Double Perovskite Mn⁴⁺-Doped La₂CaSnO₆/La₂MgSnO₆ Phosphor for Near-Ultraviolet Light Excited W-LEDs and Plant Growth

Abstract

Non-rare earth doped oxide phosphors with far-red emission have become one of the hot spots of current research due to their low price and excellent physicochemical stability as the red component in white light-emitting diodes (W-LEDs) and plant growth. Herein, we report novel Mn⁴⁺-doped La₂CaSnO₆ and La₂MgSnO₆ phosphors by high-temperature solid-phase synthesis and analyzed their crystal structures by XRD and Rietveld refinement. Their excitation spectra consist of two distinct excitation bands with the dominant excitation range from 250 to 450 nm, indicating that they possess strong absorption of near-ultraviolet light. Their emission is located around 693 and 708 nm, respectively, and can be absorbed by the photosensitive pigments Pr and Pfr, proving their great potential for plant growth. Finally, the prepared samples were coated with 365 nm UV chips to fabricate far-red LEDs and W-LEDs with low correlation color temperature (CCT = 4958 K/5275 K) and high color rendering index (R_a = 96.4/96.6). Our results indicate that La₂CaSnO₆:Mn⁴⁺ and La₂MgSnO₆:Mn⁴⁺ red phosphors could be used as candidate materials for W-LED lighting and plant growth.

1. Introduction

As the latest generation of lighting sources, light-emitting diodes (LEDs) have been successfully used in many fields and have contributed to the development of these fields, especially in agriculture, because of their outstanding advantages such as high efficiency, environmental protection, and energy reduction [1,2,3,4,5]. As the most basic source of energy for the survival and reproduction of all living things on earth, the role played by light in the growth of plants is self-evident [6,7,8,9,10]. However, not all wavelengths of light are absorbed by the pigments in plants, with visible light (380–720 nm) and near-infrared light (720–800 nm) being the absorption ranges [11,12,13,14]. In contrast, near-ultraviolet (n-UV) light (290–380 nm) plays a negligible role in the plant growth process. In order to enable plants to utilize the energy of n-UV light, there is an urgent need for a light-converting phosphors that can convert n-UV light to visible or near-infrared light. Therefore, it is crucial to find phosphors that can be excited by n-UV light.

In recent years, Mn⁴⁺ ion has attracted much attention due to its abundant reserves and low prices. It is well known that Mn⁴⁺ ion is a good non-rare earth activator, which usually occupies an octahedral coordination environment in the lattice and has outstanding optical properties, including a broad absorption band covering the UV to visible region and characteristic red luminescence in the wavelength range from red to near-infrared light [15,16,17,18]. Intriguingly, the luminescence of the Mn⁴⁺ ions is right within the absorption range of the photosensitive pigments Pr (red light-absorbing form) and Pfr (far-red light-absorbing form) in plants. The photosensitive pigment is one of the most important photoreceptors for plant photomorphogenesis, and its role is indispensable for the developmental process of plants. It is not only involved in regulating plant growth and development, but also in responding to biotic and abiotic stresses. The absence of photosensitive pigments leads to altered plant resistance to biotic stresses such as pathogenic bacteria and pests, as well as abiotic stresses such as low temperature, high temperature, drought, and salt, while the resistance of plants to these adverse stresses can be improved by changing the light quality (e.g., adjusting the red far-red light ratio). Therefore, Mn⁴⁺-doped phosphors can be applied in plant growth.

There have been many reports on Mn⁴⁺-doped phosphors [5]. However, there have been no reports on the use of La₂CaSnO₆ and La₂MgSnO₆ crystals for the synthesis of red or deep-red luminescent phosphors. In this work, we report a series of novel red-emitting double perovskite structures of La₂CaSnO₆:Mn⁴⁺ (LCSO:Mn⁴⁺) and La₂MgSnO₆:Mn⁴⁺ (LMSO:Mn⁴⁺) phosphors obtained by high-temperature solid-phase reactions. The phase purity and crystal structures were characterized by XRD and Rietveld refinement. The luminescence properties of the prepared phosphors were investigated, including photoluminescence properties, temperature-dependent luminescence properties, quantum efficiency, and fluorescence lifetime. Furthermore, by combining with n-UV LED chips and commercial phosphors, both W-LEDs can produce white light with high CRI and low CCT. This paper demonstrates the potential of La₂CaSnO₆:Mn⁴⁺ and La₂MgSnO₆:Mn⁴⁺ phosphors for applications in W-LEDs and plant growth.

