Ln₃Al₅O₁₂:Mn⁴⁺ (Ln = Y and Lu): non-rare-earth doped red phosphor for improving light properties of white light emitting diodes

Abstract

We fabricated a series of Y₃Al₅O₁₂:Mn⁴⁺ and Y_1−yLu_yAl₅O₁₂:Mn⁴⁺ phosphors by a solid state reaction. The phase and the optical properties of the synthesized phosphors were investigated. Under the excitation at 465 nm, Y₃Al₅O₁₂:Mn⁴⁺ phosphors show emission bands locating at deep red regions, which is induced by the spin- and parity-forbidden ²E_g → ⁴A_1g transitions of Mn⁴⁺. The substitution of Y³⁺ by Lu³⁺ decreases the lattice parameter and thus strengthens the crystal field strength, which gives rise to the blue shift of emission band for Y_1−yLu_yAl₅O₁₂:Mn⁴⁺ phosphors. Due to the compensation of red light by Y₃Al₅O₁₂:Mn⁴⁺ or Y_1−yLu_yAl₅O₁₂:Mn⁴⁺ phosphor, the values of correlated-color-temperature for fabricated LEDs are decreased, which leads to the suitable application for them in indoor illumination.

1 Introduction

The well known two drawbacks of the white light emitting diodes generated by the blue chip and Y₃Al₅O₁₂:Ce³⁺ phosphors are the high correlated-color-temperature (CCT) and poor color-rendering index (CRI) [1, 2]. And these two drawbacks are mainly induced by the absence of red spectral contribution. To improve the light properties of white light emitting diodes, one of widely used methods is to mix a highly efficient red phosphor showing strong blue absorption with Y₃Al₅O₁₂:Ce³⁺ yellow phosphor [3,4,5,6,7,8]. In past years, most of attempts for the red phosphors always focus on rare earth ions doped phosphors. But we know that the storage of rare earth elements are lacking. As a result, researchers try their best to look for elements with abundant storage those can be used in red phosphors. And in recent years, Mn⁴⁺ is found to be a good candidate because that Mn⁴⁺ activated red phosphors are much cheaper and possess desirable spectral features [7].

Over the last several years, Mn⁴⁺ doped aluminate garnet (Ln₃Al₅O₁₂, Ln = Lu, Y, Gd) phosphors have been paid more and more attentions [8,9,10]. It is known that Mn⁴⁺ prefers to occupy octahedral site to obtain high luminescence efficiency. In Ln₃Al₅O₁₂ hosts, Mn⁴⁺ priors to substitute octahedral Al³⁺ site due to the little distinction of radius between Al³⁺ (0.535 Å) and Mn⁴⁺ (0.530 Å), which gives large opportunities for Ln₃Al₅O₁₂:Mn⁴⁺ phosphors to maintain the rigid garnet structure, decrease defect formation and guarantee the high luminescence efficiency of Mn⁴⁺ [8]. In this work, we synthesized a series of Ln₃Al₅O₁₂:Mn⁴⁺ (Ln = Y and Lu) phosphors by the solid state reaction and investigated the luminescent properties of the synthesized phosphors.

2 Materials and method

A series of Y₃Al₅O₁₂:x%molMn⁴⁺ (x = 0.1, 0.2, 0.3 and 0.4) and Y_1−yLu_yAl₅O₁₂:0.3%molMn⁴⁺ (y = 1, 2 and 3) phosphors were synthesized by a solid state reaction. The chemical reagents of Y₂O₃ (99.9%), Lu₂O₃ (99.9%), Al₂O₃ (99.9%) and MnO₂ (99.99%) were used as raw materials in the synthesis of Ln₃Al₅O₁₂:Mn⁴⁺ (Ln = Y and Lu) phosphors. In the synthesis, chemical reagents with stoichiometric ratio were weighted and mixed in an agate mortar with the addition of a little amount of ethyl alcohol. After a thorough blending, the mixture was transferred into a platinum crucible and calcined at 1500 °C for 6 h in a muffle furnace. After the system cooled to room temperature naturally, the product was ground for subsequent measurements.

