Synthesis, Crystal Structure, and Luminescence Properties of Tunable

Aug 28, 2017 - The temperature quenching property of the phosphor was also investigated, and the good thermal stability of the phosphors was analyzed ...
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Synthesis, Crystal Structure, and Luminescence Properties of Tunable Red-Emitting Nitride Solid Solutions (Ca1−xSrx)16Si17N34:Eu2+ for White LEDs Hang Chen, Jianyan Ding, Xin Ding, Xicheng Wang, Yaxin Cao, Zhengyan Zhao, and Yuhua Wang* Key Laboratory for Special Function Materials and Structural Design of the Ministry of Education, Lanzhou University, Lanzhou 730000, PR China S Supporting Information *

ABSTRACT: A series of nitride solid solutions (Ca1−xSrx)16Si17N34:0.03Eu2+ were successfully synthesized through the conventional solid-state method. The electronic crystal structure and photoluminescence characteristics were studied in detail. The excitation in the near-ultraviolet and blue regions of the samples shows a broad band in the 250−550 nm range, which can match well with the n-UV and blue lighting-emitting diode chips. Partial substitution of Ca2+ by Sr2+ results in a redshift emission, and the impacts of Sr content on the luminescence were researched in detail. Under 410 nm excitation, the phosphor exhibited tunable red emission from 616 to 653 nm by changing the concentration of Sr2+. Based on the crystal data, the emission can be fitted into three distinguished Gaussian components, which are attributed to the different Eu2+ luminescence centers occupied in three disparate Ca2+ (Sr2+) lattice sites. The temperature quenching property of the phosphor was also investigated, and the good thermal stability of the phosphors was analyzed through the activation energy for thermal quenching. And the obtained CCT values from 2642 to 2817 K are suitable for a warm white light region. All the results indicated that the phosphors have possible application in the warm white light-emitting diodes. thermal stability; examples include M2Si5N8:Eu2+/Ce3+,20,26,27 CaSiN2:(Eu2+/Ce3+),28−30 and MSiN2:Eu2+ (M = Sr, Ba).31 From earlier work, we can see that under the excitation of UV a visible wave CaSiN2:Eu2+ can emit orange−red light. And the cubic type of CaSiN2:Eu2+ reveals a broad photoluminescence band peaking at around 620 nm.28 Besides, CaSiN2 has a relatively excellent heat and chemical stability against air compared to the MSiN2 (M = Sr, Ba); therefore, cubic type of CaSiN2 has been the concern of many researchers. But in fact, single crystals of the cubic type of CaSiN2 phase has not yet been synthesized up to now. From the report by Hick S M et.al, we can see that the cubic type of CaSiN2 reported in the early literatures is actually Ca16Si17N34.32 It can be seen from the previous studies that the luminescent intensity of the phosphors needs improvement to produce high quality photoluminescence display. Besides, there is no detailed explanation for the luminescence mechanism. Recently, some studies showed that the congener cation substitution can not only increase the emission intensity, but also adjust the location of emission spectra.33−35 However, to our knowledge, no any research was reported about the effects of substitution of Ca2+ by Sr2+ on crystal structure, luminescent properties of Eu2+ ions and the phosphorescent mechanism of Ca16Si17N34 (CSN) so far.

1. INTRODUCTION Since solid-state luminescent materials became feasible substitutes to replace traditional light sources,1,2 the white light-emitting diode (w-LED) has attracted a lot of attention due to the advantages of the LED, such as high efficiency, environmental friendliness, and designable features.3−6 In the last years, a large proportion of w-LED lighting devices are mainly using a blue InGaN chip combined with a orangeyellow-emitting phosphor, i.e., YAG: Ce3+ phosphor.7−9 Unfortunately, though it can easily obtain a higher efficiency using the phosphor, the problem is that the output light of the w-LEDs cannot achieve the color balance compared with true color because of the absent red light component. The high color temperature (correlated color temperature is above 7000 K) and poor color rendering index (CRI, Ra < 80)10−13 are the barriers which restricts their use in room lighting. Therefore, in order to overcome the disadvantages, it is urgent to find some new orange or red luminescent materials for high efficient white LEDs. In recent years, in order to obtain warm white output, rareearth doped compounds have been investigated, such as metasilicate,14−16 germanate,17 phosphate,18 borate,19 oxyfluoride,20 nitridosilicate,21−24 related SiAlONs,25 and so on. As a kind of important luminescent materials, nitridosilicate materials with rare-earth ions are widely investigated due to their excellent luminescent properties and high chemical and © XXXX American Chemical Society