2. Results and Discussion

2.1. Structural Properties

Figure 1a,b show the XRD patterns of LCSO:xMn⁴⁺ (x = 0.002, 0.004, 0.006, 0.008, 0.010) and LMSO:yMn⁴⁺ (y = 0.002, 0.004, 0.006, 0.008, 0.010) samples. Obviously, the diffraction peaks of the Mn⁴⁺-doped samples with different concentrations match with those of the standard data and no impurity peaks are found, indicating that all the Mn⁴⁺ ions are dissolved in the main lattice. We tested the X-ray photoelectron spectra (XPS) of LCSO:0.002Mn⁴⁺ and LMSO:0.002Mn⁴⁺, and the results are shown in Figure 1c,d. The signals of La, Sn, Ca, Mg, and O elements were clearly detected. The XPS spectra of Mn 2p are shown in Figure 1e,f, and it can be observed that the binding energy peak of Mn 2P_1/2 of both are located near 654 eV, and the binding energy peak of Mn 2p_3/2 of both is located near 642 eV, which proves that the oxidation state of Mn is +4.

To further explore the crystal structures of LCSO and LMSO, the crystal structure data of Ca₂LaTaO₆ (ICSD 251945) and La(Mg_0.5Sn_0.5)O₃ (ICSD 156359) were used as starting base models for LCSO:0.004Mn⁴⁺ and LMSO:0.010Mn⁴⁺ Rietveld structure refinement, and the results are shown in Figure 2a,b [19,20]. The refinement results showed that LCSO:0.004Mn⁴⁺ and LMSO:0.010Mn⁴⁺ were pure phases. The detailed crystallographic data are shown in

Table 1. Main crystallographic parameters of LCSO:0.004Mn⁴⁺ and LMSO:0.010Mn⁴⁺.

Compound	La2CaSnO6:0.004Mn4+	La2MgSnO6:0.010Mn4+
Space group	P21/n	P21/n
a, Å	5.74019	5.63776
b, Å	5.95969	5.72036
c, Å	8.25804	8.02015
α = γ	90°	90°
β	89.99°	90.06°
V	282.506	258.650
Crystal system	Monoclinic	Monoclinic
Z	2	2
Rwp, %	6.08	8.26
Rp, %	4.83	5.33
χ2	1.008	2.173

.

The crystal structures of LCSO and LMSO were obtained by refining the crystallization results in the P2₁/n space group, and the results are shown in Figure 2c,d. La₂CaSnO₆ and La₂MgSnO₆ have the structural general formula A₂BB’O₆, where the A, B, and B’ sites are occupied by La, Ca/Mg, and Sn atoms, respectively. The B site binds to O²⁻ to form CaO₆, MgO₆, and SnO₆ octahedra. The CaO₆/MgO₆ octahedron and the SnO₆ octahedron have a common oxygen atom at the apex of both, and these two octahedra are staggered to form a long-range-ordered structural layer. The A site occupies the gap between the [BO₆] octahedra and coordinates with twelve O²⁻. In the LCSO and LMSO crystal structures, there are three cations available for Mn⁴⁺ occupation. Considering the three aspects of ionic radius, valence state and coordination number, among the four ions La³⁺ ( r = 1.1 Å, CN = 7), Ca²⁺ ( r = 1 Å, CN = 6), Mg²⁺ ( r = 0.72 Å, CN = 6), and Sn⁴⁺ ( r = 0.69 Å, CN = 6), the Sn⁴⁺ site is considered the most suitable Mn⁴⁺ ( r = 0.53 Å, CN = 6) doping site.

