Effects of Impurity Doping on the Luminescence Performance of Mn⁴⁺-Doped Aluminates with the Magnetoplumbite-Type Structure for Plant Cultivation

Abstract

Mn⁴⁺ activated LaMgAl₁₁O₁₉ (LMA/Mn⁴⁺) with red emitting phosphor was obtained by sintering under air conditioning. The X-ray diffraction pattern Rietveld refinement results reveal that three six-fold coordinated Al sites are substituted by Mn⁴⁺ ions. Furthermore, the chemical valence state of manganese in the LMA host was further confirmed through X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). Photoluminescence emission (PL) and excitation (PLE) spectra of LMA/Mn⁴⁺ as well as the lifetime were measured, and the 663 nm emission is ascribed to the ²E_g→⁴A_2g from the 3d³ electrons in the [MnO₆]⁸⁻ octahedral complex. The emission spectrum matches well with the absorption of phytochrome. Temperature-dependent PL spectra show that the color changes of the phosphor at 420 K are 0.0110 for Δx and −0.0109 for Δy. Moreover, doping Zn²⁺ and Mg²⁺ ions in the host enhances the emission intensity of Mn⁴⁺ ions. These results highlight the potential of LMA/Mn⁴⁺ phosphor for a light-emitting diode (LED) plant lamp.

1. Introduction

Indoor agriculture has attracted considerable attention because of its relatively stable grow environment without outside interference [1,2]. Studies show that light distribution in blue (400–500 nm) and red (600–690 nm) regions has significant implications for plants as it affects the photosynthetic reaction along with the developmental processes of flowering [3,4,5]. Very recently, phosphor converted light emitting diodes (pc-LEDs) have been recognized as the primary artificial light source for indoor plant growth because of its unique advantages over the traditional gas-discharge lamps, such as simple fabrication technology, being power-economical, low radiant heat output, and the ease of controlling the spectral composition [6,7,8,9,10].

Currently, LED grow light could be produced by the blue-emitting LED, which is prepared though packing of a red phosphor on the GaN chip surface using silicone [11]. Moreover, the plant growth lamps are adjustable according to demand by changing the spectra and luminous efficacy of the phosphor. In particular, the radiation at 650–750 nm wave-bands is necessary for plant growth, thus red phosphor has a major effect on the plant lamps [4,7,12]. It is well known that the transition metal ion Mn⁴⁺ (3d³ electronic configuration) presents broad and intense absorption in the near ultraviolet (n-UV) and blue region, resulting from the ⁴A₂→(⁴T₁, ²T₂, and ⁴T₂) spin-allowed transitions, and emits red to near-infrared (NIR) light, arisen by the transition from ²E→⁴A₂ in an octahedral coordination environment [13,14,15,16,17]. Furthermore, the Mn⁴⁺ ion doped oxide red phosphors perform well as red luminescent materials for LED plant lamps, as demonstrated in Ba₂TiGe₂O₈ [5], La(MgTi)_1/2O₃ [18], NaLaMgWO₆ [7], Li₂MgZrO₄ [8], and so on. Besides, LaMgAl₁₁O₁₉ (LMA) with a magnetoplumbite structure exhibits excellent physical and chemical stability and contains an amount of [AlO₆] octahedral sites to accept doped Mn⁴⁺ ions into the LMA lattice [19]. The LaMgAl₁₁O₁₉ host doped with Mn²⁺ ions has been reported in a great deal of research [20,21,22,23]. However, the related luminescence property of LaMgAl₁₁O₁₉/Mn⁴⁺ (LMA/Mn⁴⁺) phosphor is rarely reported.

In this study, a promising red-emitting Mn⁴⁺-activated LMA phosphor for plant growth will be reported. The chemical valence state of manganese in the LMA host was investigated by electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). As expected, the LMA/Mn⁴⁺ phosphor shows a broad excitation band in the UV-blue region, and a narrow red light emission band peaking at 663 nm, which, in accordance with the absorption band of phytochrome, is observed just under 467 nm. The optimal doping concentration of Mn⁴⁺ ion is 1 mol %, and the influence of the doping concentration of Mn⁴⁺ ions on luminescent property is discussed. By introducing impurities to the LMA host, such as Zn²⁺ and Mg²⁺ ions, the luminescent efficiency is significantly improved. All these results demonstrate the great potential of LMA/Mn⁴⁺ phosphors for application in the agriculture industry as red-emitting luminescent materials.