Received: April 23, 2017

A

DOI: 10.1021/acs.inorgchem.7b01021 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry In this work, we have been synthesized a series of Sr2+ substituted nitride solid solutions with the formula (Ca0.97−xSrx)16Si17N34:0.03Eu2+ (0 ≤ x ≤ 0.30) (CSSN) by conventional solid-state method under N2/H2 atmosphere at atmospheric pressure. The crystal structure and luminescent performance of the CSSN phosphors powers were investigated in detail. Besides, the influence of various contents of Sr2+ and Ca2+ on the photoluminescence performance and possible phosphorescent mechanism were discussed.

2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis. The nitridosilicate solid solutions, CSSN:Eu2+ were synthesized through conventional solid-state reactions. The raw materials Sr3N2 (>99.5%), Ca3N2 (>95%), EuF3 (99.999%), and Si3N4 (99.5%) were adequately ground in an agate mortar for 30 min in a nitrogen-filled glovebox (both O2 and H2O are under 1 pm) to form homogeneous mixture. Then the obtained mixture was sintered at 1450 °C for 5 h in a reducing atmosphere at constant pressure (5% H2/95% N2). And the products were cooled down to room temperature, and then the products were ground into a uniform power for further measurements. 2.2. Characterization. All the measurements were conducted at room temperature.The X-ray powder diffraction (XRD) (D2 PHASER X-ray Diffractometer, Germany) with a graphite monochromator using Cu Kα radiation (λ = 1.54056 A), operating at 30 kV and 15 mA was employed to identify the phase formation and crystal structure of the samples. The Rietveld method36 was employed to perform the structure refinement by the program General Structure Analysis System (GSAS).37,38 The band structure for the sample was calculated with density functional theory (DFT) and performed with the Cambridge Serial Total Energy Package (CASTEP) code.39 The theoretical basis of density was used for local-density approximation (LDA).40 The photoluminescence (PL) and PL excitation (PLE) spectra were obtained by using a FLS-920T fluorescence spectrophotometer equipped with a 450 W Xe light source and double excitation monochromators. The luminescence intensities of the samples depending on the temperature were carried out by using an aluminum plaque with cartridge heaters; the temperature was measured by thermocouples inside the plaque and controlled by a standard TAP-02 high temperature fluorescence controller. The diffusion reflectance spectra (DRS) were measured on a UV−vis spectrophotometer (PE Lambda 950). The powder morphology was investigated by transmission electron microscopy (TEM, FEI Tecnai F30) and scanning electron microscopy (SEM; S-3400, Hitachi, Japan).

Figure 1. XRD patterns of CSSN:0.03Eu2+ (0 ≤ x ≤ 0.30).