2.2. Photoluminescence Properties

The excitation and emission spectra of La₂CaSnO₆:0.002Mn⁴⁺ and La₂MgSnO₆:0.002Mn⁴⁺ at room temperature are shown in Figure 3a,b. The photoluminescence excitation (PLE) spectra monitored at 693 and 708 nm show two distinct excitation bands with the dominant excitation range from 250 to 450 nm and the weak excitation range from 450 to 600 nm, due to the Mn⁴⁺-O²⁻ charge transition band (CTB), ⁴A_2g → ⁴T_1g, ⁴A_2g → ²T_2g, and ⁴A_2g → ⁴T_2g transitions [21,22]. As shown in Figure 3c,d, the Gaussian fit analysis of the energy-based PLE spectra of these two samples shows that the above four effects correspond roughly to 28,508 cm⁻¹, 25,943 cm⁻¹, 24,514 cm⁻¹, and 18,768 cm⁻¹ for LCSO:0.002Mn⁴⁺, and 28,816 cm⁻¹, 26,381 cm⁻¹, 25,458 cm⁻¹, and 19,342 cm⁻¹ for LMSO:0.002Mn⁴⁺. Both exhibit intense red emission under 365 nm UV excitation due to the spin-prohibited ²E_g→⁴A_2g leap of the Mn⁴⁺ ions in the octahedral environment, with the peak emission near 693 nm for LCSO: 0.002Mn⁴⁺ and around 708 nm for LMSO: 0.002Mn⁴⁺, which can be absorbed by the photosensitive pigments Pr and Pfr [23]. These results prove that both phosphors are well suited for n-UV chips and are potential red-emitting phosphors, as well as able to be applied in plant growth. In addition, the emission spectra of both were found to show several sharp peaks. These peaks consist of the anti-Stokes emission, the zero-phonon line (ZPL) of the ²E_g → ⁴A_2g transition, and the Stokes emission [10,16]. Figure 3e,f show the emission spectra of LCSO:0.002Mn⁴⁺ and LMSO:0.002Mn⁴⁺ at 7 K. The ZPL of the ²E_g → ⁴A_2g transition was found to occur at 678 nm (14,749 cm⁻¹) and 692 nm (14,451 cm⁻¹), respectively.

The photoluminescence (PL) spectra of a series of different concentrations of LCSO:xMn⁴⁺ (x = 0.001–0.010) and LMSO:yMn⁴⁺ (x = 0.001–0.010) were tested as shown in Figure 4a,b. All samples with different doping concentrations show no absence and shift of emission peaks, only changes in emission intensity. From Figure 4c,d, it can be seen that LCSO:xMn⁴⁺ (x = 0.001–0.010) and LMSO:yMn⁴⁺ (y = 0.001–0.010) have the same trend, and the emission intensity increases with the increase in doping concentration from 0.001 to 0.002 and decreases with the increase from 0.002 to 0.01, and the optimal doping concentration for both was determined as 0.002. When the doping concentration exceeds 0.002, the emission intensity starts to decrease, and the concentration quenching appears and becomes more severe with the increase in the Mn⁴⁺ content. The concentration quenching includes two mechanisms: exchange interaction and multistage–multistage interaction. To investigate which mechanism causes the concentration quenching, we calculated the critical distance R_c by the following equation [24,25]:

$${\mathrm{R}}_{\mathrm{c}}\approx 2{\left(\frac{3\mathrm{V}}{4{\mathsf{\pi }\mathrm{X}}_{\mathrm{c}}\mathrm{N}}\right)}^{\frac{1}{3}}$$

(1)

where V is the volume of the unit cell, x_c is the critical concentration, and N represents the number of sites that the Mn⁴⁺ ions can occupy per unit cell. For the LCSO host, V = 282.506 ų, N = 2, and x_c = 0.002; the critical distance R_c is calculated to be 51.29 Å. For the LMSO host, V = 258.650 ų, N = 2, and x_c = 0.002; the critical distance R_c is calculated to be 49.81 Å. Because the exchange interaction is effective only when the distance between activators is shorter than 5 Å, the concentration quenching mechanism of the Mn⁴⁺ ions in phosphors is dominated by multipole–multipole interactions. According to Van Uiter’s report, the emission intensity (I) per activator ion follows the equation [26]:

