) The PL spectra of LMN:xMn 4+ .The inset shows the dependence of the PL intensity on the Mn 4+ concentration level for the LMN:xMn 4+ phosphors.

Figure 5 .
Figure 5.The PL spectra for (a) LMN:Mn 4+ ,yB and the relative point line diagram; and (b) LMN:Mn 4+ ,B,R + (R = K, Na, Li) phosphors.(c) The normalized luminescence intensity of different phosphors.(d) Tanabe-Sugano energy level diagram of a Mn 4+ ion in the LMN host.

Figure 5 .
Figure 5.The PL spectra for (a) LMN:Mn 4+ ,yB and the relative point line diagram; and (b) LMN:Mn 4+ ,B,R + (R = K, Na, Li) phosphors.(c) The normalized luminescence intensity of different phosphors.(d) Tanabe-Sugano energy level diagram of a Mn 4+ ion in the LMN host.

Figure 7 .
Figure 7. Temperature-dependent PL spectra and the corresponding contour maps of the ther evolution of (a,d) LMN:Mn 4+ , (b,e) LMN:Mn 4+ ,B, and (c,f) LMN:Mn 4+ ,B,Li + .(g) Relative integra intensity as a function of the temperature.(h) Thermal quenching activation energy of these ph phors.(i) Configurational coordinate diagram for the Mn 4+ ions (The black dashed arrow represe the transition process generated by heating and the red dashed arrow represents the non-radia transition).

Figure 7 .
Figure 7. Temperature-dependent PL spectra and the corresponding contour maps of the thermal evolution of (a,d) LMN:Mn 4+ , (b,e) LMN:Mn 4+ ,B, and (c,f) LMN:Mn 4+ ,B,Li + .(g) Relative integrated intensity as a function of the temperature.(h) Thermal quenching activation energy of these phosphors.(i) Configurational coordinate diagram for the Mn 4+ ions (The black dashed arrow represents the transition process generated by heating and the red dashed arrow represents the non-radiative transition).

Figure 8 .
Figure 8.(a) CIE chromaticity coordinates diagram of (i) the LMN host and (ii) LMN:Mn 4+ , (iii) LMN:Mn 4+ ,B, and (iv) LMN:Mn 4+ ,B,Li + phosphors; (b) EL spectrum of the red LED device driven by a 410 nm chip compared with the absorption spectra of plants to natural light.