Site Occupancy Preference, Enhancement Mechanism, and Thermal

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The site occupancy preference, the enhancement mechanism and the thermal resistance of Mn4+ red luminescence in Sr4Al14O25 : Mn4+ for warm WLEDs Mingying Peng, Xuewen Yin, Peter A. Tanner, Mikhail G Brik, and Pengfei Li Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00226 • Publication Date (Web): 19 Mar 2015 Downloaded from http://pubs.acs.org on March 24, 2015

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Chemistry of Materials

The site occupancy preference, the enhancement mechanism and the thermal resistance of Mn4+ red luminescence in Sr4Al14O25 : Mn4+ for warm WLEDs Mingying Peng,*a Xuewen Yin,a Peter A. Tanner,b M. G. Brik,c,d,e and Pengfei Lia a

The China-Germany Research Center for Photonic Materials and Device, The State Key Laboratory of Luminescent Materials and Devices, School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, P.R. China

b

Department of Science and Environmental Studies, The Hong Kong Institute of Education, 10 Lo Ping Road, Tai Po. New Territories. Hong Kong S.A.R. P.R. China c

College of Sciences, Chongqing University of Posts and Telecommunications, Chongqing 400065, P.R. China d

e

Institute of Physics, University of Tartu, Ravila 14C, Tartu 50411, Estonia

Institute of Physics, Jan Dlugosz University, Armii Krajowej 13/15, PL-42200 Czestochowa, Poland *Email: [email protected]

Red phosphors play an indispensable role in phosphor-based warm white light emitting diodes (WLEDs). We demonstrated recently that the non-rare-earth phosphor Sr4Al14O25:Mn4+ exhibits red luminescence even more intensely than the commercial Mn4+ phosphor 3.5MgO.0.5MgF2.GeO2:Mn4+ upon blue excitation. Herein, on the basis of crystal field calculations employing the exchange charge model, we identify the energy levels of three types of Mn4+ ions situated at Al3+ sites in the Sr4Al14O25 crystal lattice and find that the doped manganese ions occupy preferentially the Al4 and Al5 more highly covalent sites rather than the Al6 site. We report that the Mn4+ luminescence can be enhanced upon the inclusion of Mg2+ in the synthesis reaction. The mechanisms for this effect comprise the lower nonradiative decay rate from the 2

Eg state due to the reduction in energy migration along Mn4+ ions to killer sites and 1

ACS Paragon Plus Environment


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also the morphology evolution from orderly layered smooth nanosheets to irregular nanoparticles disorderly compacted in porous bundles. Interestingly, various other phases are formed upon the addition of Mg2+. The resistance of Mn4+ photoluminescence in the phosphor to thermal impact has also been studied and no obvious thermal degradation after a cycle experiment by heating and cooling the sample between 25 oC and 300 oC was found. As proof of concept, a warm perception WLED has been made when the phosphor was applied to the package of a blue LED chip and YAG:Ce.

1 Introduction The world’s continuous growth in population and its industrialization unavoidably lead to an ever-increasing demand for energy. If the solutions to this crisis only rely on the combustion of fossil fuels, severe carbon emissions will ensue, and lead to climate change. More and more countries have realized this, in addition to the emerging shortage of conventional fuels, and have consequently made stimulus policies and new financial investments on finding either new alternative clean and persistent energy sources or new technologies for energy saving. The white light emitting diode (WLED) has been recognized as one of the energy efficient technologies with high performance and long term stability, and a popular approach has combined an InGaN blue LED and the yellow conversion phosphor YAG:Ce3+.1-3 The innate deficiency of this design is the weak device emission in the red spectral region yielding poorer color rendering ability and higher color temperature.1,5,6 It is 2


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therefore logical to supplement a red phosphor into this system to improve the quality of illumination. The red phosphor should be excitable by blue light (for instance with a wavelength between 450 nm to 480 nm) and not by green light.1,6-9 In this way the composite system can efficiently avoid the reabsorption of the as-generated white light. This idea has stimulated research on red phosphors activated by rare earths, with outstanding candidates being divalent europium-doped (oxy)nitrides.10-14 However, besides economic considerations, the excitation of Eu2+ can extend continuously to the green and even red spectral ranges due to the location of its 4f7 → 4f65d transitions,1, 8-14

so that Eu2+ doped (oxy)nitrides are not the most desirable phosphors for this

purpose. We have found that Sr4Al14O25:Mn4+ can be one of the more promising candidates. It is easily synthesized by a standard solid state reaction at 1200 oC in air

8,15

and it

can be excited by wavelengths from 250 to 500 nm, thereby emitting red light between 600 nm to 760 nm. The Mn4+ ions prefer to substitute at octahedral Al3+ sites in the AlO6 layers in the crystal, and these are regularly spaced from each other by the units of [Al10O23] all made up by corner-shared AlO4 tetrahedra in one dimension parallel to axis a.8 This distribution can help to isolate Mn4+ ions from nonradiative perturbations, thereby enabling high luminescence efficiency. We also noticed that the inclusion of Mg2+ into the phosphor can intensify Mn4+ photoluminescence (PL) 8 but the reason as yet remains unclear. We also did not know whether the PL could be continuously increased by further addition of Mg2+. Besides, for actual application in a WLED, it is necessary to study the resistance of Mn4+ PL to thermal impact. This is 3