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

(2)

where x is the activator concentration, I/x is the emission intensity per activator concentration, K and β are the constants for the same excitation conditions, and θ = 6, 8, and 10 represent the dipole–dipole(d-d), dipole–quadrupole(d-q), and quadrupole–quadrupole(q-q) interactions. Figure 5a,b show a plot of log(I/x) versus log(x), with −θ/3 being the slope. For LCSO, a slope of −1.62 gives a θ of 4.86, which is closer to 6. For LMSO, a slope of −1.93 gives a θ of 5.79, again close to 6. These results suggest that dipole–dipole interactions dominate the concentration quenching mechanism of Mn⁴⁺ emission in LCSO and LMSO.

Figure 5c,d present luminescence decay curves of LCSO:xMn⁴⁺ (x = 0.002, 0.006, 0.015) and LMSO:yMn⁴⁺ (y = 0.002, 0.006, 0.015). All decay curves can be described by fitting the effective decay time τ, which is calculated as [27]:

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

(3)

where I(t) means the luminescence intensity of Mn⁴⁺ at moment t. As the doping concentration of Mn⁴⁺ ions increases, the effective lifetimes were calculated to be 0.289, 0.238, and 0.171 ms for LCSO:xMn⁴⁺ (x = 0.002, 0.006, 0.015) and 1.179, 1.011, and 0.635 ms for LMSO:yMn⁴⁺ (y = 0.002, 0.006, 0.015), respectively. The decrease in the effective lifetimes is due to the increase in non-radiative energy transfer with the increase in Mn⁴⁺ doping concentration [28].

In addition, we tested the internal quantum yields (IQYs) of LCSO:0.002Mn⁴⁺ and LMSO:0.002Mn⁴⁺, as shown in Figure 6. The IQYs were 22.90% for LCSO:0.002Mn⁴⁺ and 35.48% for LMSO:0.002Mn⁴⁺ under 365 nm excitation. The IQY of LMSO: 0.002Mn⁴⁺ is higher than some previously reported Mn⁴⁺-doped samples, as shown in

Table 2. The luminescence properties of La₂(Ca/Mg)SnO₆:Mn⁴⁺ and other Mn⁴⁺-doped phosphors.

Phosphor Composition	λex (nm)	λem (nm)	IQY	References
Ba2YNbO6:Mn4+	360	695	29.2	[17]
Ca2YSbO6:Mn4+	470	680	19.72	[18]
SrLaGa3O7:Mn4+	380	715	13.83	[21]
La2MgTiO6:Mn4+	350	710	32.5	[29]
Li2MgTiO4:Mn4+	460	676	32	[30]
Li2MgZrO4:Mn4+	335	670	32.3	[31]
LCSO:Mn4+	365	693	22.90	This work
LMSO:Mn4+	365	708	35.48	This work

.

2.3. Crystal Field Analysis and Nephelauxetic Effect

In general, the Tanabe–Sugano energy level diagram can be used to explain the energy level splitting of the Mn⁴⁺ ions in a six-coordinate octahedral environment and its spectral properties, as shown in Figure 7a. When irradiated with an external light source, the electrons in the ground state ⁴A_2g absorb energy and jump to excited states ⁴T_1g, ⁴T_2g, and ²T_2g, and then, through a non-radiative energy conversion process, to the lowest excited state ²E_g, and finally return to the ground state; the energy is expressed in the form of red light. In this process, the crystal field strength (D_q) strongly influences the energy level transition of the Mn⁴⁺ ions due to the unique 3d³ electron configuration of Mn⁴⁺ [32]. The crystal field strength is calculated from the energy difference of ⁴A_2g → ⁴T_2g by the following equation [33,34]:

$${\mathrm{D}}_{\mathrm{q}}=\frac{\mathrm{E}\left({}_{}{}^4\mathrm{A}{}_{2\mathrm{g}}-{}_{}{}^4\mathrm{T}{}_{2\mathrm{g}}\right)}{10}$$