bigger ions (Sr2+) replaced the smaller ions (Ca2+), the lattice would be expanded. The Rietveld analysis for CSSN:Eu2+ (x = 0 and 0.20) was obtained by the structural data of the Ca16Si17N34 reported by Hick et al.32 All the parameters refined are converging to the residual factors Rwp = 10.78%, Rp = 9.50% and Rwp = 13.33%, Rp = 10.97% for (Ca0.97Eu0.03)16Si17N34 and (Ca0.77Sr0.20Eu0.03)16Si17N34, respectively. The XRD data of Rietveld analysis patterns are presented in Figure 2. The red crosses and black solid lines are experimental patterns and calculated patterns, respectively. The positions of the Bragg reflections of the calculated pattern are shown by the pink short vertical lines. The difference between the calculated and experimental patterns is plotted at the bottom. From the obtained samples with different doping concentrations, there is no detectable impurity phase observed. The related parameters of crystallography and refinement about the samples are shown in Table 1. The crystalline structure of CSSN is the same as CSN which is cubic with the space group F −43m (216). The substitution of Sr2+ for Ca2+ caused the cell volume to increase from 3340.57 to3361.92 Å. The atomic coordinates are provided in the Table S1 in the Supporting Information, and Table 2 summarizes some selected bond distances. The refined crystal structure for the CSSN phosphor crosssectional view in the [001] direction is shown in Figure 3(a). The structure of CSSN consists of SiN4 tetrahedra which share corners. In the structure, there are 3 distinct Ca (Sr/Eu) sites in the unit as shown in Figure 3(b). The Ca1 (Sr1/Eu1) and Ca3 (Sr3/Eu3) are located at distorted octahedral sites which are coordinated by six nitrogen atoms, while the Ca2 (Sr2/Eu2) is situated at a cubic site which is coordinated by eight nitrogen atoms. In Figure 4(a) and (b), the typical SEM image and lowmagnification TEM image of CSSN phosphor are given. The particles of the sample with good dispersion exhibit rod shapes and diameters about 1−5 μm. Furthermore, it is obviously found that some rods have quadrilateral prism shape. Meanwhile, in order to understand the distribution of the element, a more detailed elemental analysis (mapping analysis)

3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Phase Characteristics. Figure 1 shows the X-ray diffraction patterns of the CSSN:Eu2+ phosphors with different Sr2+ content (0 ≤ x ≤ 0.3). All the diffraction peaks of the products are well matched with the pattern simulated from the reported crystallographic data, and there are no other peaks of other allotropic forms or the raw materials are detected up to x = 0.30, indicating that the solid solution is obtained. However, there are visible impurity peaks in the patterns beyond x = 0.30, which suggests that the concentration of Sr2+ in the CSSN phosphors is limited to x = 0.30. From the intensity of the X-ray peaks, we can see that the intensity decreased when the Sr2+ content is greater than 0.20, which indicates that the crystallinity of the CSSN is reduced. The peak positions of the products CSSN:Eu2+ are slightly shifted to lower 2θ degree following the increasing of Sr2+ content. The shift of the diffraction position is attributed to lattice expansion. It is known that the ionic radius of Sr2+ (1.12 Å, coordination number = 6) is bigger than that of Ca2+ (0.99 Å, coordination number = 6) in the host lattice.33 So when the B

DOI: 10.1021/acs.inorgchem.7b01021 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Rietveld refinement of (Ca0.97−xSrxEu0.03)16Si17N34 for (a) x = 0 and (b) x = 0.2. Red crosses: measured diffractogram; bottom blue line: difference profile; black line: calculated diffractogram; pink bars: positions of the reflections.

Table 1. Structural Data for (Ca0.97−xSrxEu0.03)16Si17N34 (x = 0 and 0.20) Formula

(Ca0.97Eu0.03)16Si17N34

Crystal system Space group Cell parameters (Å) Cell volume (Å3) Z Radiation/pm 2θ range/deg Refinement Reliability factors

(Ca0.77Sr0.20Eu0.03)16Si17N34

Cubic F4̅3m(216) a = 14.9488 a = 14.9806 3340.57 3361.92(16) 4 Cu Kα (λ = 1.5418 Å) 5 ≤ 2θ ≤ 85 Rietveld refinement with GSAS software Rp = 9.50% Rp = 10.97% Rwp = 10.78% Rwp = 13.33% χ2 = 1.172 χ2 = 1.231