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because when the driving current increases, the chip temperature easily becomes higher than 150 oC. For instance we have observed the temperature of 156 oC for a LuVO4:Bi3+ coated LED lamp at 80 mA.16 The aims of this work were to first identify the site occupancy preference of Mn4+ ions once doped into the compound of Sr4Al14O25 by calculation, and then to elucidate the mechanism on how Mg2+ addition can enhance Mn4+ PL. Then an evaluation of the thermal properties of the phosphor could be carried out. These aims have been achieved by help of a theoretical investigation using the exchange charge model (ECM) in addition to multiple instrumental methods: X-ray diffraction (XRD), static and dynamic PL, high temperature PL, diffuse reflection, and electron paramagnetic resonance (EPR). Finally, a successful prototype of a warm WLED with the application of the Mn4+ phosphor is demonstrated. 2. Experimental 2.1 Synthesis The phosphors were synthesized by a standard solid state reaction.8 The optimal preparation conditions of 7 h at 1200 oC in air with 5 mol% excess boric acid as flux, which were deduced in a previous study, were followed using the SrCO3, Al(OH)3, H3BO3, MnCO3 and MgO raw materials.8 The concentration of Mn was kept throughout at the critical concentration value of 0.1 mol% Mn.8,15 Individual batches of 15 g were weighed according to Sr4Al14(0.999-x)O25:0.1%Mn, xMgO, 5% B (x = 0, 0.1%, 0.3%, 0.5%, 0.6%, 0.7%) and mixed homogeneously in an agate mortar. Samples were heated up to 700 oC in alumina crucibles with the rate of 2 oC min-1, held for 2 h to thermally decompose the starting reagents, and then cooled down to 4


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room temperature naturally. After intermediate blending to improve the homogeneity of mixing, the reactants were heated at 5 oC min-1 and sintered at 1200 oC for 7 h in air, and finally cooled down to room temperature in the furnace. The samples were crushed and ground for subsequent measurements. 2.2. Instrumental methods XRD patterns of the samples were recorded by a Rigaku D/max-IIIA X-ray diffractometer (40 kV, 1.2° min-1, 40 mA, Cu-Kα1, λ = 1.5405 Å). Static excitation and emission spectra, dynamic emission decay spectra and quantum efficiency were measured between 10 K and 300 K by an Edinburgh Instruments fluorospectrometer FLS 920 equipped with a red-sensitive photomultiplier (Hamamatsu R928P) in a Peltier-cooled housing in the single photon counting mode and with the aid of an integration sphere. A microsecond pulsed xenon flash lamp µF900 with an average power of 60 W was available to record the emission decay curves for lifetimes in the range of 1 µs to 10 s. A 450 W ozone-free xenon lamp was used as the excitation source for steady-state measurements. The calibrations for quantum efficiency were made by the Edinburgh Instrument and the measurements were repeated three times at room temperature for each excitation scheme and then averaged overall. High temperature PL spectra were measured between 25 oC and 300 oC by a Jobin Yvon Triax 320 fluorospectrometer equipped with double excitation monochromators and a homemade high temperature sample heater. The morphology of the samples was characterized using a Hitachi S-3700N scanning electron microscope (SEM) with energy dispersive X-ray spectrometry (EDS). Electron paramagnetic resonance (EPR) 5



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spectra were obtained with an X-band Bruker A300 spectrometer at a frequency 9.44 GHz under the microwave power of 21.42 W at 85 K. A series of LED lamps were fabricated by depositing the red Mn4+ phosphor and/or Y3Al5O12:Ce3+ on a blue LED chip with emission at 450 nm. The color rendering index (CRI) and color temperature (CT) as well as Commission International de l’Eclairage (CIE) chromaticity were evaluated by a photo-electricity test system (V2.00 LED spec system). Measurements were performed at room temperature unless otherwise specified. 3. Results and discussion 3.1 Crystal field calculations of Mn4+ energy levels and analysis of the Mn4+ site occupancy The standard crystal field Hamiltonian for an ion with an unfilled 3d orbital was used to calculate the energy levels of the Mn4+ ions in Sr4Al14O25:17 p

H=

∑ ∑ B kp O kp ,

(1)

p = 2, 4 k = − p

where O kp are the linear combinations of the irreducible tensor operators acting on the angular parts of the impurity ion’s wave functions (the exact definition of the operators used in the exchange charge model (ECM) can be found in ref. 17), and B kp are the crystal field parameters (CFPs) which can be calculated directly from the experimental crystal structure data. The Hamiltonian (1) is defined in the space spanned by all wave functions of the free ion’s LS terms. The ECM allows the expression of the CFPs as a sum of two terms:17

B pk = B pk , q + B pk , S ,

(2)

with

6


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B pk ,q

=

− K kp e 2

r

p

∑ qi i

V pk (θ i , ϕ i ) Ri

p +1

,

(3)

and

B kp , S

=

K kp e 2

k 2(2 p + 1) 2 2 2 V p (θ i , ϕ i ) . ∑ G s S ( s ) i + Gσ S (σ ) i + γ p Gπ S (π ) i Ri 5 i