(4)

in addition, the Racah parameter B can be evaluated by the following equation [35]:

$$\frac{{\mathrm{D}}_{\mathrm{q}}}{\mathrm{B}}=\frac{15\left(\mathrm{x}-8\right)}{{\mathrm{x}}^2-10\mathrm{x}}$$

(5)

the value of x can be calculated as follows [35]:

$$\mathrm{x}=\frac{\mathrm{E}\left({}_{}{}^4\mathrm{A}{}_{2\mathrm{g}}-{}_{}{}^4\mathrm{T}{}_{1\mathrm{g}}\right)-\mathrm{E}\left({}_{}{}^4\mathrm{A}{}_{2\mathrm{g}}-{}_{}{}^4\mathrm{T}{}_{2\mathrm{g}}\right)}{{\mathrm{D}}_{\mathrm{q}}}$$

(6)

the Racah parameter C can be acquired by the expression [35]:

$$\frac{\mathrm{E}\left({}_{}{}^2\mathrm{E}{}_{\mathrm{g}}-{}_{}{}^4\mathrm{A}{}_{2\mathrm{g}}\right)}{\mathrm{B}}=\frac{3.05\mathrm{C}}{\mathrm{B}}+7.9-\frac{1.8\mathrm{B}}{{\mathrm{D}}_{\mathrm{q}}}$$

(7)

From the excitation spectrum and emission spectrum in Figure 3, the peak energies of the ⁴A_2g → ⁴T_1g, ⁴A_2g → ⁴T_2g, and ²E_g → ⁴A_2g are 25,943, 18,768, and 14,749 cm⁻¹ for LCSO:0.002Mn⁴⁺ and 26,381, 19,342, and 14,451 cm⁻¹ for LMSO:0.002Mn⁴⁺, respectively. Substituting the above data into Equation (4)–(7), the values of D_q, B, C, and D_q/B were calculated to be 1876.8 cm⁻¹, 707.37 cm⁻¹, 3160.88 cm⁻¹, and 2.65 for LCSO:0.002Mn⁴⁺ and 1934.2 cm⁻¹, 684.67 cm⁻¹, 3107.66 cm⁻¹, and 2.83 for LMSO:0.002Mn⁴⁺, respectively. The D_q/B values of both are greater than two and prove that the Mn⁴⁺ ions possess strong crystal fields in LCSO and LMSO hosts. In addition, the ²E_g → ⁴A_2g transition of the Mn⁴⁺ ions is hardly affected by the crystal field strength as shown by the Tanabe–Sugano energy level diagram. What can affect such a transition is a nephelauxetic effect that depends on the covalent relationship between the Mn⁴⁺ ions and the ligand [23,36,37,38]. Brik et al. introduced the nephelauxetic ratio (β) to describe the nephelauxetic effect, which was calculated as follows [34]:

$$\mathsf{\beta }=\sqrt{{\left(\frac{\mathrm{B}}{{\mathrm{B}}_0}\right)}^2+{\left(\frac{\mathrm{C}}{{\mathrm{C}}_0}\right)}^2}$$

(8)

where B_o is 1160 cm⁻¹ and C_o is 4303 cm⁻¹ represent the Racah parameters of Mn⁴⁺-free ions. The β values of LCSO and LMSO were calculated as 0.955 and 0.933. On the basis of previous experience with a large number of Mn⁴⁺-doped crystals, a rule of thumb was established to predict the emission wavelength of Mn⁴⁺ with the following equation [32]:

$$\mathrm{E}\left({}_{}{}^2\mathrm{E}{}_{\mathrm{g}}-{}_{}{}^4\mathrm{A}{}_{2\mathrm{g}}\right)=16261.92\mathsf{\beta }-880.49\pm \mathsf{\sigma }$$

(9)

where σ is 332 cm⁻¹, a value close to the typical energy of phonon modes [12,32]. As shown in Figure 7b, the ²E_g energy levels calculated by this empirical formula are in the range 14,318–14,982 cm⁻¹ and 13,960–14,624 cm⁻¹ for LCSO:0.002Mn⁴⁺ and LMSO:0.002Mn⁴⁺. The experimental values of the ²E_g → ⁴A_2g transition are 14,430 cm⁻¹ for LCSO:0.002Mn⁴⁺ and 14,124 cm⁻¹ for LMSO:0.002Mn⁴⁺, both of which are within the range.

