Color-Tunable Phosphor [Mg1.25Si1.25Al2.5]O3N3:Eu2+âA New

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Functional Inorganic Materials and Devices

Color-tunable phosphor [Mg1.25Si1.25Al2.5]O3N3:Eu2+ - A new modified polymorph of AlON with double sites related luminescence and low thermal quenching Junyi Li, Jianyan Ding, Yaxin Cao, Xufeng Zhou, Bo Ma, Zhengyan Zhao, and Yuhua Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 5, 2018

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ACS Applied Materials & Interfaces

Color-tunable phosphor [Mg1.25Si1.25Al2.5]O3N3:Eu2+ - A new modified polymorph of AlON with double sites related luminescence and low thermal quenching Junyi Li, Jianyan Ding, Yaxin Cao, Xufeng Zhou, Bo Ma, Zhengyan Zhao, Yuhua Wang* National & Local Joint Engineering Laboratory for Optical Conversion Materials and Technology, Key Laboratory for Special Function Materials and Structure Design of the Ministry of the Education, Lanzhou University, Lanzhou, 730000, P.R. China. *Correspondence: Yuhua Wang, Fax:+86-931-8913554, E-mail: [email protected] Abstract Aluminum oxynitride (AlON) was commonly used in functional ceramic materials, including phosphors for white light-emitting diodes (LEDs). In the current work, a new polymorph of AlON structure, single phase [Mg1.25Si1.25Al2.5]O3N3, has been devised and synthesized through the solid-state reaction at a rather low temperature of 1550°C. Its structure has been calculated by the Rietveld refinement. The [Mg1.25Si1.25Al2.5]O3N3 crystallizes in trigonal with lattice parameters of a=b=3.0312 Å, c=41.5758Å, V=330.83Å3, respectively, and it is formed by Mg2+ and Si4+ ions replacing partical Al3+ ions of Al5O3N3. The photoluminescence(PL) spectra of a series of Eu2+ doped [Mg1.25Si1.25Al2.5]O3N3 show a tunable light ranging from cyan to orange with a fullspectrum-covered emission and a wide excitation band with two peaks located at 290nm and 335nm. This is resulted from the two possible sites offered by the cation substitution for Eu2+ to occupy and thus broadening the emission spectra, which significantly enrich the monotonous luminescent properties of conventional AlON phosphors. Additionally, the energy transfer from one site to another has been identified using the decay curves and time-resolved emission spectra(TRES). The SEM and TEM characterization confirmed the sample’s great crystallinity and the thermal stability with more than 85% of the initial intensity at 250°C further indicates its potential in white LED applications. Keywords: AlON phosphors, color-tunable, full-spectrum-covered emission, low thermal quenching, cation substitution , energy transfer.

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Introduction Phosphor-converted(pc-) white light-emitting diodes (LEDs), regarded as a potential substitute in illumination for conventional incandescent and fluorescent lightings, have drawn significant attention recently owing to the low cost, high quality, prolonged service time, prominent efficiency, as well as the environmentally friendly characteristic.

1-3

Traditionally, there are two common methods to gain the white light, one is to combine the yellow phosphor Y3Al5O12: Ce3+ (YAG: Ce3+) with InGaN blue chip, another is to excite the mixture of tricolor (red, green and blue) phosphors by near-ultraviolet (n-UV) LED chips. However, both of the two approaches have non-negligible disadvantages that the former sustains a high correlated color temperature(CCT) and a poor color rendering index (CRI) caused by the absence of the red region, while the latter requires difficult production of various phosphors and has problems of white light stability.4-8 Among massive phosphor compounds, rare-earth-actived nitride/oxynitride phosphors are widely investigated because of their strong absorption in the UV to blue band, red-shifted emission

spectra,

excellent

(Ba,Sr,Ca,)2Si5N8:Eu2+,9-10

efficiency

(Sr

and

thermal

,Ca)AlSiN3:Eu2+,11-12

stability.