Table 2. Selected Bond Distances of M−N (M = Eu/Ca/Sr) for CSSN Bond

(Ca0.97Eu0.03)16Si17N34

(Ca0.77Sr0.2Eu0.03)16Si17N34

M1-N1 M1-N1 M1-N2 M1-N2 M1-N5 M1-N5 M2-N2 M2-N2 M2-N2 M2-N2 M2-N5 M2-N5 M2-N5 M2-N5 M3-N2 M3-N2 M3-N2 M3-N6 M3-N6 M3-N6

2.5584 2.5584 2.3813 2.3813 2.3472 2.3472 2.6659 2.6659 2.6659 2.6659 2.6689 2.6689 2.6689 2.6689 2.7062 2.7062 2.7062 2.2982 2.2982 2.2982

2.6109 2.6109 2.5145 2.5145 2.4408 2.4408 2.7262 2.7262 2.7262 2.7262 2.6000 2.6000 2.6000 2.6000 2.7112 2.7112 2.7112 2.2109 2.2109 2.2109

Figure 3. (a) Crystal structure, viewed through the [001] plane, and (b) coordination of the Ca(Eu/Sr) ions in CSSN.

Figure 4. (a) SEM image, (b) TEM image, and (c) N/Ca/Si/Sr mapping image of CSSN.

has been performed, as shown in Figure 4(c). From the mapping analysis, we can see that Sr ions as the replacements of Ca ions are evenly distributed among the samples. C

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Figure 5. (a) Band structure of the CSN host; (b) TDOS and PDOS of the CSN host lattice.

Figure 5(a) shows the DFT calculations of CSN on the basis of crystal structure data. Besides, the LDA was employed as the theoretical basis of the density function. From the band structure, it is obvious that the compound possesses a direct band gap of 2.61 eV with the conduction band bottom and the valence band top which are both located at the Brillouin zone G point. Figure 5(b) exhibits the calculated total density of states (TDOS) and partial density of states (PDOS) of CSN. As seen, the density states of the CSN can be divided into four energy regions. The valence band consists of the N-2s2p, Si-3s3p, and Ca-3s3p states and mainly distributes into three energy regions. The Ca-3s state is at the lowest energy part, and the lower energy ranges are N-2s, partial Si-3s3p, and Ca-3p states from −11.5 to −21.5 eV. And the upper section of the valence band is primarily determined by N-2p and partial Si-3s3p states, and Ca-3s3p3d states from the Fermi energy level to −10 eV. Among them, N-2p and Si-3p are mainly contributory, ranging from −5.5 to −4 eV, which are originating from the chemical Si−N bond. In the conduction band, the major components are Ca-3d states, as well as a small number of the Ca-3s3p and N2p states, ranging from 2.74 to 4.97 eV. Because of a major contribution of Ca-3d on the bottom of the conduction bands, the Ca−N bond mainly determines the band gap of the CSN. 3.2. Optical Properties. The diffuse reflection spectra of CSSN were measured for undoped host and Eu2+ doped with different Sr2+ contents from 0 to 0.30, as shown in Figure 6. Because of the host absorption, there is a strong absorption ranging from 200 to 300 nm for both undoped sample and doped samples. For the doped samples, there are strong absorption bands in the range of 300 nm through 550 nm, which are attributed to the 4f → 5d transition of the Eu2+ ion. With the increasing of the Sr2+ content, the absorption becomes stronger; especially when the concentration increased to 0.20, the absorption has a significant enhancement. Furthermore, the body color changes from grayish to orange following the increment of Sr2+. The band gap of the CSN host can be calculated by the under equations: (αhν)2 = A(hν − Eg )

α=

(1 − R )2 2R

Figure 6. DRS of CSSN and CSSN:0.03Eu2+ (0 ≤ x ≤ 0.3).