2.4. Photoluminescence Thermal Stability

Figure 8a,b show the emission spectra of LCSO:0.002Mn⁴⁺ and LMSO:0.002Mn⁴⁺ with temperature under 365 nm excitation. The increase in temperature leads to an increase in lattice vibrations, which increases the nonradiative transition and produces a thermal quenching effect with a significant decrease in the intensity of the emission peak [39]. The poor thermal stability of both is due to the significant Stokes shift of Mn⁴⁺ ions in the crystal environment. In addition, the normalized emission intensity curves of the two samples (Figure 8c) indicate that the quenching temperatures (T_50%) of the LCSO:0.002Mn⁴⁺ and LMSO:0.002Mn⁴⁺ samples are about 331 and 397 K, respectively. The LCSO:0.002Mn⁴⁺ phosphor exhibits a more serious decay trend between the two. To better understand the thermal quenching properties, the activation energy (E_a) is introduced to represent the energy difference between the cross-relaxation point and the excited state ²E_g, which can be calculated by the Arrhenius formula [40]:

$$\ln \left(\frac{{\mathrm{I}}_0}{\mathrm{I}}-1\right)=\mathrm{lnA}-\frac{{\mathrm{E}}_{\mathrm{a}}}{\mathrm{kT}}$$

(10)

where I₀ is the initial emission intensity, I is the emission intensity at temperature T, A is a constant, E_a is the activation energy, and k is the Boltzmann constant equal to 8.626 × 10⁻⁵ eV/K. The specific linear relationship is described in Figure 9. E_a of LCSO:0.002Mn⁴⁺ is 0.38 eV, and E_a of LMSO:0.002Mn⁴⁺ is 0.40 eV. LCSO matrix structure is less stable than LMSO, resulting in lower activation energy and, thus, more prone to non-radiative processes, which eventually leads to a poor thermal stability. This situation is reflected in the configuration of the Mn⁴⁺ ions coordinate diagram (Figure 8d), indicating that smaller activation energy can excite the electron to reach the cross-relaxation point between the ground and excited states, followed by a radiation-free transition back to the ground state causing the quenching, thereby resulting in a lower quenching temperature [41].

2.5. Electroluminescence Spectrum of the Fabricated LED Devices

To demonstrate the potential application of these two red phosphors in plant growth, we prepared two far-red LED models with electroluminescence spectra and luminescence photographs, as shown in Figure 10a,b. The CIE coordinates of (0.6431, 0.3006) and (0.4948, 0.2909) for both under a voltage of 3.36 V and a current of 20 mA are shown in Figure 10d (numeral 1 and numeral 2), respectively. Both far-red LED devices emitted bright far-red light under 365 nm excitation, and their emission spectra matched well with the absorption spectra of plant photosensitive pigments Pr and Pfr.

In addition, to demonstrate the potential application of these two red phosphors in W-LEDs, three W-LED models were prepared, and their electroluminescence spectra, luminescence photographs, and performance parameters (color rendering index, correlated color temperature, and lumen efficiency) are shown in Figure 10c,e,f. The CIE coordinates of (0.3716, 0.4052), (0.3486, 0.3770), and (0.3386, 0.3648) for the three with a voltage of 3.36 V and a current of 20 mA are shown in Figure 10d (numeral 3, numeral 4, and numeral 5), respectively. Comparing Figure 10e,f with Figure 10c, we found that the addition of these two phosphors was able to overcome the problem of missing far-red light, and the color rendering index increased from 93 to 96.4 and 96.6. The above results indicate that LCSO:0.002Mn⁴⁺ and LMSO:0.002Mn⁴⁺ red phosphors can be used as candidates for W-LEDs.