Ca-α-SiAlON:

For

instance,

Eu2+,13

β-

SiAlON:Eu2+,14 and so on. Currently, aluminum oxynitride (AlON), known as a sosoloid substance in the binary system of Al2O3-AlN, has gained increasing interest. It is largely used in ceramic materials and thermodynamic stable electromagnetism windows due to its wide band gap, transparence in the visible light range and outstanding mechanical performances at both room and high temperatures.15-16 These properties indicate that AlON has a great possibility for uses as a host matrix of advanced phosphors for white light LED applications as well.17 Till now, there are some reports about photoluminescent properties in γ-AlON and other polymorph of AlON such as Al10O3N8 and Al5O6N,18-22 showing good brightness and thermal stability. Nevertheless, all these phosphors have simplex emission spectra which are mostly located at blue to green region with narrow band, thus it needs to mix with at least two different color of phosphors to obtain the white light, which has a greater risk of unstability. Furthermore, most AlON phosphors demand a


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rather harsh synthesis condition with high temperature around 2000°C and high pressure up to 0.5-1Mpa. These two disadvantages vastly restrict their production and application. As far as we know, the monotonous luminescence of conventional AlON phosphors are mainly caused by the single site or channel occupation of rare earth ions, and the high synthesis temperature for AlON is largely owing to the extremely high melting point of AlN and Al2O3. In order to provide an idea to solve these problems, we focused on a new polymorph of AlON, Al5O3N3, which has been obtained under a relatively low temperature of 1650°C not long ago,23 and a compound named [Mg1.25Si1.25Al2.5]O3N3 which has been mentioned but not introduced in detail.24 It is reasonably inferred that partical Al3+ ions of Al5O3N3 could be replaced by Mg2+ and Si4+ ions to form [Mg1.25Si1.25Al2.5]O3N3. Such cation substitution could not only offer multiple sites for rare earth ions to occupy and widen the emission spectra, but also significantly reduced the synthesis temperature by some other oxide raw materials with lower melting point rather than Al2O3, which have been proved by several literature where the synthesis is within 900°C.25-26 Based on the above considerations, Eu2+-doped [Mg1.25Si1.25Al2.5]O3N3 was successfully prepared by solid state reaction at 1550°C with no Al2O3 in raw materials. The Rietveld refinement was applied to calculate its structure, and the double sites related luminescent properties along with mechanisms have been studied thoroughly. The PL and PLE spectra and excellent thermal stability confirmed its prospect in white light LED applications. 2. Experiment 2.1. Raw Materials and Preparation A series samples of [Mg1.25Si1.25Al2.5]O3N3:xEu2+ (abbreviated to MSAON:xEu2+, where the x is mole percent) was synthesized through solid-state reaction. The crude materials were AlN(aluminium nitrate) (Aldrich, ≥ 99.50%), Si3N4 (Aldrich, ≥ 99.50%), MgO (A.R. [Analytical Reagent]), SiO2 (A.R.), Eu2O3 (99.99%). The reactants were weighed in stoichiometric proportions and mixed with 2% wt of H3BO3(A.R.) acting as a flux



agent. The mixture was finely ground in an agate mortar and then was put into an Al2O3 copple. Then, the mixture was sintered at 1550°C for 4h with a flowing gas of N2/H2 in a horizontal tubular furnace. Finally, the samples were furnace-cooled down to room temperature and ground again, yielding crystalline powder for further measurement. 2.2. Characterization and Measurement The phase purity of a series of as-synthesized MSAON:xEu2+ phosphors have been analysed by the powder X-ray diffraction (XRD) with a Rigaku D/max–24009 diffraction and Ni-filtered Cu-Kα ray. In addition, the XRD data were gathered from 10° to 80° with the count time of 0.1 seconds/step and the step width of 0.03°. The Rietveld refinement results were educed with the general structure analysis system (GSAS),27 and the XRD data for refinement are gathered with the count time of 0.5 seconds/step and the step width of 0.01°. The component and morphology of the powder were observed by scanning electron microscopy (SEM, S-340, Hitachi, Japan) and transmission electron microscopy (TEM). The Perkin Elmer 950 spectrometer with BaSO4 white background as the standard reference for reflection measurement was used to measure the diffuse reflectance spectra (DRS) of un-doped and Eu2+ activated MSAON. The PL and PL excitation spectra of samples were tested by a FLS-920T fluorescence spectrophotometer with a 450W Xe source and double excitation monochromators. The X-ray photoelectron spectroscopy (XPS) was collected on a Kratos AXIS Ultra DLD spectrometer. The luminescence decay curves and TRES spectra were gained by FLS-920T fluorescence spectrophotometer with nF900 nanosecond flashlamp. All the previous measurements were proceeded at room temperature. Besides, thermal quenching spectra were measured by heating apparatus (TAP-02) with PL equipment, while the thermoluminescence (TL) curves were tested by a FJ-427A TL meter (Beijing Nuclear Instrument Factory). 3. Results and discussion 3.1. Structure and phase analysis