2. Experimental Details

2.1. Sample Preparation

Sample LaMgAl₁₁O₁₉/xMn⁴⁺ (LMA/xMn⁴⁺) phosphors were prepared by the solid-state reaction method. The starting raw materials were La₂O₃, Al₂O₃, MgO, MnCO₃, and R (R = Li₂CO₃, Na₂CO₃, MgO, CaO, ZnO, GeO₂). LaMgAl_11−x−yO₁₉/xMn⁴⁺/yR (x = 0.02%~5.0%; y = 1% mol) was synthesized by calcination of the mixture of starting materials at 1600 °C for 6 h in an ambient atmosphere. The samples were prepared and then ground in an agate mortar.

2.2. Sample Characterization

Structural characterizations were executed by X-ray diffraction (XRD) measurements (X’ Pert PRO, Cu K_α, λ = 1.5418 Å, PANalytical, Holland). The morphology and grain size of the LMA/0.01Mn⁴⁺ were investigated using a scanning electron microscope (SEM, Zeiss Sigma500, Jena, Germany). The program Material Studio (MS 5.5, Biovia, San Diego, CA, USA) was used to analyze the crystal structure and atomic position. The photoluminescence (PL) and photoluminescence excitation (PLE) were recorded by Edinburgh Instruments (FLS 980; Livingston, UK) equipped with 450 W xenon lamps as a lighting source. The quantum efficiency (QE) measurements were performed by the spectrophotometer with a barium sulfate coated integrating sphere. The QE, which is defined as the ratio of the total number of photons emitted (I_em) to the number of photons absorbed (I_abs), is expressed as

$$\eta =\frac{I_{em}}{I_{abs}}=\frac{{\displaystyle \int L_S}}{{\displaystyle \int E_R-{\displaystyle \int E_S}}}$$

where L_S is the emission spectrum of the sample, and E_S and E_R are the spectra of the excitation light with and without the sample in the integrating sphere, respectively. The diffuse refection (DR) spectra of the samples were measured by a UV–Vis–NIR spectrophotometer (Lambda 950, Pzserkin Elmer, Canton, MA, USA), using BaSO₄ as a standard reference. The electron paramagnetic resonance measurement was carried out using a Bruker A300 (Rheinstetten, Germany) with the microwave frequency fixed at 9.8 GHz. The X-ray photoelectron spectroscopy (Thermo ESCALAB 250XI, Waltham, MA, USA) was conducted by Thermo Fisher Scientific. The first-principles calculations for LMA were performed by CASTEP (MS 5.5, Biovia, San Diego, CA, USA) [24], a plane-wave pseudopotential total energy package based on density functional theory (DFT) [25]. The Perdew–Burke–Emzerhoff (PBE) function within the generalized gradient approximation (GGA) form was adopted to describe the exchange-correlation energy. The optimized norm-conserving pseudopotentials in the Kleinman–Bylander [26] form for all the elements were used to model the effective interaction between atom cores and valence electrons. The high kinetic energy cutoff 1000 eV and dense 5 × 1 × 5 Monkhorst–Pack [27] k-point meshes in the Brillouin zones were chosen for LMA.

3. Results and Discussion

3.1. Phase Purity and Crystal Structure Analysis

The XRD patterns of LMA/xMn⁴⁺ (x = 0, 0.01, 0.02, and 0.05) were conducted and are shown in Figure 1a. It is obvious that all the XRD patterns of the as-prepared samples are consistent with the standard data of LaMgAl₁₁O₁₉ (PDF #78-1845), indicating that there is not an observable change of the crystal structure with Mn⁴⁺ ion doping. Then, the influence of the doping concentration of Mn⁴⁺ ion on the crystal structure was studied by comparing the dominant diffraction peak at 36.16°. Apparently, the diffraction peak gradually shifts to the lower angles with the increased doping concentration of Mn⁴⁺ ion, which is the result of the bigger ions Mn⁴⁺ occupying the Al³⁺ ion sites in the LMA host lattice.