Here, α is the absorption coefficient, A is a constant, hν refers to the incident photo energy, and n = 2 for a direct band gap.41 As displayed in the right bottom corner inset of Figure 6, the obtained Eg value is about 3.74 eV by extrapolating the value of hν to the photo energy axis linearly. From the results, it is obvious that there is a major difference value between the experimental result and the calculated result. The difference value is usual and inherent for the LDA method because of the discontinuity of the exchange-correlation energy and the inappropriate structural model, as well as the actual crystal being a defect structure instead of a perfect structure model.42,43 The PLE and PL spectra of the CSSN:0.03Eu2+ (0 ≤ x ≤ 0.30) are shown in Figure 7. The PLE spectrum covers the spectrum in the range of 250−550 nm, indicating that there is a large crystal field splitting of Eu2+ 5d levels. The broad excitation spectrum shows that it has the strong absorption of n-UV and blue light, which means that they can be combined well with the n-UV and blue LED chips. From the excitation spectrum, we can see that there is one distinct band centered at 410 nm as well as three shoulders at 299, 382, and 460 nm, respectively. From the experimental results, it is indicated that there is no significant change of the excitation bands of CCSN:0.03Eu2+ by the fractional substitution of Ca by Sr.

(1)

(2) D

DOI: 10.1021/acs.inorgchem.7b01021 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. PLE and PL spectra of CSSN:0.03 Eu2+ (0 ≤ x ≤ 0.30).

Figure 8. (a) Crystal structure of the CSSN host lattice; (b) detailed Eu2+ crystal site environments in CSSN.

compressive stress around the activators, which can change the crystal field, and the symmetry also decreased and the detailed Eu2+ crystal sites environment in CSSN was shown in Figure 8. And the improvements of parity selection rules and likelihood of electron transition can enhance the emission intensity.45,46 As for the peak position, there is a continuous redshift from 616 to 652 nm following the increase of Sr content (0 ≤ x ≤ 0.300). The phenomenon is in contrast to the proleptic blueshift because of the larger size of Sr2+.47 The redshift can be explained by the fact that the CSSN structure still exists while a portion of the Sr2+ ions replace Ca2+ ions.48 In order to adapt these larger ions, there is no increase or even slight decrease for the distances among Sr2+ (or Ca2+) (which is confirmed as shown in Table 2), and as a consequence, the increasing crystal field splitting leads to an emission peak redshift. However, this can only explain the slight redshift when a few Sr2+ ions replace Ca2+ ions. Because there are three different crystallographic sites occupied by the alkaline earth atoms, three emission bands are expected. As a matter of fact, the PL spectrum can be fitted into three Gaussian components I1, I2, and I3, as shown in Figure 9. From the fitting results and the refined structural data, three emission bands can approximately be assigned to three Ca2+ sites. From the earlier report, we know that the higher

From Figure 7, it is obvious that the PL of the samples shows one broad band varying from 616 to 653 nm with a full width at half-maximum of ∼115 nm by changing the Sr content, which is because of the allowed transitions of Eu2+ ions between the excited 5d and the ground 4f levels. The intensity of the emission peak has a great enhancement following the increase of the Sr content. The radii of Eu3+ (r = 0.95 Å) and Eu2+ (r = 1.14 Å) are similar to those of Ca2+ (r = 0.99 Å) and Sr2+ (r = 1.13 Å), respectively.33 Therefore, in the Sr-rich environment, more Eu3+ are transformed to Eu2+ compared to the Ca-rich environment, and the result is that the more Eu2+ ions lead to the increase of the intensity of emission.33 On the other hand, it is known that the introduction of object ions will result in lattice distortion, even defect, in the samples. In order to balance the defect, the host will self-limit the entrance of the object ions and the object ions would move to the surface lattice.44 So, as for the object ions, only a small number of Eu2+ could enter into the crystal lattice. Accordingly, when the larger Sr2+ ions replace the Ca2+ ions, the coordination polyhedrons around the Ca2+ were distorted, which can decrease the lattice mismatch led by the introduction of Eu2+. The result is that there will be more Eu2+ contained in the crystal lattice and the intensity of the emission was enhanced. Meanwhile, when other ions were codoped in the crystal lattice, there will be a E