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From the chemical formula of MSAON and the principle of charge balance, it could be Fig. 1 The experimental (sphere), calculated (cyan line), the Bragg diffraction positions (pink line) and the difference (purple line) by the Rietveld refinement of MSAON.

deduced that a quarter mole of Mg2+ and Si4+ respectively substituted the Al3+ of Al5O3N3 to

form

the

structure

of

MSAON.

Thus,

the

Rietveld

refinement

of

[Mg1.25Si1.25Al2.5]O3N3:0.02Eu2+ has been processed using the (GSAS) program27 based on the crystal data of Al5O3N323. As exhibited in Fig. 1, the experimental XRD points are accordant with the calculated image and all of the peaks match the reflection condition. The reliability factors are Rwp =13.30%, Rp = 10.61% and χ2 = 2.429. The result implies that MSAON crystallizes in trigonal with the space group R-3m (166) and the lattice parameters of a=b=3.0312Å, c=41.5758Å, V=330.83Å3, respectively. The precise data is listed in Table 1. Table 1 The crystal data of [Mg1.25Si1.25Al2.5]O3N3:Eu2+ from the Rietveld refinement Chemical Formula Crystal system Space group Cell parameters Cell ratio Cell volume Z

[Mg1.25Si1.25Al2.5]O3N3 Trigonal R-3m (166) a= 3.0312Å, c=41.5758Å a/b=1.0000, b/c=0.0729, c/a=13.7160 330.83Å3 3

Atom

X/a

Y/b

Z/c

Wyck.

Site

S.O.F.

Al1 Mg1

0 0

0 0

0 0

3a 3a

-3m -3m

0.5 0.25

Si1

0

0

0

3a

-3m

0.25

O1

0

0

0.0808

6c

3m

0.5



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N1

0

0

0.0808

6c

3m

0.5

Al2

0

0

0.2661

6c

3m

0.5

Mg2

0

0

0.2661

6c

3m

0.25

Si2

0

0

0.2661

6c

3m

0.25

O2

0

0

0.1954

6c

3m

0.5

N2

0

0

0.1954

6c

3m

0.5

Al3a

0

0

0.1282

6c

3m

0.333

Mg3a

0

0

0.1282

6c

3m

0.167

Si3a

0

0

0.1282

6c

3m

0.167

Al3b

0

0

0.1436

6c

3m

0.167

Mg3b

0

0

0.1436

6c

3m

0.083

Si3b

0

0

0.1436

6c

3m

0.083

O3

0

0

0.3077

6c

3m

0.5

N3

0

0

0.3077

6c

3m

0.5

The crystal structure of MSAON is drawn in Fig. 2. As can be seen, it has a long layered centrosymmetric unit cell with three kinds of [Mg/Si/Al]-[O/N] polyhedrons. The edgeshared C1 (C is an abbreviated of cation, representing Mg2+/Si4+/Al3+) octahedrons and corner-shared C2 tetrahedrons make up a layered network through the corner-shared way. Further more, the layers connect with each other with edge-shared irregular C3 octahedrons. This structure is consistent with Al5O3N323 and similar to a series of AlON compounds18-22, indicating its novel chemical stability. As shown in Fig. 2(c), the average ionic bond lengths in C1 octahedron and C2 tetrahedron are 2.049Å and 1.812 Å respectively, and the space size along with a high synthesis pressure is sufficient for Eu2+ (1.17Å CN=6) to replace Mg2+ (0.72Å CN=6) and act as luminescent centers. The radius distinction of Al3+(0.53Å, CN=6) and Si4+(0.40Å CN=6) with Eu2+ are too large and their valence states are not same, so the occupation of Al3+ and Si4+ will not be prior considered. The irregular C3 octahedron is actually formed by two face-shared tetrahedron with a rather short cation distance. Although the Eu2+ may also occupy these sites, cross-relaxation could happen between them and exhibit no light-emitting. In general, there could be two luminescent centers in MSAON which will be investigated systematically in the further discussion.