The phase was confirmed by X-ray powder diffraction (XRD) technique in a D8 advance X-ray diffractometer (Bruker, German) using Cu Kα radiation (λ = 0.15418 nm). The ultraviolet–visible (UV–vis) absorption spectra of the phosphors were recorded on a Cary 5000 UV–vis spectrophotometer. The excitation and emission spectra were measured by an Edinburgh Instrument FLS920 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The emission spectra of the fabricated LEDs were measured by a PMS-80 LED spectrophotocolorimeter.

3 Results and discussion

Figure 1 presents the XRD patterns of the synthesized Y₃Al₅O₁₂:0.3mol%Mn⁴⁺, Y₂LuAl₅O₁₂:0.3mol%Mn⁴⁺, YLu₂Al₅O₁₂:0.3mol%Mn⁴⁺, Lu₃Al₅O₁₂:0.3mol%Mn⁴⁺ phosphors and the standard data of Y₃Al₅O₁₂ (JCPDs card no. 88-2047) and Lu₃Al₅O₁₂ (JCPDs card no. 73-1369). The ionic radius of Y³⁺ (1.019 Å) is larger that of Lu³⁺ (0.977 Å), which induces the decrease of lattice parameter with the increase of Lu³⁺ and the shifts of diffraction peaks to larger 2θ angles. In the XRD patterns of the synthesized phosphors, there are no diffraction peaks corresponding to other phase, suggesting that Lu³⁺ and Mn⁴⁺ have entirely incorporated into the structure of Y₃Al₅O₁₂. Herein, Lu³⁺ and Mn⁴⁺, respectively, substitute Y³⁺ and Al³⁺ sites due to the similar ionic radii.

Fig. 1

XRD patterns of Y_1−yLu_yAl₅O₁₂:0.3%molMn⁴⁺ (y = 0, 1, 2 and 3) phosphors and the standard XRD data of Y₃Al₅O₁₂ and Lu₃Al₅O₁₂

The luminescence of a phosphor has close relation with the crystal structure information of the host. Both Y₃Al₅O₁₂ and Lu₃Al₅O₁₂ have the cubic phase and belong to the space group of ${\text{Ia}}\bar {3}{\text{d}}(230)$. The parameters for Y₃Al₅O₁₂ and Lu₃Al₅O₁₂ are a = b = c = 12.008 Å and a = b = c =11.906 Å, respectively. Figure 2 shows the crystal structures of Y₃Al₅O₁₂ and Lu₃Al₅O₁₂ of the 1 × 1 × 1 unit cell. In Ln₃Al₅O₁₂ (Ln = Y and Lu), Ln³⁺ ions coordinate with 8 O²⁻ ions and form the polyhedral shape, but Al³⁺ ions have two different lattice sites and coordinate with 6 and 4 O²⁻ ions to form AlO₆ octahedral and AlO₄ tetrahedral shapes. The AlO₆ octahedral and AlO₄ tetrahedral shapes connect with each other by means of sharing the same O²⁻ ion. And in Y₃Al₅O₁₂ and Lu₃Al₅O₁₂, the bond lengths between Mn and ligand O are 1.921 and 1.864 Å, respectively. As the atom number per cell keeps constant, the decrease of lattice parameter means the shortening of distance between atoms. This weakens the Mn interaction between atoms. So, Mn⁴⁺ will experience a stronger crystal field as the content of Lu increases.

Fig. 2

Unit cell representations of Y₃Al₅O₁₂ and Lu₃Al₅O₁₂ crystal structures

Figure 3 shows the absorption spectra of Y₃Al₅O₁₂:x%molMn⁴⁺ (x = 0.1, 0.2, 0.3 and 0.4) phosphors in the UV–vis range. Y₃Al₅O₁₂:Mn⁴⁺ phosphors shows obvious absorption bands in UV–vis range. There are a strong absorption band in the range of 225–400 nm and a relatively weak band in the range of 425–525 nm. The wide and strong absorption band in the range of 225–400 nm originates from the Mn⁴⁺ → O²⁻ charge transfer, the spin-allowed ⁴A_2g → ⁴T_1g and ⁴A_1g → ⁴T_2g transitions, but the absorption band in the range of 425–525 nm corresponds to the ⁴A_2g → ⁴T_2g transition [11,12,13]. The absorption intensity increases in UV range and the absorption edge in UV range shifts to longer wavelength with the increase of Mn⁴⁺ doping concentration. Unlike in UV range, the absorption intensity increases with the increasing Mn⁴⁺ doping concentration but the absorption edge does not vary.