DOI: 10.1021/acs.inorgchem.7b01021 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 9. Deconvolution of the PL band of CSSN:0.03Eu2+ into three Gaussian components (0 ≤ x ≤ 0.30).

crystal field corresponds to the smaller bond length, while the lower crystal field is related to the larger bond length.49 Thus, it should be noted that the lowest-energy emission peak (I1) ought to be certainly attributed to the partially occupied nature of the Ca2 site (coordination number 8, average M−N bond length 2.6678 Å). On the other hand, it is well-known that the a asymmetric and strong crystal field decreases the energy of the lowest excited state more, resulting in photon absorption and emission at a longer wavelength.41 So the Ca1 (coordination number 6, average M−N bond length 2.4290 Å), which occupies the sites in a severely distorted octahedron of lower asymmetric, should be ascribed to the lowest-energy emission peak (I3). And another emission peak (I2) is naturally attributed to the Ca3, which is a distorted octahedral site with the average M−N bond length 2.5020 Å. From Figure 9, it is obvious that all the peak positions have a red-shift following the increase of Sr concentration. As mentioned above, all three Ca2+ ions locate in the distorted octahedral sites, or elongated cubic site. With the increase of Sr2+, the larger Sr2+ ions replace the smaller Ca2+ ions, and the distortions become larger and larger, which leads to the emission spectral shift to the long wave direction. This could be the main reason for the emission spectrum redshift. From the fitting bands, it is apparent that the intensity ratio of the I1, I2, and I3 is varying with the various Sr2+ contents. Without the addition of Sr2+, the Eu2+ ions were distributed randomly on the three Ca sites, but due to the severely distorted octahedron around the Ca1 site, there are a small number of Eu2+ ions that occupy the Ca1 sites, which is the reason for the low intensity of I3. Although the larger Sr2+ ions can enlarge the sites, the Eu2+ ions are still inclined to take

up the relatively symmetric Ca2 and Ca3 sites rather than the Ca1 sites. To investigate the effect of the decay time for CSSN:Eu2+, the lifetimes dependent on Sr2+ content are measured at room temperature, and the decay time spectra are shown in Figure 10. These decay curves can be fitted into a three exponential equation:50 ⎛ −t ⎞ ⎛ −t ⎞ ⎛ −t ⎞ I(t ) = A1 exp⎜ ⎟ + A 2 exp⎜ ⎟ + A3 exp⎜ ⎟ ⎝ τ2 ⎠ ⎝ τ1 ⎠ ⎝ τ3 ⎠

(3)

Figure 10. Decay lifetime of the CSSN:Eu2+ (0 ≤ x ≤ 0.30) under excitation at 410 nm. F

DOI: 10.1021/acs.inorgchem.7b01021 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 11. (a) Temperature dependence of the PL spectra of the CCSN:0.03Eu2+ (x = 0) phosphor; (b) temperature dependent luminescence intensity of CCSN:0.03Eu2+ (0 ≤ x ≤ 0.3) (the inset exhibits the activation energies of the phosphors); (c) thermal quenching data of the CCSN:0.03 Eu2+ phosphor and the commercial phosphors YAG:Ce3+ and Sr2Si5N8:Eu2+.

where I is the luminescence intensity; t is the time; A1, A2, and A3 are fitting constants; and τ1, τ2, and τ3 are the decay times for exponential components. Through the above data, it is easy to calculate the average decay time (τ) using the formula below:51 τ=