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Fig. 2 Crystal structure of MSAON: (a) (110) projection, (b) three-dimensional view, (c) polyhedron structure.

The phase identification of MSAON:xEu2+ (0.02 ≤ x≤ 0.10) phosphors analysed by XRD is shown in Fig. 3. It is observed that all the peaks are well paralle to the calculated lines, indicating that the samples show excellent crystallinity and the doping of Eu2+ induce no obvious impurity phase. Meanwhile, the slight shift towards smaller angle in the diffraction peaks reveals the successful incorporation of larger Eu2+ replacing smaller Mg2+ as activators into the MSAON host lattice.



The morphology of MSAON has been viewed by SEM in Fig. 4(a). The particles show a good dispersibility and evenly distribute across the screen resulted from the hightemperature solid state reaction. According to a close look at Fig. 4(b), the particles have a trend of schistose growth although they present irregular morphology, indicating the existence of preferential orientation of its crystalline grain. Its particle size distribution has been calculated by the Nano Measurer program additionally. The results depicted in Fig. 4(c) shows that the particle size of MSAON ranges from 1.5μm to 4.5μm with an average diameter of 2.61μm, which agrees well with the encapsulation requirement of (n)UV-LED chips.

Fig. 3 The XRD patterns of MSAON:xEu2+ (0.02 ≤ x≤ 0.10).

Fig. 4 (a) and (b) SEM morphology of MSAON, (c) The particle size distribution of MSAON.


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The typical low-magnification TEM image of MSAON has been shown in Fig. 5(a). The sample presents a schistose shape which is in agreement with the result of SEM. Fig. 5(b) is the High-resolution(HR-) TEM image of MSAON. As can be seen, the selected area exhibit finely single crystal structures with great crystallinity. The measured interplanar spacings are 0.251nm and 0.256nm, which is close to the calculated interplanar distances _

of (015) and (114) (0.250nm and 0.255nm respectively). The EDS analysis spectrum shown in Fig. 5(c) verifies the existance of Mg, Si, Al, O, N, C and Cu in the sample. The C and Cu components belong to the carbon film and copper grid supporting the sample.

Fig. 5 (a) Typical low-magnification TEM image of MSAON, (b) High-resolution TEM image of MSAON (c) EDS spectrum of MSAON, the inset table shows the normalized at% of each element,（d-h）Elemental EDSmapping image of MSAON.

The inset table of Fig. 5(c) also provides the normalized at% of each element. Considering that EDS is a semiquantitative analysis, the results are not entirely similar but pretty close to the original chemical formula of MSAON. The elemental EDSmapping images of MSAON displayed in Fig. 5(d-h) can offer further evidence of all the elements and they uniformly distribute on the selected region of the sample.



3.2. Luminescent properties Fig. 6 shows the DRS of the Eu2+ un-doped and doped MSAON, the excitation spectrum of MSAON:0.02Eu2+ has also been exhibited to make a comparison. Obviously, the absorption ranging from 200 to 270nm could be arised from the host lattice, which shows an ashen body color. The light yellow body color and the wide absorption band from 270nm to 500nm could be attributed to the 4f-5d electron transition of divalent Eu ions, which agrees with the excitation spectrum of MSAON:0.02Eu2+. The band gap of MSAON can be obtained through the relationship of [F(R)hν]0.5 with photon energy, where F(R)=(1 − R)2/2R is the Kubella−Munk function, R is the reflection gained in the DRS. by applying the methods provided by Cao et al.28 in the inset image of Fig. 6, the band gap of MSAON is approximately calculated to be 4.09eV by making tangent to F(R)=0.

Fig. 6 The DRS and PLE of MSAON:xEu2+; the inset shows [F(R)hv]2 vs photo energy hν for MSAON.