Fig. 3

Absorption spectra of Y₃Al₅O₁₂:x%molMn⁴⁺ (x = 0.1, 0.2, 0.3 and 0.4) phosphors in the UV–vis range

Figure 4 presents the excitation spectrum of Y₃Al₅O₁₂:0.3mol%Mn⁴⁺ phosphor and emission spectra of Y₃Al₅O₁₂:x%molMn⁴⁺ (x = 0.1, 0.2, 0.3 and 0.4) phosphors. In the excitation spectrum monitored at 667 nm, there are two obvious excitation bands peaking at about 324 and 465 nm, which originates from the ⁴A_2g → ⁴T_1g and ⁴A_1g → ⁴T_2g transitions of Mn⁴⁺ in the octahedral coordination. The wide range of 275–525 nm for the excitation spectrum of Y₃Al₅O₁₂:Mn⁴⁺ suggests that the phosphor can be excited by the lights in UV, near-UV and blue regions. It is also worth noting that the excitation band of Y₃Al₅O₁₂:Mn⁴⁺ hardly overlaps with the emission band of Y₃Al₅O₁₂:Ce³⁺ (broad yellow emission band peaking at 530–540 nm) [2, 14, 15], suggesting that the risk of photon re-absorption usually taking place between the nitride red phosphor and the Y₃Al₅O₁₂:Ce³⁺ yellow phosphor is highly reduced. Under the excitation at 465 nm, Y₃Al₅O₁₂:x%molMn⁴⁺ (x = 0.1, 0.2, 0.3 and 0.4) phosphors show emission bands locating at deep red light region, which comes from the spin- and parity-forbidden ²E_g → ⁴A_1g transitions of Mn⁴⁺. The emission intensity increases firstly with the increasing Mn⁴⁺ concentration up to x = 0.3 and then decreases with the further increasing Mn⁴⁺ concentration. The decrease of emission intensity is induced by the concentration quenching. Concentration quenching mechanism leads to the inability to increase the emission intensity by simply increasing Mn⁴⁺ ions. Moreover, it can be seen that the peaks of emission bands do not shift with the changing Mn⁴⁺ concentration. It is known that Mn⁴⁺ substitutes the site of Al³⁺ and these two ions have similar ionic radii in Y₃Al₅O₁₂. The similar sizes keep the lattice without any constriction or expansion. As a result, there is the invariability of the field splitting and thus the locations of emission peaks do not shift.

Fig. 4

Excitation spectrum of Y₃Al₅O₁₂:0.3mol%Mn⁴⁺ phosphor and emission spectra of Y₃Al₅O₁₂:x%molMn⁴⁺ (x = 0.1, 0.2, 0.3 and 0.4) phosphors

Figure 5 shows the emission spectra of Y_1−yLu_yAl₅O₁₂:0.3%molMn⁴⁺ (y = 0, 1, 2 and 3) phosphors under the excitation at 465 nm. All of phosphors show emission bands corresponding to the spin- and parity-forbidden ²E_g → ⁴A_1g transitions of Mn⁴⁺. The Lu³⁺ substitution does not lead to the obvious change of emission band shape but induces the slightly blue shifts of emission peaks. For Y₃Al₅O₁₂:0.3%molMn⁴⁺, the peak of the strongest emission band locates at 667 nm. But for Y₂LuAl₅O₁₂:0.3%molMn⁴⁺, YLu₂Al₅O₁₂:0.3%molMn⁴⁺ and Lu₃Al₅O₁₂:0.3%molMn⁴⁺ phosphors, the peaks of the strongest emission bands shift to 665, 664 and 662 nm, respectively. Generally, the Mn⁴⁺ luminescence has a close relation with the crystal strength and site symmetry of the host. Energy of the Mn^{4+: 2}E_g → ⁴A_2g transition is mainly determined by the nephelauxetic effect which is correlated with the wave-function overlap between the Mn⁴⁺ ions and the ligand [16,17,18]. As discussed above, the Lu³⁺ substitution decreases the lattice parameter and shortens the band length between Mn⁴⁺ and ligand O²⁻. That is to say, Mn⁴⁺ experiences a stronger crystal field in Y_1−yLu_yAl₅O₁₂:0.3%molMn⁴⁺ (y = 1, 2 and 3) phosphors. The stronger crystal field strength induced by the shorter band length between Mn⁴⁺ and ligand O² makes the energy-level splitting more, which gives rise to the blue shift of emission band for Lu³⁺ substituted Y₃Al₅O₁₂:Mn⁴⁺ phosphors.