A1τ12 + A 2 τ22 + A3τ32 A1τ1 + A 2 τ2 + A3τ3

the samples with x = 0 as a representative are shown in Figure 11(a). From Figure 11(a), it is obvious that the emission intensity gradually decreases with increasing temperature. Besides, the emission spectrum peaking at 616 nm has shifted to 605 nm when the temperature increases from room temperature to 250 °C. As shown in Figure 11(b), a series of the temperature dependent emission spectra were also investigated. The PL spectra tested under various temperatures show the same patterns, and decrease little by little following the increasing temperature. This behavior can be attributed to the thermally active phonon-assisted tunneling from the excited states of the lower-energy emission band to those of the higherenergy emission band in the configuration coordinate diagram.49,53 As a sample, the temperature dependent emission spectra of the CCSN:0.03Eu2+ (x = 0.2) and the commercial Sr2Si5N8:Eu2+ phosphor, YAG:Ce3+ phosphor are shown in Figure 11(c). The samples have a higher T50 (thermal quench temperature) than the commercial YAG:Ce3+. In order to further investigate the thermal process, the thermal quenching data was fitted by the Arrhenius equation. On the basis of the classic theory of thermal quenching, the PL intensity related to temperature can be expressed by the following expression:53,54

(4)

From the above equation, the lifetimes (τ) are calculated to be 0.883, 0.919, 0.934, 1.064, 1.140, 1.214, and 1.236 μs, for different Sr2+ contents (0 ≤ x ≤ 0.30), respectively. Obviously, the decay times gradually increase with the increasing Sr2+ content, and this is because of the changing local coordination environments of Eu2+. From a previous study,52 we know that the high energy peak has a long decay time while low energies have short decay time. The increased Sr2+ ions cause all the three emission peaks to shift to long wavelength, so that the decay time increases following the increasing Sr2+ which is consistent with the emission. A good thermal stability is a very important factor for phosphors which can be considered in the practical application. The PL spectra of the temperature of the emission spectra of CCSN:0.03Eu2+ (0 ≤ x ≤ 0.30) under 410 nm excitation for G

DOI: 10.1021/acs.inorgchem.7b01021 Inorg. Chem. XXXX, XXX, XXX−XXX

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enhance the Stokes shift,47 and the thermal quenching mechanism is a thermally activated crossover from the high vibrational levels of the excited state to the ground state. So that the thermal quenching temperature will decrease with increasing temperature. As shown in Figure 11(b), the experimental results are fitted well with the theory when x ≤ 0.15. As mentioned above, the Sr (Eu) ions prefer to occupy larger Ca2 an Ca3 sites not Ca1 sites. When more Sr ions were added, the more Eu ions entered into the Ca2 and Ca3 sites. So the concentration of Eu2+ in those sites is higher, and the frequency of the energy transition between the activator ions at different sites increased greatly. And the transfer process of electrons is not confined to ② and ④, but also ⑤. Thus, the emission intensity decreased slowly; this could be the reason why the thermal quenching temperature with x = 0.2 is a little higher, but further increasing the Sr content, due to the large lattice distortion and poor crystallinity, the thermal stability decreases quickly with the increasing temperature. Figure 13 shows the chromaticity diagram of the (CIE) for CSSN (0 ≤ x ≤ 0.3) phosphors, which exhibits the varied

I0

( −ΔkTE )

1 + A exp

(5)

where I0 is the initial intensity, I(T) is the intensity at a given temperature T, A is a constant, ΔE is the activation energy for thermal quenching, and k is the Boltzmann’s constant. The relationship of intensity and temperature is shown in the inset of Figure 11(b). A series of lines were obtained. Based on eq 5, the activation energy ΔE was calculated which is listed as shown in the inset of Figure 11(b), and all the ΔE are above 0.23 eV. The high activation energy means that CSSN has good heat stability, which is good for practical application. To understand the temperature quenching property of a phosphor in depth, the configuration coordinate diagram was employed to account for the mechanism of thermal quenching, as shown in Figure 12. Curve g is the ground state of Eu2+,

Figure 12. Configurational coordinate diagram of the excited states and the ground state of Eu2+. Figure 13. CIE chromaticity coordinates of CCSN:0.03Eu 2+ phosphors.