The PL and PLE spectra of MSAON:0.02Eu2+ in Fig. 7(a) shows that when excited at 335nm, the PL intensity around 550nm is stronger, causing a cyan to orange lightemitting. Monitored at 460nm, the broad excitation band can be obtained with two peaks at 290nm and 335nm. Along with the DRS, we can reasonably infer that they are both resulted from the electron transition of Eu2+. As displayed in Fig. 7(b) and (d), two


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shapes of emission spectra excited by different peaks can all be fitted into two wellseparated Gaussian curves due to the Eu2+ occupying two Mg2+ cation sites, which is discussed in the above Rietveld refinement. To ensure the ascription of the two luminescent centers, the crystal field strength (Dq) around Eu2+ was calculated by the formula below29: Dq =Ze2r4/(6R5)

(1)

where e is the electron charge, r is the d-wave function, Z is the anion charge and R is the bond length. The average bond length of C1 sites (2.094 Å) is larger than C2 sites (1.812 Å) from the Rietveld refinement, leading to a smaller crystal field splitting and the shorter wavelength of C1 sites. Therefore, we can conclude from the above analysis that the shorter Gaussian peaks belong to C1 sites while the longer belong to C2 sites. The double peaks of PLE spectrum could also be assigned similarly in Fig. 7(c).

Fig. 7 (a) The PL and PLE spectra of MSAON:0.02Eu2+; (b), (c) and (d) the Gaussian peaks fitting for PL and PLE spectra of MSAON:0.02Eu2+;

The wide excitation and emission band of MSAON: Eu2+ is assigned to the 4f-5d electron transition of Eu2+. The characteristic emission with the peak around 610nm of Eu3+ has never been found. On the basis of the following ionic reaction equation:



6Eu3++2N3-→6Eu2+ +2N2

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(2)

plenty of N3- in this oxynitride matrix around Eu ions provide a reducing atmosphere, protecting Eu2+ from being oxidized to Eu3+ and leading to the broad emission spectra. The full XPS spectrum of MSAON: Eu2+ has been plotted in Fig. S1, confirming the existence of Eu 3d state as well as other elements which is in agreement with the EDS results. By scanning the fine XPS spectrum of Eu 3d in Fig. S2, it is evidently that the Eu2+ peaks at ~1125 and 1155eV play a major role, while the Eu3+ peaks at ~1135 and 1165eV are much weaker.30 Since XPS is a surface sensitive technology, we can firmly believe that although there are a tiny bit of un-reduced Eu3+ attaching on the surface of the sample, most Eu ions exhibit divalent state owing to the reducing atmosphere in the host lattice.

Fig. 8 The energy level diagram of Eu2+ doped MSAON.

For the purpose of further explicating the two different luminescent centers related emission, the energy level diagrams of MSAON: Eu2+ have been drawn in Fig. 8 based on the PL, PLE and DRS spectra. The electrons situated at the 4f ground state can be actived to the splitted 5d levels of Eu2+(way ① ), corresponding to the excitation band. The high-energy electrons can assemble on the low-energy excitation level(way②) and carry on the spin-lattice relaxation, which causes the energy loss via the lattice vibration.


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Ultimately, the electrons would jump to the 4f levels of Eu2+ again from the lowest energy level(way ③) with the light-emitting. In the current process, the different stokes shift(ST) and crystal field splitting(CFS) of Eu2+ in the two cation sites is the origin of the wide emission spectra. To find the influence of Eu2+ concentration on luminescent intensity and energy transfer mechanism, a series of PL spectra of MSAON: xEu2+ has been scanned in Fig. 9. Along with the inset image, it is clear that the intensity of the peak shows a decline and the wavelength displays a red shift with the increase of Eu2+ content. That is to say the intensity difference between the two luminescent centers becomes smaller. When doped with low concentration of Eu2+, the Eu2+ could tend to occupy the looser C1 sites, which causes the stronger intensity of shorter wavelength. With the increase of Eu2+ content, more Eu2+ have to enter the more compact C2 sites, leading to the smaller difference between the two peak intensities. This phenomenon could realize a tunable emitting color from cyan to orange. Moreover, the red region in the PL spectra ranging from 600 to 750nm could solve the drawbacks that the white light realized by the encapsulation of YAG:Ce3+ and blue chips lacks red wave band.



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Fig. 9 The series of PL spectra of MSAON:xEu2+ ,the inset image shows the change of peak Intensity and wavelength with different Eu2+ content.