(

)

(4)

The first term B pk , q is due to the Coulomb (point charge) interaction between the impurity ion and the lattice ions enumerated by index i with charges qi and spherical coordinates, Ri , θ i , ϕ i (with the reference system centered at the impurity ion itself). The averaged values r p , where r is the radial coordinate of the d electrons of the optical center (also known as the moments of the 3d electron density), can be obtained either from the literature or calculated numerically. The values of the numerical factors K kp , γ p , the expressions for the polynomials V pk and the definitions of the operators O kp can all be found in Ref. 17. The second term of Eq. (2) B pk , S is proportional to the overlap between the wave functions of the central ion and the ligands and thus describes all covalent effects. The S ( s), S (σ ), S (π ) terms correspond to the overlap integrals between the d-functions of the central ion and p- and s-functions of the ligands: S ( s) = d 0 s0 , S (σ ) = d 0 p 0 , S (π ) = d1 p1 . The G s , Gσ , Gπ entries are dimensionless adjustable parameters of the model, whose

values are determined from the positions of the first three absorption bands in the experimental spectrum. These parameters can be approximated by a single value, i.e., G s = Gσ = Gπ = G , which then can be estimated from one absorption band only. This

is usually a sufficient approximation.17 The summation in Eq. (4) is extended only to 7



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the nearest neighbors of an impurity ion (i.e., six ligands in the case of an octahedral impurity center), since the overlap with the ions from the further (second, third, etc.) coordination spheres can be safely neglected. The ECM employs a small number of fitting parameters, which is one of its strongest features. Besides, it enables the calculation of the CFPs and energy levels of impurity ions in crystals without invoking any a priori assumptions about the impurity center symmetry. The reliability and vitality of the ECM is confirmed by its successful applications to the calculations of energy levels of rare earth and transition metal ions.17, 18 However, the absolute error in calculated energy levels may be up to a thousand cm-1. All calculations were performed using the structural data from Ref. 19. According to this reference, Sr4Al14O25 crystallizes in the Pmma space group (No. 51), with the lattice constants a, b, c (in Å): 24.745, 8.474 and 4.881, respectively. There are two formula units in one unit cell. After doping, the Mn4+ ions occupy the Al3+ positions. There are six inequivalent Al sites in one unit cell. However, three of these are 4-fold coordinated by oxygen ions, and, as such, are not occupied by Mn4+ ions, which exhibit the preference to enter the octahedral sites. The three remaining aluminum sites (the so called Al4, Al5, and Al6 sites 19) are in principle suitable for occupation by Mn4+. The crystal field calculations can aid a more precise identification of the impurity sites. The CFPs were calculated using a large cluster consisting of 22542 ions around an impurity site, to ensure proper convergence of the crystal lattice sums in the equations. 8


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All non-zero CFPs are given below in Table 1. The Mn4+-O2 overlap integrals required for the calculations of the CFPs were taken from Ref. 18. With these CFPs and the Racah parameters B = 790 cm-1, C = 3192 cm-1 (the value of the ECM parameter G was determined from the first absorption band corresponding to the 4A2g → 4T2g transition and turned out to be equal to 7.0) the energy levels of Mn4+ were obtained and these are listed in Table 2. The deduced energy levels from the 10 K 2Eg → 4A2g emission spectra Fig. 6(a) in ref. 8 are: Al4 site (C2 symmetry) 15361, 15384 cm-1; Al5 site (C2h symmetry) 15457 cm-1; Al6 site not detected. We find that our original statement concerning the temperature shifts of the bands due to the Al4 and Al5 sites is incorrect because the room temperature spectrum exhibits a strong band at 15337 cm-1 with a weak shoulder at low energy at 15314 cm-1. This shoulder was previously disregarded. The distinction between the sites Al4 and Al5 is made from the emission spectra on the grounds that: (i) the splitting of the 2Eg → 4A2g transition at site Al4 is constant (23 cm-1) on going from room temperature to 10 K, but the relative intensity of the two bands changes in accordance with temperature; (ii) the temperature shift of a weak band at 15457 cm-1 assigned to the C2h site is similar (46 cm-1 from room temperature to 10 K) but its intensity change relative to the lower energy bands does not follow the Boltzmann Law. Hence a theoretical confirmation of site occupation is not available from the data for the 2Eg → 4A2g transition due to inherent approximations in the calculation. The

calculated energy

levels are superimposed

onto the

experimental

excitation/emission spectra in Fig. 1. The calculated energy for the lowest 4A2g → 4T1g 9