Fig. 5

Emission spectra of Y_1−yLu_yAl₅O₁₂:0.3%molMn⁴⁺ (y = 0, 1, 2 and 3) phosphors

The LEDs based on GaN blue chips were fabricated by mixing Y₃Al₅O₁₂:Ce³⁺ phosphor (GaN-YAG:Ce³⁺ LED) with Y₃Al₅O₁₂:Mn⁴⁺ phosphor (GaN-YAG:Mn⁴⁺ LED) or Lu₃Al₅O₁₂:Mn⁴⁺ phosphor (GaN-LuAG:Mn⁴⁺ LED). Figure 6 gives the electro-luminescence spectra of the fabricated LEDs under the excitation of 20 mA current. All of electro-luminescence spectra consist of emission bands locating at blue and yellow regions, which, respectively, originate from the blue chip and the Y₃Al₅O₁₂:Ce³⁺ phosphor. The emission bands locating at deep red region for GaN-YAG:Mn⁴⁺ and GaN-LuAG:Mn⁴⁺ LEDs are induced by the mixed Y₃Al₅O₁₂:Mn⁴⁺ phosphor or Lu₃Al₅O₁₂:Mn⁴⁺ phosphor. The CIE coordinates for GaN-YAG:Ce³⁺, GaN-YAG:Mn⁴⁺ and GaN-LuAG:Mn⁴⁺ LEDs are (0.325, 0.338), (0.359, 0.327) and (0.366, 0.319), respectively. And the CCT values for GaN-YAG:Ce³⁺, GaN-YAG:Mn⁴⁺ and GaN-LuAG:Mn⁴⁺ LEDs are 5841, 4290 and 3928, respectively. These results demonstrate that the mixed Y₃Al₅O₁₂:Mn⁴⁺ phosphor or Lu₃Al₅O₁₂:Mn⁴⁺ phosphor can decrease the CCT values of fabricated LEDs. Generally, the warm white light is suitable for home applications as the value of CCT less than 5000 K and the cold white light is suitable for commercial lighting purposes if the value of CCT higher than 5000 K [19, 20]. Therefore, we can conclude that Y₃Al₅O₁₂:Mn⁴⁺ phosphor or Lu₃Al₅O₁₂:Mn⁴⁺ phosphor is the promising commercial red phosphor in warm white LEDs.

Fig. 6

Electro-luminescence spectra of the fabricated LEDs under the excitation of 20 mA current

4 Conclusion

Y₃Al₅O₁₂:Mn⁴⁺ and Y_1−yLu_yAl₅O₁₂:Mn⁴⁺ phosphors were synthesized successfully by a solid state reaction. In Y₃Al₅O₁₂ or Y_1−yLu_yAl₅O₁₂ hosts, Mn⁴⁺ ions substituted Al³⁺ sites and formed solid solutions. Under the excitation at 465 nm, the synthesized Y₃Al₅O₁₂:Mn⁴⁺ and Y_1−yLu_yAl₅O₁₂:Mn⁴⁺ phosphors show emission bands corresponding to the spin- and parity-forbidden ²E_g → ⁴A_1g transitions of Mn⁴⁺. Due to the smaller ionic radius of Lu³⁺, the substitution of Y³⁺ by Lu³⁺ decreases the lattice parameter and thus strengthens the crystal field strength, which gives rise to the blue shift of emission band for Lu³⁺ substituted Y₃Al₅O₁₂:Mn⁴⁺ phosphors. The electro-luminescence spectra of LEDs fabricated by mixing Y₃Al₅O₁₂:Ce³⁺ phosphor with Y₃Al₅O₁₂:Mn⁴⁺ or Y_1−yLu_yAl₅O₁₂:Mn⁴⁺ show obvious emission band locating at red region, which decreases the correlated-color-temperature of white light. Y₃Al₅O₁₂:Mn⁴⁺ and Y_1−yLu_yAl₅O₁₂:Mn⁴⁺ phosphors are suitable for applications in warm white LEDs due to the compensation of red light.