The decay curves of MSAON: xEu2+ (0.02 ≤ x≤ 0.10) with the excitation at 335nm and monitored at 460nm have been shown in Fig. 10 (a). It is obvious that the curves can be well fitted by a second-order exponential curve by the biexponential formula,31 I =A1exp(-t/τ1) + A2exp(-t/τ2)

(3)

where I is the luminescent intensity, A1 and A2 are fitting constants, t is the time, τ1 and τ2 are short and long lifetimes for exponential components, respectively. Therefore, the average lifetimes of MSAON: xEu2+ (0.02≤x≤0.10) can be calculated by the following formula,32 τ=(A1τ12 + A2τ22)/(A1τ1 + A2τ2)


(4)

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Fig. 10 (a) The decay curves of MSAON: xEu2+ (0.02 ≤ x≤ 0.10); (b) the emission time map of MSAON: 0.02Eu2+; (c) and (d) Gaussian fitted curves of 273.44ns and 1509.91ns TRES spectra.

and listed in Fig. 10(a). They falls down with the increase of Eu2+ concentration, indicating that the radiationless transition gets stronger and energy transfer has occurred between Eu2+. The TRES of MSAON: 0.02Eu2+ have also been measured and displayed as an emission time map in Fig. 10(b). It can be obviously observed that the curves present double peaks which are consistent with the PL spectra and change with the decay time. By paying attention to 273.44ns and 1509.91ns TRES spectra in Fig. 10(c) and (d), the well fitted Gaussian peaks reveal that the emission intensity of C2 site increases obviously, which could further testify the existence of the two luminescent centers and possible energy transfer from C1 sites to C2 sites. As mentioned in the beginning of this work, thermal stability is an urgent requirement for the phosphors. Thus the temperature quenching PL spectra of MSAON: Eu2+ has been



scanned in Fig. 11(a). Apparently, the sample displays an excellent thermal stability that at a rather high temperature of 250°C ，the peak intensity of MSAON: Eu2+ still keeps more than 85% of the headmost intensity at room temperature, which is far more stable than YAG:Ce3+ 33. It can be further observed that the peak wavelength shows a blue shift of about 10nm and the peak intensity have a trend of rising up after 200°C. Meanwhile, the emission intensity around 600nm which belongs to the C2 site decreases faster than the the C1 site. In a continuous heating test in Fig. 11(b), the sample’s peak intensity keeps in high level and nearly unchanged at both 150°C and 250°C. Although the thermal stability is not completely reversible owing to the prolonged heating process, it can still restore to 80% of the initial intensity after cooling down to room temperature again. In order to clearly explain these interesting phenomenons, the energy level and configurational coordinate diagram of MSAON: Eu2+ has been exhibited in Fig. 11(d). Based on the above discussions, the relative locations of C1 and C2 sites were presumably drawn. The electrons could be actived from the ground state to the excited states and drop afterwards(way① and ②) with light-emitting. When the temperature goes up, more electrons can absorb the thermal activation energy ΔE and jump directly to the ground states(way③) with a non-radiative transition, which could be the main origin of thermal quenching. In this process, the activation energy ΔE1 of C1 sites is obviously larger than ΔE2 of C2 sites due to the shorter emission wavelength, therefore the thermal stability of C1 sites is much better than C2 sites. With the further raise of the temperature, the electrons could also transfer from tighter C2 sites to looser C1 sites(way④) and emit light along the way① again, causing the blue shift of the PL spectra.


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Fig. 11 (a)The temperature quenching PL spectra of MSAON: Eu2+, the inset shows the relationship between the temperature and emission intensity; (b) the relationship between the heating time and emission intensity; (c) the TL curve of MSAON: Eu2+; (d) the energy level and configurational coordinate diagram of MSAON: Eu2+.