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transition is appreciably different for Mn4+ at site Al6 (444 nm) compared with site Al4 (332 nm) and site Al5 (326 nm). This spin-allowed transition is the strongest and hence the experimental data are more consistent with the calculated data for sites Al4 and Al5. Therefore, we assume that the manganese ions preferably occupy the more covalent8 Al4 and Al5 sites after doping, rather than the Al6 site. The oxygen octahedron at the Al5 site is compressed along the C2 axis (by 4.45% in terms of the 1.929 Å bond), whereas the oxygen octahedron of the Al6 site is elongated (by 11.9%, taking the 1.918 Å bond). That is why the B21 parameter is so large for the Al6 site. Moreover, since the character of deformation is opposite (compression at Al5 and elongation at Al6), the signs of B21 and B20 are also opposite. As for the Al4 site neglecting the small difference between the 1.92 Å and 1.93 Å bonds, it can be described then as a slightly compressed octahedron - like the Al5 site, and the signs of the B20 and B21 CFP for the Al4 and Al5 sites are the same, but opposite to those for the Al6 site. The monoexponential lifetime fit for emission when employing 325 nm excitation becomes poorer for (eleven) longer wavelength measurements from 620 nm up to 700 nm, with the adjusted coefficient of determination changing from 0.995 to 0.987, and the fitted lifetime decreases by 37% from 1.43 ms at 620 nm to 0.90 ms at 700 nm. Presumably this is not due to refractive index change but is an indication of biexponential decay and from such fits the mean fast lifetime is 0.37±0.08 ms and the mean slow lifetime is 1.54±0.09 ms. These values are consistent with those determined from eleven measurements using 335 nm excitation in the same 10


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wavelength range: 0.41±0.07 ms and 1.54±0.09 ms. The weight of the short lifetime in the biexponential fit increases to longer wavelengths. As mentioned above, there are several zero phonon lines for the emission spectrum and the two emitting states are not in thermal equilibrium so presumably the two lifetimes correspond to emission from these two states. Under this explanation, the effect of Mn4+ ions in other phases upon the emission spectrum is not prominent and is not taken into account.

3.2 The role Mg2+ plays in enhancing Mn4+ PL in Sr4Al14O25:Mn4+ In CaAl12O19:Mn4+ 20-22 the introduction of Mg2+ ions increased the PL intensity of Mn4+ and this was attributed to the formation of Mg2+ - Mn4+ ion pairs replacing Mn4+ - Mn4+ pairs and hence decreasing the nonradiative depopulation of the 2Eg state. This is because energy migration is faster between the Mn4+ - Mn4+ ion pairs and the probability of the energy terminating at a killer site is greater. It was also found that the inclusion of either Mg2+ or Ca2+ into CaAl12O19:Mn4+ could lead to the disappearance of Mn2+,23 which was postulated as a precursor charge compensator: Mn4+ - Mn2+. Furthermore, the new phases Al2O3 and MgAl2O4 were formed upon Mg2+ addition to CaAl12O19:Mn4+.22 Upon the addition of Mg2+ to Sr4Al14O25:Mn4+ at the expense of Al3+, the PL intensity increases first until the content of Mg2+ rises to 0.5%, so that it is intensified by up to 46% as Figs. 3a and 3b show. Then the intensity weakens after the Mg2+ content is higher than 0.5%. Nevertheless, the PL intensity is still stronger than the sample without Mg2+ co-dopant even when the content of Mg2+ reaches 0.7%. 11



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The emission lifetime changes with the Mg2+ content as shown in Fig. 3b, which employed 325 nm excitation whilst monitoring the emission at 652 nm, at which wavelength the monoexponential fits have adjusted coefficients of determination of 0.998 for x = 0 up to 0.7. The lifetime of Sr4Al14O25:Mn4+/Mg2+ lies between 1.41 ms and 1.42 ms, and it is longer than that of 1.40 ms for the analogous Mg2+-undoped sample of Sr4Al14O25:Mn4+. Since the radiative lifetime of the sample with concentration of [Mn4+] → 0 was found to be 1.48 ms from the [Mn4+] = 0 intercept of the plot of lifetime versus [Mn4+],8 the increase in lifetime for the 0.7% sample is due to a decrease in nonradiative rate of ~30%. The elongation of lifetime implies the reduced content of Mn4+ - Mn4+ pairs due to the formation of Mg2+ - Mn4+ along with MgO addition. This is similar to the situation in CaAl12O19:Mn4+/Mg2+

20-22

and it

contributes to the enhancement of Mn4+ emission in the co-doped samples. When Mg2+ is built into Sr4Al14O25:Mn4+, there are some very slight changes in the positions of features in the emission spectra, Fig. 3a. The strongest band (at 652 nm in Sr4Al14O25:Mn4+) moves ~20 cm-1 to low energy when Mg2+ is incorporated and also there is a shift of the broad maximum at 665 nm to low energy at 666 nm (i.e., by ~20 cm-1). Presumably these minor changes are due to the incorporation of Mn4+ in other new phases which are formed. The intensity ratio of emission bands due to Mn4+ at the C2h and C2 sites of Sr4Al14O25 does not change markedly so that the introduction of Mg2+ does not lead to major changes in local site symmetry of Mn4+. The major question is: why can co-doping Mg2+ into the sample enhance the Mn4+ emission significantly? A study of the products from reaction of SrCO3 with Al2O3 in 12