To achieve crystallographic data of the prepared samples, LMA/0.01Mn⁴⁺ was selected as the representative to carry out XRD Rietveld refinement (Figure 1b). The refining adopted the crystallographic data of LaMgAl₁₁O₁₉ (ICSD #48171) as an initial model and it converged to R_wp = 11.5%, demonstrating the high reliability of the refined results [28]. Comparing the different profiles between the experimental and calculated ones, we can confirm that the sample has a hexagonal structure belonging to the P63/mmc space group. The lattice parameters of a = 5.5950(6) Å, c = 21.9680(2) Å, α = 90°(4), γ = 120°(8), and cell volume V = 594.86(3) ų are consistent with reference data, as listed in

Table 1. Crystallographic data and refinement parameters of the LMA/0.01Mn⁴⁺.

Atom	x	y	z	Occ	U	Coordination Number
La	0.33333	0.66667	0.75000	1	0.00911
Mg	0.33333	0.66667	0.02720	0.5	0.00240	4
Al1	0.00000	0.00000	0.00000	1	0.00190	6
Al2	0.00000	0.00000	0.25000	1	0.02305	5
Al3	0.33333	0.66667	0.02720	0.5	0.00240	4
Al4	0.33333	0.66667	0.18950	1	0.00228	6
Al5	0.16740	0.33480	0.89200	1	0.00304	6
O1	0.00000	0.00000	0.15100	1	0.00278
O2	0.33333	0.66667	0.94200	1	0.00329
O3	0.18000	0.36000	0.25000	1	0.00709
O4	0.15200	0.30400	0.05300	1	0.00455
O5	0.50500	0.010000	0.15100	1	0.00380
Symmetry:	hexagonal		Space group:		P63/mmc
Lattice parameters: a = b = 5.5950 Å; c = 21.9680 Å; α = β = 90°; γ = 120°; V = 594.86 Å3

.

Many previous studies have proven that Mn⁴⁺ ions in octahedral geometry will produce red emission [11]. Hence, the more [Al(O/F)₆] octahedral sites the structure has, the greater the capacity it will offer to adopt Mn⁴⁺ ions, which is beneficial to emit efficient red light. As shown in the LMA structure built based on the refining results (Figure 2), the coordination environments of Al₁₋₅ atoms can be observed. It is worth noting that there are three six-fold coordinated Al sites (Al₁, Al₄, and Al₅), which are identical to the coordination situation in other Mn⁴⁺ ion doped oxide hosts. Correspondingly, the Mn⁴⁺ ions could substitute for the Al₁, Al₄, and Al₅ sites in the LMA host to form the [MnO₆]⁸⁻ octahedral complex.

In order to assess the manganese (IV) center obtained in the phosphor, the EPR spectrum of LMA/0.01Mn⁴⁺ at 77 K was conducted. As shown in Figure 3a, the typical hyperfine sextet doubly degeneracy energy levels are observed, indicating the Mn ions possess a strong crystal field. The center of the signals corresponding to the g value is 2.11 based on the following equation:

$$hv=g\beta H$$

(1)

where h is the Planck constant (6.626620 ×10⁻²⁷ erg/s), ν is the microwave frequency, g is the nondimensional spectral splitting factor (g value), and β is the Bohr magneton (9.27410×10⁻²¹ erg/G). The result verifies that the magnetic dipolar transition of Mn⁴⁺ ions occupies symmetric octahedral sites. Meanwhile, XPS was further used to confirm the chemical valence state of manganese in LMA/Mn⁴⁺. The XPS spectra of LMA/0.01Mn⁴⁺, LMA/0.05Mn⁴⁺, MnCO₃, and MnO₂ are displayed in Figure 3b, in which MnCO₃ and MnO₂ are used as the reference standards for Mn²⁺ and Mn⁴⁺ ions, respectively. The peaks of the as-synthesized sample could be assigned to the Mn 2p_3/2 and are close to the peaks of MnO₂. Meanwhile, the peak intensity increases with the increasing content of Mn as shown in Figure 3b. All these results suggest that the oxidation state of manganese in the LMA host is +4 [29,30].

Scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) were performed to further display the detailed morphological feature and elements of LMA/0.01Mn⁴⁺ particles. As shown in Figure 4a, the size of the selected particles is about 5–15 μm, illustrating the good crystallization of the as-synthesized sample. The EDS analysis of the phosphors is presented in Figure 4b. All the targeted peaks of the elements (lanthanum (La), magnesium (Mg), aluminum (Al), and oxygen (O)) could be clearly observed. The elemental mapping was carried out to further confirm the uniform distribution of elements. The EDS image shows the presence of La, Al, Mg, and O in the sample, as well as the homogenous distribution of the corresponding elements in the phosphor (Figure 4c).