When the temperature exceeds 200°C, electrons in the deep traps could be excited(way⑤) and thus causes the uptrend of emission intensity. The TL curve shown in Fig. 11(c) verifies the existence of traps in MSAON: Eu2+. The trap depth could be performed with the following approximate formula,34,35



ET =TM/500

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(5)

where ET(eV) is the energy gap between the trap levels and the conduction band of the matrix, and TM(K) stands for the temperature corresponding to the maximum value of the TL peak. The depth of the traps was calculated to be 0.738eV by the TL peak located at 96°C, indicating that the traps are relatively deep. The chromaticity coordinates of MSAON: xEu2+ (0.02≤x≤0.10) have been plotted in Fig. 12. Obviously, the coordinate of MSAON: Eu2+ can be tuned by increasing Eu2+ concentration from (0.2686,0.2885) to (0.2990, 0.3100) with a cyan to orange lightemitting and a decreasing correlated color temperature (CCT), and they are all close to the cold white light region. Seeing that the emission band of MSAON: Eu2+ has already covered the full visible spectrum, only a tiny enhancement in red region intensity is needed to obtian a warm white light. So as displayed in the inset photograph and spectrum in Fig. 12, the sample was mixed with high-stable commercial red phosphor CaAlSiN3:Eu2+, which successfully leads to a warm white light with the coordinate of (0.3363,0.3147) and a further decreased CCT.


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Fig. 12 The chromaticity coordinates of MSAON: xEu2+ (0.02 ≤ x≤ 0.10), the inset shows the PL spectrum of MSAON: Eu2+ mixed with CaAlSiN3:Eu2+.

4. Conclusion In general, the new color-tunable phosphor [Mg1.25Si1.25Al2.5]O3N3: Eu2+ has been successfully synthetized through a solid state reaction at a rather low temperature. A compact structure similar to AlON compounds has been obtained and the cation substitution accommodates two suitable cation sites for Eu2+ to occupy. The fullspectrum-covered emission band with the light-emitting ranging from cyan to orange can be tuned. The double luminescence centers and the energy transfer between them have been proved by a series of measurements including decay curves, time-resolved emission spectra and thermal quenching spectra. [Mg1.25Si1.25Al2.5]O3N3: Eu2+ also shows an excellent thermal stability with more than 85% of the initial intensity at 250°C. Additionally,

warm

white

light

was

realized

by

the

double-mixture

of

[Mg1.25Si1.25Al2.5]O3N3: Eu2+ and CaAlSiN3: Eu2+. All the results implied that [Mg1.25Si1.25Al2.5]O3N3: Eu2+ is a potential candidate for full-spectrum w-LEDs



applications and ulteriorly provided an innovation to modify the luminescent properties of AlON phosphors further at the academic level. Supporting Information The full and fine XPS spectra of [Mg1.25Si1.25Al2.5]O3N3:Eu2+.

Acknowledgements The authors appreciate the Gansu Province Development and Reform Commission along with the National Natural Science Funds of

China (Grant No. 51672115 and No.

51502122). References (1) Wang, L.; Xie, R. J.; Suehiro, T.; Takeda, T.; Hirosaki, N. Down-Conversion Nitride Materials for Solid State Lighting: Recent Advances and Perspectives Chem. Rev. 2018, 118(4), 1951-2009. (2) Lin, C. C.; Liu, R. S. Advances in Phosphors for Light-emitting Diodes J. Phys. Chem. Lett. 2011, 2, 1268−1277. (3) Ye, S.; Xiao, F.; Pan, Y. X.; Ma, Y. Y.; Zhang, Q. Y. Phosphors in Phosphorconverted White Light-emitting Diodes: Recent Advances in Materials, Techniques and Properties Mater. Sci.Eng. R. 2010, 71, 1−34. (4) Ding, J. Y.; Seto, T.; Wang, Y. C.; Cao, Y. X.; Li, H., Wang, Y. H. The New Mode of Energy Transferring between Mn2+ and Eu2+ in Nitride Based Phosphor SrAlSi4N7 with Tunable Light and Excellent Thermal Stability Chem.–An Asian Journal 2018.07. (5) Zhao, M.; Liao, H. X.; Ning, L. X.; Zhang, Q. Y.; Liu, Q. L.; Xia, Z. G. Nextgeneration Narrow-Band Green-Emitting RbLi(Li3SiO4)2:Eu2+ Phosphor for Backlight Display Application Adv. Mater., 2018, 1802489.


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Color-Tunable Phosphor [Mg1.25Si1.25Al2.5]O3N3:Eu2+âA New

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