while curves e1, e2, and e3 are the excited states of Eu1, Eu2, and Eu3, respectively. The electrons can be excited to the excited states from g to e1, e2, and e3 under excitation of UV or blue light. At room temperature, a large amount of electrons return to the ground along ways ② and ③, and along with the radiation of energy hν. But with the increasing of the temperature, every chemical bond vibrates with its fundamental or multiple modes of frequency, and more electrons transit to the ground along ways ② and ④ because of the high vibrational energy which is greater than the activation energy ΔE. The transition does not emit a photon, so that the luminescence efficiency is apparently decreased. On the other hand, because the ΔE1 is much smaller than ΔE2 and ΔE3, when the temperature increases, the electrons at the excited state of Eu1 can more easily return to the ground state along ways ④ than along ways ③. In other words, more electrons transit to the ground state by nonradiative transition instead of radiative transition, and the intensity of the emission peak at long wavelength decreases quickly following the rising temperature. As a result, the emission spectra of the products shifted toward blue with rising temperature. In this work, the doping of Sr2+ greatly changes the nephelauxetic effect and the crystal field, which are reflected in the variation of activation energy. In the theory, the substitution of smaller ions by larger ions can

chromaticity points from orange (0.5964, 0.4075) to red (0.6595, 0.3397) by changing the Sr content under 410 nm excitation. It is indicated that the phosphors can be applicable in pc-WLED. Furthermore, as representatives, the insets of Figure 13 are the typical digital photographs of the CSSN (x = 0, 0.25) phosphors under excitation of a 365 nm UV lamp. In order to test the quality of light, a series of the correlated color temperature (CCT) values of the samples have been estimated by using the McCamy empirical formula:55 CCT = −449n3 + 3525n2 − 6823n + 5520.33

where n =

x − xe y − ye

(6)

is the inverse slope of the line ye = 0.186 and xe

= 0.332 are the epicenter and the (x, y) is the CIE color coordinates. If the CCT values are larger than 5000 K, this signifies cold white light, whereas values less than this signify warm white light. The obtained CCT values from 2642 to 2817 K are suitable for a warm white light region. Meanwhile, the chromaticity coordinates of commercial phosphor (YAG:Ce3+) are (0.451, 0.526), attributed to yellow, with the CCT of 3500 K. The lower CCT and higher color saturation of CCSN H

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Inorganic Chemistry

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indicate that it can be considered a possible phosphor for warm w-LED.

4. CONCLUSION A series of Eu2+ doped nitride solid solution CCSN phosphors were successfully prepared by the solid-state reaction. The crystal structure, band structure, and luminescence properties of the samples have been investigated in detail. The PLE spectra show a very broad band in the wavelength range from 250 to 550 nm which can match well with n-UV and blue LED chips. The broad band in the range 550−750 nm was observed in the PL spectra which can be fitted into three Gaussian components. This is because the activators occupy the three different Ca sites. Following the increase of Sr2+ content, the emission peak was varied in the range 616−654 nm because of the varying environments around the activators. The temperature dependent PL spectra show that the samples have a good thermal stability which can be comparable to that of the commercial phosphor. The configurational coordinate diagram was employed to analyze the thermal stability. The obtained CCT values from 2642 to 2817 K are suitable for a warm white light region. All the results show that the CCSN:Eu2+ is a possible candidate for use in warm w-LED.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01021. Atomic coordinates of CSN and CSSN (PDF) Accession Codes

CCDC 1550302−1550303 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86 931 8913554. Tel: +86 931 8912772. E-mail: wyh@ lzu.edu.cn. ORCID

Yuhua Wang: 0000-0002-5982-8799 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Gansu Province Development and Reform Commission and the Fundamental Research Funds for the Central Universities (No. lzujbky-2016-234) and National Natural Science Funds of China (Grant no. 51372105).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b01021 Inorg. Chem. XXXX, XXX, XXX−XXX