3.2. Electronic Structure Calculations of LMA

The electronic structure of LaMgAl₁₁O₁₉ is presented in Figure 5a. The top of the valence band (VB) maximum and the bottom of the conduction band (CB) minimum locate at different k-points, revealing that LaMgAl₁₁O₁₉ is an indirect semiconductor with a band gap of 4.05 eV. The wide band gap demonstrates that the LaMgAl₁₁O₁₉ is a good luminescent material for accommodating the ⁴A_2g and ²E_g states of Mn⁴⁺ ion [16]. Additionally, Figure 5b shows the partial density of states (DOS) of La, Mg, Al, and O. Corresponding with the total DOS for LaMgAl₁₁O₁₉, the VB top is mainly composed of O-2p, while La-4f states make a significant contribution to the CB top.

3.3. Components Luminescent Properties of LMA/Mn⁴⁺

Figure 6a depicts the diffuse reflection (DR) spectra of LMA with different doping concentrations of Mn⁴⁺ ion. Similar to the results reported previously, there is a characteristic intense spin-allowed Mn⁴⁺/⁴A_2g→⁴T_2g transition peaking at ~470 nm and a weak recognizable spin-forbidden ⁴A_2g→²T_1g one located at 390 nm [11,16]. However, the charge transfer band is too weak to separate from the DR spectra.

Figure 6b shows the normalized photoluminescence (PL) and PL excitation (PLE) spectra of the LMA/0.01Mn⁴⁺ at room temperature. Apparently, one prominent red emission band at 663 nm caused by the anti-stokes and stokes transition ²E_g, ²T_2g→⁴A_2g of the 3d³ electrons from Mn⁴⁺ ions in the [MnO₆]⁸⁻ octahedra is observed under excitation of 467 nm [11]. The chromaticity coordinate of the LMA/Mn⁴⁺ sample in the Commission Internationale de L’Eclairage (CIE) 1931 color spaces is calculated to be (0.725, 0.274), which is beneficial to the long-day plants. The PLE spectrum of the sample comprises two ultraviolet peaks at 345 nm and 390 nm, and one distinguishable blue band centered around 470 nm derived from the inner d–d transitions of Mn⁴⁺ ions. The peak locations are the same as the absorption peaks in the DR spectra and the peaks originate from the Mn⁴⁺–O²⁻ charge-transfer band (CTB) ⁴A_2g→⁴T_1g and spin-allowed transitions ⁴A_2g→⁴T_2g of Mn⁴⁺ ions, respectively. The internal quantum efficiency of LMA/Mn⁴⁺ is measured to be 38.5% for the excitation wavelength of 465 nm, as demonstrated in Figure S1. The value is comparable to the one reported by Peng et al. [13]. These spectral features agree well with other previous studies about Mn⁴⁺-doped aluminates, suggesting that the Mn⁴⁺ ions had been successfully incorporated into the LMA host. The Tanabe–Sugano energy-level diagram illustrates the dependence of energy levels of 3d³ on the parameters of Dq, B, and C, in which Dq is a parameter that characterizes the strength of the octahedral crystal field, while B and C are Racah parameters. From the supplementary Equations (1)–(4), the values of Dq, B, and C in the LMA/Mn⁴⁺ are then determined to be 2150 cm⁻¹, 321 cm⁻¹, and 4076 cm⁻¹, respectively.

The doping concentration of Mn⁴⁺ ion dependence of the PL integrated intensity is shown in Figure 7a. Typically, the luminescence intensity increases gradually at first with the increase of the doping concentration of Mn⁴⁺ ion, then approaches a maximum value at 0.01, and finally decreases when the doping concentration of Mn⁴⁺ ion is slightly higher than 0.01. The exchange interaction or multipole–multipole interaction within the nearest Mn⁴⁺ ions are ascribed to the concentration quenching phenomenon. To clarify this point, it is necessary to calculate the critical transfer distance (R_c) among the Mn⁴⁺ ions. Upon the Blasse mechanism [31], R_c is evaluated via the following equation:

$$R_C\approx 2{\left(\frac{3V}{4\pi x_{c}Z}\right)}^{\frac{1}{3}}$$

(2)

In this work, V = 594.86 ų, N = 28, x_c = 0.01, and the critical R_c of Mn⁴⁺ ion in LMA is calculated to be ~15.9 Å. The estimated R_c value is bigger than 5 Å, hence it is inferred that an exchange interaction may not be the main possible approach.