3. Materials and Methods

3.1. Sample Preparation

All La₂CaSn_1−xO₆:xMn⁴⁺ (abbreviated as LCSO:xMn⁴⁺, x = 0–0.01) and La₂MgSn_1-yO₆:yMn⁴⁺ (abbreviated as LMSO:yMn⁴⁺, y = 0–0.01) phosphors were synthesized by conventional high-temperature solid-state reaction. The raw materials were La₂O₃ (A.R.), CaCO₃ (A.R.), MgO (A.R.), SnO₂ (A.R.), and MnCO₃ (5N), which were weighed in the appropriate chemical proportions. After 20 minutes of thorough grinding in an agate mortar, the resulting mixture was transferred to an alumina crucible, warmed up at 4 °C per minute, preheated at 900 °C for 2 hours, and then sintered at 1480 °C for 3 hours in an air atmosphere. Finally, the samples were naturally cooled to room temperature and then reground into homogeneous powder for further measurements.

3.2. Characterization

The phase purity of the sintered samples was analyzed using powder X-ray diffraction (XRD) (AXS D8 Advance, Bruker, Karlsruhe, Germany) with graphite monochromatic Cu Kα radiation (λ = 0.15405 nm) operating at 40 kV and 40 mA. Test from 10 to 80° in steps of 0.02° with a counting time of 0.14s per step. XRD Rietveld refinement was performed using the Structural Analysis System (GSAS) program (Created by Brian H. Toby, Argonne Laboratories, USA). The X-ray photoelectron spectroscopy (XPS) data were obtained through a Thermo Scientific K-Alpha instrument (Thermo Fisher Scientific, Waltham, Massachusetts, USA) with a monochromatized Al Kα line source, and the binding energy was calibrated to the C 1s peak at 284.8 eV. Photoluminescence emission (PL) and excitation (PLE) spectra were measured on a fluorescence spectrophotometer (F-7100, Hitachi, Tokyo, Japan) equipped with a 150 W xenon lamp. The luminescence decay curves and quantum yields (QY) were measured by an Edinburgh Instruments FLS-980 spectrometer (Edinburgh Instruments, Edinburgh, UK) equipped with an Edinburgh Instruments integrating sphere. The temperature-dependent photoluminescence emission (PL) spectra were tested on a phosphor excitation spectrum with thermal quenching analysis system EX-1000 (Hangzhou, China). The commercial blue phosphor BAM:Eu²⁺, green phosphor (Sr,Ba)₂SiO₄:Eu²⁺, red phosphor CaAlSiN₃:Eu²⁺, the as-synthesized La₂(Ca,Mg)SnO₆:0.002Mn⁴⁺, and UV-LED chips (365 nm) were used to fabricate W-LEDs. The luminescence characteristics of the white LED were evaluated using a Starspec SSP6612 (forward bias current, 20 mA, forward voltage, 3.36V) (Hangzhou, China).

4. Conclusions

In summary, a series of La₂(Ca/Mg)SnO₆:Mn⁴⁺ red phosphors were successfully prepared using a conventional high-temperature solid-state reaction. The Rietveld refinements revealed that both La₂CaSnO₆:Mn⁴⁺ and La₂MgSnO₆:Mn⁴⁺ were monoclinic phases and the Mn⁴⁺ ions replace the Sn⁴⁺ ions. The Tanabe–Sugano energy level diagrams indicate that the Mn⁴⁺ ions were in a strong crystal field. In addition, LCSO:0.002Mn⁴⁺ and LCSO:0.002Mn⁴⁺ quantum yields were 22.90% and 35.48%, respectively. The thermal quenching phenomena of LCSO:0.002Mn⁴⁺ and LMSO:0.002Mn⁴⁺ were explained by the conformational coordinate diagram of the Mn⁴⁺ ions. Finally, the obtained far-red LED devices emitted bright far-red light that matched well with the absorption spectra of plant photosensitive pigments Pr and Pfr. Additionally, the prepared W-LEDs had R_a of 96.4 and 96.6 and CCT of 4958 K and 5275 K, respectively. These results show that LCSO:0.002Mn⁴⁺ and LMSO:0.002Mn⁴⁺ red phosphors can be used as the red component of W-LEDs as well as in plant growth.