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different molar ratios

24

postulated that the PL of Sr4Al14O25:Mn4+ is enhanced by

coupling with SrAl2O4:Mn4+. We do not find this mechanism to be physically realistic and also the assignments of the relevant transitions differ from herein. Besides the above-mentioned Mn4+ pair reduction, two other possible enhancement scenarios are (i) the scavenging of hydroxyl and aquo systems by Mg2+, and (ii) the effects of MgO addition upon crystallinity and particle size. Our FT-IR spectra show that (i) is not significant. Regarding (ii), SEM images show that Sr4Al14O25:Mn4+ consists of orderly stacked parallel sheets with a thickness less than 100 nm. The bigger surfaces of sheets are smooth and flat without notable pores and cracks (see Fig. 5). When 0.3% MgO is co-doped into the sample, the sheet surfaces become rough and smaller particles are created which sticking on the surfaces. The sheets, when the content of MgO increases to 0.5%, are cracked into small particles, which range from tens of nanometers to 300 nm, along with some pores (diameter less than 100 nm) on the surfaces, and they are also no parallel to each other. When the amount of MgO increases further to 0.7%, thin sheets gradually grow into thick solid structures on scale of micrometers (see Fig. 5). The increase in decay lifetime with grain size has also been found for other systems (e.g., in the study of LiNdP4O12

25

and references

therein) and various explanations have been put forward. It is also documented that an increase in particle size and crystallite size (up to a saturation limit of ~500 nm) provides an increase in luminescence intensity and quantum efficiency.26 For these reasons we consider that the morphology for the 0.5% MgO sample is beneficial to the increase of extraction efficiency of emitted light. 13



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The EPR spectra of the samples containing different MgO content in the present study were measured and illustrated as Fig. 4a. The center of the signals corresponds to the g value of 2.0201, and the sextet resonance spectrum is associated with magnetic dipolar transitions of Mn2+ in a symmetric octahedral site. The results depicted in Fig. 4a show that although octahedral Mn2+ is detected in the sample without Mg2+, its concentration is much lower than in the Mg doped samples, and it increases

with

MgO%

increase.

At

the

same

time,

the

absorption

of

Sr4Al14O25:Mn4+/Mg2+ decreases slightly compared to that of Sr4Al14O25:Mn4+ (see Fig. 4b). The X ray diffractograms of each sample (Fig. 3c) show that the phases of SrAl2O4 and SrAl12O19 instantly appear when MgO is introduced, although Sr4Al14O25 still dominates the intensity. Other phases which are formed (not shown in the figure) are Cmma-SrAl4O7 (2θ at 26.77o, 29.90o) and C12/C2-SrAl4O7 (2θ at 30.64o, 32.53o). The new phases soon become dominant when the amount of MgO continues to increase, for example as illustrated for x = 0.7% in Fig. 3. Since the monoclinic phase of SrAl2O4 comprises two types of seven-coordinated strontium ions and four types of tetrahedral aluminum sites, the octahedral red emission center of Mn4+ cannot survive in this compound. The content of Mn2+ increases in Fig. 4a with increasing Mg2+ because it substitutes preferentially at the expense of octahedral Al3+ rather than Sr2+ or tetrahedral Al3+ in the phases such as SrAl12O19 or Cmma-SrAl4O7, in view of size match. For instance, the ionic radii of octahedral Mn2+ and Al3+ ions are 0.67 Å and 0.535Å, respectively, while those of tetrahedral Mn2+ and Al3+ ions are 0.66 Å and 0.39 Å, respectively, and that of the 12-oxygen-coordinated Sr2+ in SrAl12O19 is 1.44 14


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Å. The ions Mn2+ and Mn4+ can both be stabilized in SrAl12O19 and Cmma-SrAl4O7, in which Al3+ occupies both octahedral and tetrahedral sites, but the intensity of emission of Mn4+ (peaking at 658 nm) is much weaker than that of Mn4+ in Sr4Al14O25:Mn4+. It is noted that in cases where Mn4+ PL enhancement occurs, there are impurity phases coexisting with the dominant phase: such as the phases of Al2O3 and MgAl2O4 in CaAl12O19:Mn4+/Mg2+.22 We also observed similar phenomena in Mn4+-doped SrMgAlxOy, where SrMgAl10O17:Mn4+ and Al2O3:Mn4+ are co-precipitated as the aluminum content x increases. The emissions of SrMgAl10O17:Mn4+ and Al2O3:Mn4+ can be intensified by 2.6 and 222 times, respectively.27 The contribution of the co-existing phases to the enhancement of PL requires further investigation in future.

3.3 Quenching of high temperature Mn4+ PL from Sr4Al14O25:0.1%Mn, 0.5%Mg Fig. 6 shows the temperature dependence of Mn4+ PL integrated intensity upon the yoyo processes of heating and cooling over the temperature range from 25 to 300 oC and using different excitation schemes. The intensity is integrated over the spectral range of 590 to 750 nm and it is normalized as compared to the case at 25 oC. The corresponding emission spectra are displayed for two excitation wavelengths in Fig. 7. All of the graphs in Fig. 6 show similar trend irrespective of whether UV or blue excitation was employed. The emission intensity generally decreases all the way along the temperature increase to 300 oC. For blue excitations, the emission is half quenched at ~150 oC. When the temperature decreases from 300 oC to 25 oC, the emission restores its intensity generally, so the quenching and recovery are reversible 15