Thus, the concentration quenching mainly relies on the electric multipolar interaction, which is confirmed by Dexter’s theory [32]. The type of interaction between Mn⁴⁺ ions can be expressed by the following equation:

$$\frac{I}{x}=K{\left[1+\beta {\left(x\right)}^{\frac{\theta }{3}}\right]}^{-1}$$

(3)

where x is the activator concentration; K and β are constants; and θ is an indication of the electric multipolar character in which θ = 6, 8, 10 corresponds to dipole–dipole, dipole–quadrupole and quadrupole–quadrupole, respectively. Figure 7b depicts the dependence of log(x) and log(I/x) for the LMA/Mn⁴⁺ phosphors. The linear slope is calculated to be ~2.12 and the value of θ is determined as approximately 6. Consequently, the quenching mechanism is a dipole–dipole interaction in LMA/Mn⁴⁺.

Generally, as a red color converter in blue-chips, the thermal stability of the sample is a significant parameter in fundamental research, because the chip temperature usually undergoes a high temperature (> 423K), which would degrade emission intensity and color quality. Figure 8a gives the temperature-dependent emission spectra of LMA/0.01Mn⁴⁺ spanning from 300 K to 480 K under excitation of 467 nm. Obviously, the emission intensity of the as-prepared sample decreases monotonically with the increase of temperature. However, there is no distinct peak position shift over 380 K. The integrated emission intensity is presented in Figure 8b. When the sample is heated to 420 K, the integrated intensity retains only 36% of the emission intensity at 300 K. The thermal quenching behavior may be affected by the oscillator strength and the distributed killer centers, which could strengthen the nonradiative energy relaxation. Despite an obvious drastic emission loss, the chromaticity coordinate variations at 420 K are 0.0110 and −0.0109, respectively, well satisfying the requirement in LED applications [33]. Meanwhile, the temperature-dependent PL decay curves of LMA/0.01Mn⁴⁺ at 663 nm were further measured. As shown in Figure 8c, the decay lifetimes seem to be multiexponential. We found that there are three six-fold coordinated Al sites (Al₁, Al₄, and Al₅) in the structure of LMA. When Mn⁴⁺ ions substitute for the Al₁, Al₄, and Al₅ sites in the LMA host, three [MnO₆]⁸⁻ octahedral complex luminescence center formed, which would contribute to the decay time with multiexponential function. Meanwhile, the doping concentration of Mn⁴⁺ ions also affect the decay process via energy transfer. The distance between the activated centers is 2.658 to 5.59Å, hence the energy transfer would lead to the deviation from linear of the decay curves. Moreover, the decay lifetimes also strongly rely on the temperature. The lifetimes are calculated according to the equation below:

$$\tau =\frac{{\displaystyle {\int }_0^{\infty }I\left(t\right)tdt}}{{\displaystyle {\int }_0^{\infty }I\left(t\right)dt}}$$

(4)

where τ is the decay time and I(t) is the luminescence intensity at time t. Based on Equation (4), the average lifetime of LMA/0.01Mn⁴⁺ decreases gradually from 1.35 ms at 300 K to 0.46 ms at 480 K, which is consistent with the change rule of the emission intensity. The decay of Mn⁴⁺ in the matrix becomes faster and faster, owing to the increased nonradiative energy migration among Mn⁴⁺ ions at a higher temperature.

The activation energy (ΔE) for thermal quenching can be determined by the following equation [34]:

$$I_T=I_0/\left[1+c\exp \left(-∆E/kT\right)\right]$$

(5)

where I₀ and I_T stand for the initial emission intensity and the luminescence intensity at temperature T, respectively; c is a constant for a designated host; and k is the Boltzmann constant. Thus, the activation energy of the LMA/Mn⁴⁺ phosphor is calculated to be 0.38 eV. The thermal quenching mechanism can be explained by the configurational coordinate diagram, as shown in Figure 8d.