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when the random errors during PL measurements are considered. Therefore, the cycle experiments imply no severe thermal degradation for Sr4Al14O25:0.1%Mn, 0.5%Mg. The activation energy ∆E was evaluated by the equation IT = I0/(1+Aexp(-∆E/kT)), where IT is the intensity at temperature T, I0 is the initial intensity, A is the frequency factor, and k is the Boltzmann constant.28 In the graph employing 470 nm excitation, a fit with A = 1690 and ∆E = 0.097 eV (782 cm-1) is indicated. The rationale is that the quenching can be described by the configuration coordinate scheme. After excitation from the ground state 4A2 to the 4T1 or 4T2 excited state, corresponding to UV or blue absorption, respectively, nonradiative relaxation to the lowest excited state 2E occurs, with radiative transit to the ground state 4A2 releasing red light.20-24, 27, 29-32 When the temperature increases, thermal excitation from 2E can occur. This excitation can occur via the crossover point of the states of 4A2 and 2E (leading to nonradiative decay to the ground state) and/or to the excited 2T1g state with energy transfer to Mn2+. Both of these alternatives lead to the simultaneous decrease in the emission intensity and lifetime of Mn4+ as Figs. 6 and 7 depict. Figure 7 shows that when the Mn4+ emission is quenched a weak, broad emission appears between 570-650 nm. The emission could arise from octahedral Mn2+ sites, as determined to be present from the EPR spectrum, Fig. 4a.13,30,33 This emission, however, is not observed at room temperature under 300-340 nm excitation which would be directly into Mn2+ 4T2, 4E, 4T1 states so that this hypothesis is unlikely. The emission lifetime is in the microsecond range and further more detailed investigations are required to investigate its origin. 16


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3.4 WLED fabrication using Sr4Al14O25:0.1%Mn,0.5%Mg The internal quantum efficiency was measured to be 38% and 35% for the excitation wavelengths of 380 nm and 470 nm, respectively, for Sr4Al14O25:0.1%Mn, 0.5%Mg (SAM, hereafter). SAM was dispersed into silicon resin and coated with(out) YAG:Ce (YAG, hereafter) onto blue InGaN LED chips (BLED, hereafter) which were packed by a surface mount technology. The devices are depicted in Fig. 8. Figure 8a shows the unlit lamp with BLED, YAG and SAM, whereas Fig. 8b depicts the bright pink light from the lamp with BLED and SAM, exhibiting a luminous efficacy of 6.61 lm W-1. The emission spectrum is illustrated at the bottom of Fig. 8b. When SAM was introduced into the lamp combining BLED and YAG, the red component light was complemented into the device (see Figs. 8c-f) and the color temperature (CT) was lowered accordingly from 6913 K. With the inclusion of 1.5 g SAM, emission with a warm perception was achieved with CT = 3846 K, with the efficiency dropping from 126 lm W-1 to 69 lm W-1. In future, the tradeoff needs to be balanced between luminescence efficacy and the color quality of this device.

4. Conclusions Crystal field calculations indicate that the site occupancy preference of Mn4+ in Sr4Al14O25 is at the Al4 and Al5 higher covalent sites rather than the Al6 site. This type of ESM calculation could be a universal method to predict the site occupancy preference of Mn4+ in other compounds. Addition of Mg2+ can enhance the Mn4+ PL from Sr4Al14O25:Mn4+ up to about 0.5% Mg2+ added. The enhancement is largely due to a reduction in the nonradiative decay rate from the 2Eg state, with the replacement 17



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of Mn4+ - Mn4+ neighbors by Mn4+ - Mg2+ pairs, and in addition the morphology evolution from orderly stacked smooth nanosheets to disorderly distributed smaller but compacted particles. Interestingly, at least four new phases are formed upon the addition of Mg2+ at the expense of Al3+. Upon the yoyo processes of heating and cooling, Sr4Al14O25:Mn4+ does not show thermal degradation - no matter which wavelength of light is used for excitation in the spectral blue or UV spectral range. It indeed shows thermal quenching, and it can be understood in the frame of the configuration coordinate diagram of Mn4+. WLED devices have been made with warm perception with the inclusion of Sr4Al14O25:Mn4+ into the system of BLED and YAG. We aim to improve the performance of color gamut or luminous efficacy of the Sr4Al14O25:Mn4+ - based WLED in future by locally modifying composition around Mn4+ ions and optimizing morphological properties.

Acknowledgements The authors would like to acknowledge financial support from the National Natural Science Foundation of China (Grant No. 51322208 and 51132004), Guangdong Natural Science Foundation for Distinguished Young Scholars (Grant No. S20120011380), the Department of Education of Guangdong Province (Grant No. 2013gjhz0001) and Fundamental Research Funds for the Central Universities (Grant No. 2013ZG004). We would also thank Professor Philippe Boutinaud for useful correspondence concerning the forms of SrAl2O4.