In a further experiment, the influence of M (M = Li⁺, Na⁺, Mg²⁺, Ca²⁺, Zn²⁺, Ge⁴⁺) ion on the emission intensity of Mn⁴⁺/LMA was studied (Figure S2). The ionic radii of these M ions are 0.590 Å for Li⁺ (CN = 4), 0.99 Å for Na⁺ (CN = 4), 0.720 Å for Mg²⁺ (CN = 6), 1.00 Å for Ca²⁺ (CN = 6), 0.60 Å for Zn²⁺ (CN = 4), and 0.530 Å for Ge⁴⁺ (CN = 6), respectively [35]. When the impurity ions were co-doped into LMA/Mn⁴⁺, the characteristic peaks in the emission spectra were not changed. The integrated PL intensities of the samples into which the impurity ions were incorporated are presented in Figure 9a. Therein, Mg²⁺ and Zn²⁺ ions have the ability to enhance luminescence of LMA/0.01Mn⁴⁺, Na⁺ and Ca²⁺ ions have no obvious impact on Mn⁴⁺ ion luminescence, and the Li⁺/Ge⁴⁺ doped LMA/0.01Mn⁴⁺ phosphors show weaker luminescence intensities under excitation of the same blue light. Specifically, the ratio of the emission yields of LMA/0.01Mn⁴⁺ phosphors with and without Zn²⁺ ion doping are measured to be 2.04, in favor of the conclusion that the luminescence intensity of LMA/Mn⁴⁺ can be greatly strengthened via impurity doping. Moreover, the effect of co-dopants for improving Mn⁴⁺ luminescence follows the order of Zn²⁺ > Mg²⁺ > Na⁺. As discussed in Figure 8c, the substitution of Mn for the Al₁, Al₄, and Al₅ sites in the LMA host is attributed to the multiexponential function; the co-doped model also presents similar decay curves. Similarly, the decay curves were tested to verify this phenomenon. Figure 9b shows the lifetime of Mn⁴⁺ ion in phosphor with co-doping Zn²⁺ and Mg²⁺ ions, and the result suggests that the lifetime becomes longer. The introduction of Mg²⁺/Zn²⁺ ions increases the PL intensity of Mn⁴⁺ ions, which is attributed to the replacement of Mn⁴⁺–Mn⁴⁺ pairs by Mg²⁻–Mn⁴⁺ and/or Zn²⁺–Mn⁴⁺ ion pairs. Hence, the nonradiative depopulation of the ²E_g state is decreased because the energy migration between the Mn⁴⁺–Mn⁴⁺ ion pairs is faster than Mg²⁻–Mn⁴⁺ and/or Zn²⁺–Mn⁴⁺ ion pairs, and thus the probability of the energy terminating at a killer site of the Mn⁴⁺–Mn⁴⁺ ion pairs is greater. Moreover, the formation of Mg²⁺–Mn⁴⁺/Zn²⁺–Mn⁴⁺ pairs along with the impurities of Mg²⁺/Zn²⁺ ions not only lengthen the emission lifetimes of the co-doped samples compared with that of the LMA/0.01Mn⁴⁺ sample, but also lead to the decrease in the nonradiative rate. [13,16].

4. Conclusions

In summary, a red emitting phosphor LMA/Mn⁴⁺ for an LED plant growth lamp was synthesized by the solid state reaction method and sintering at 1600 °C in the air directly. The crystal structure of LMA/0.01Mn⁴⁺ was studied by the XRD Rietveld refinements. XPS and EPR spectra demonstrate that the chemical valence of manganese is +4. The LMA/Mn⁴⁺ phosphor exhibits a red emission peaking at 663 nm, which is attributed to the ²E→⁴A₂ transition of Mn⁴⁺ ion in the [MnO₆]⁸⁻ octahedral environment, and the QE is 35.8% under the blue-light excitation. The optimal doping concentration of Mn⁴⁺ ion is 0.01 mole and the quenching mechanism is a dipole/dipole interaction. Moreover, the chromaticity coordinate variations at 420 K are 0.0110 and −0.0109, respectively. Additionally, the luminescence of LMA/Mn⁴⁺ could be greatly enhanced via impurity doping Zn²⁺ or Mg²⁺ ions. These properties make LMA/Mn⁴⁺ a promising red emitting phosphor for plant growth LEDs.