References 1. Shang, M. M.; Li, C. X.; Lin, J. Chem. Soc. Rev. 2014, 43, 1372-1386. 2. Tanner, P. A. Chem. Soc. Rev. 2013, 42, 5090-5101. 3. Zhang, R.; Lin, H.; Yu, Y. L.; Chen, D. Q.; Xu, J.; Wang, Y. S. Laser Photon. Rev. 2014, 8, 158-164. 4. Wang, B.; Lin, H.; Xu, J.; Chen, H.; Wang, Y. ACS Appl. Mater. Interfaces 2014, 6, 22905-22913. 5. Lin, H.; Wang, B.; Xu, J.; Zhang, R.; Chen, H.; Yu, Y.; Wang, Y. ACS Appl. Mater. Interfaces 2014, 6, 21264-21269.

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6. Lin, C. C.; Liu, R. S. J. Phys. Chem. Lett. 2011, 2, 1268-1277. 7. Kang, F. W.; Yang, X. B.; Peng, M. Y.; Wondraczek, L.; Ma, Z. J.; Zhang, Q. Y.; Qiu, J. R. J. Phys. Chem. C 2014, 118, 7515-7522. 8. Peng, M. Y.; Yin, X. W.; Tanner, P. A.; Liang, C. Q.; Li, P. F.; Zhang, Q. Y.; Qiu, J. R., J. Am. Ceram. Soc. 2013, 96, 2870-2876. 9. Peng, M. Y.; Wondraczek, L. Opt. Lett. 2010, 35, 2544-2546. 10. Xie, R. J.; Hirosaki, N.; Suehiro, T.; Xu, F. F.; Mitomo, M. Chem. Mater. 2006, 18, 5578-5583. 11. Pust, P.; Wochnik, A. S.; Baumann, E.; Schmidt, P. J.; Wiechert, D.; Scheu, C.; Schnick, W. Chem. Mater. 2014, 26, 3544-3549. 12. Yeh, C. W.; Chen, W. T.; Liu, R. S.; Hu, S. F.; Sheu, H. S.; Chen, J. M.; Hintzen, H. T. J. Am. Chem. Soc. 2012, 134, 14108-14117. 13. Lv, W. Z.; Jiao, M. M.; Zhao, Q.; Shao, B. Q.; Lu, W.; You, H. P. Inorg. Chem. 2014, 53, 11007-11014. 14. Guo, N.; Zheng, Y. H.; Jia, Y. C.; Qiao, H.; You, H. P. J. Phys. Chem. C, 2012, 116, 1329-1334. 15. Peng, M. Y.; Liang, C. Q.; Zheng, J. Y.; Qiu, J. R. Chinese Patent ZL 102732250B, 2012. 16. Kang, F. W.; Peng, M. Y.; Zhang, Q. Y.; Qiu, J. R. Chem. Eur. J. 2014, 20, 11522-11530. 17. Spectroscopy of solids containing rare-earth ions; Kaplyanskii, A. A.; Macfarlane, B. M., Ed.; Elsevier: North-Holland, Amsterdam, 1987. 18. Optical Properties of 3d-Ions in Crystals: Spectroscopy and Crystal Field Analysis; Avram, N. M.; Brik, M. G., Eds.; Springer and Tsinghua University Press: Beijing, 2013. 19. Wang, D.; Wang, M. Q.; Lu, G. G. J. Mater. Sci. 1999, 34, 4959-4964. 20. Pan, Y. X.; Liu, G. K. Opt. Lett. 2008, 33, 1816-1818. 21. Brik, M. G.; Pan, Y. X.; Liu, G. K. J. Alloy. Compd. 2011, 509, 1452-1456. 22. Pan, Y. X.; Liu, G. K. J. Lumin. 2011, 131, 465-468. 23. Murata, T.; Tanoue, T.; Iwasaki, M.; Morinaga, K.; Hase, T. J. Lumin. 2005, 114, 207-212. 24. Chen, L.; Zhang, Y.; Liu, F.; Zhang, W.; Deng, X.; Xue, S.; Luo, A.; Jiang, Y.; Chen, S. Phys. Stat. Sol. A 2013, 210, 1791-1796. 25. Marciniak, L.; Stefanski, M.; Tomala, R.; Hreniak, D.; Strek, W. Opt. Mater. 2015, 41, 17-20. 26. Wang, W.-N.; Widiyastuti, W.; Ogi, T.; Wuled Lenggoro, I.; Okuyama, K. Chem. Mater. 2007, 19, 1723-1730. 27. Peng, M. Y.; Cao R. P.; Qiu, J. R. Chinese Patent ZL 201110425704.X, 2011. 28. Zhang, M.; Wang, J.; Zhang, Z.; Zhang, Q.; Su, Q. Appl. Phys. B 2008, 93, 829-835. 29. Ye, T. N.; Li, S.; Wu, X. Y.; Xu, M.; Wei, X.; Wang, K. X.; Bao, H. L.; Wang J. Q.; Chen, J. S. J. Mater. Chem. C 2013, 1, 4327-4333. 30. Li, P. F.; Peng, M. Y.; Yin, X. W.; Ma, Z. J.; Dong, G. P.; Zhang, Q. Y.; Qiu, J. R. Opt. Express 2013, 21, 18943-18948. 31. Tanner, P. A.; Pan, Z. F. Inorg. Chem. 2009, 48, 11142-11146. 32. Brik, M. G.; Srivastava, A. M. J. Lumin. 2013, 133, 69-72. 33. Lu, W.; Hao, Z. D.; Zhang, X.; Luo, Y. S.; Wang, X. J.; Zhang, J. H. Inorg. Chem. 2011, 50, 7846-7851.

Table and Figure captions: Table 1 Non-zero CFPs (all in cm-1) for the Mn4+ ions at the Al4, Al5, and Al6 positions in Sr4Al14O25. 19



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Table 2 Calculated energy levels (all in cm-1) for the Mn4+ions at the Al4, Al5, and Al6 positions in Sr4Al14O25. Fig. 1 Excitation (black line) and emission (red line) spectra of Sr4Al14O25:0.1%Mn. The calculated energy levels of Mn4+ are shown by the vertical lines (long lines – spin-quartet states; short lines – spin-doublet states). Mn4+ at Al4, Al5 and Al6 sites is labeled as wine red, olive and blue vertical lines, respectively, to show the difference in location. The comparison implies the experimental data matches the calculated levels of Mn4+ at Al4 and Al5 sites better than Al6 site. Fig. 2 The site occupancy preference of Mn4+ ions to Al4 and Al5 sites over Al6 sites in AlO6 layer of Sr4Al14O25. Fig. 3 (a) PL spectrum of Sr4Al14O25:0.1%Mn, xMgO (x = 0, 0.5%, 0.7%) upon the excitation at 325nm; (b) Dependence of ratio of Ix/I0 and lifetime on MgO content, where I0 is the emission intensity without MgO and Ix is the intensity at x; (c) XRD pattern of Sr4Al14O25:0.1%Mn, xMgO (x = 0, 0.1%,0.3%, 0.5%, 0.6%, 0.7%): Black (█), olive (█), red (█) and blue (█) rectangles denote the reflections from the precipitated phases of SrAl2O4, SrAl12O19, Cmma-SrAl4O7 and C12/C2-SrAl4O7, respectively, with the incorporation of MgO into the samples. Fig. 4 (a) EPR spectra of Sr4Al14O25:0.1%Mn, xMgO (x = 0, 0.3%, 0.5%, 0.7%) at 85 K; (b) Diffuse reflectance spectra of Sr4Al14O25 and Sr4Al14O25:0.1%Mn, xMgO (x = 0, 0.5%). Fig. 5 Microstructure evolution along with the content of Mg%. The scale bar in SEM images is one micrometer. Fig. 6 Temperature dependent relative emission intensity of Sr4Al14O25: 0.1% Mn, 0.5% Mg in heating (red curve) and cooling (blue curve) processes upon different excitation schemes: (a) 325 nm; (b) 380 nm; (c) 450 nm; (d) 470 nm. Solid blue line in (d) is fitting to the heating process by IT = I0/(1+Aexp(-∆E/kT)), where A = 1690 and ∆E = 0.097 eV. Fig. 7 The PL spectra of Sr4Al14O25:0.1%Mn, 0.5%Mg upon excitation at 325 nm at different temperatures as stated. Fig. 8 Phosphor coated LED devices: (a) Blue LED (BLED) coated with YAG:Ce (YAG) and Sr4Al14O25:0.1%Mn,0.5%Mg (SAM); (b) BLED + SAM; (c) BLED+YAG; (d) BLED+YAG+SAM (0.5 g); (e) BLED+YAG+SAM (1.0 g);(f) BLED+YAG+SAM (1.5 g). The emission spectrum of each device is listed at the bottom of each figure accordingly. The color temperature (CT), CRI and luminescence efficacy was measured at the distance of 0.316 m under a current of 20 mA and a bias voltage of 5 V. 20


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Table 1 CFPs B20 B21 B22 B40 B41 B42 B43 B44

Al4 site 369.3 801.9 -3966.9 -1387.8 27985.5 8453.9 89557.0 -14689.6

Al5 site 4067.0 7685.0 -257.1 -957.2 33732.6 10207.1 82960.1 -14929.7

Al6 site -4048.1 -16132.4 -4056.7 -993.8 20594.6 4907.5 76062.1 -17874.5

Table 2 Terms 4

A2g

2

Eg

Calculated energy Al4 site

Al5 site

Al6 site

0

0

0

15337,15421

15236,15360

15143, 15395

2

T1g

15832,16295, 16371

15746,16151,16260

15500,15669, 16110

4

T2g

22317, 22521, 22839

21541, 23356, 23607

15393,15487,20295

2

T2g

23299, 23561, 24461

23356, 24099, 24702

20295,22462, 22929

4

T1g

30147, 30159, 31775

30675, 30921, 35599

22537, 29941,31177

21



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Fig. 1

Fig. 2

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1.5

b

1.44

1.0 Ix/I0

1.42 τ (ms) 1.40

0.5 0.0

0.0

0.2

0.4 0.6 MgO%

1.38 0.8

23



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Fig. 3

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Fig. 4

25



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Fig. 5

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Fig. 6

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Fig. 7

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Fig. 8

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TOC

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Site Occupancy Preference, Enhancement Mechanism, and Thermal

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