Thermal quenching of Mn4+ luminescence in SrAl12O19:Mn4+

Author’s Accepted Manuscript Thermal quenching of Mn4+ luminescence in SrAl12O19:Mn4+ Xiu Wang, Pengfei Li, Mikhail G. Brik, Xiaoman Li, Liyi Li, Mingying Peng www.elsevier.com/locate/jlumin

PII: DOI: Reference:

S0022-2313(18)30705-1 https://doi.org/10.1016/j.jlumin.2018.10.044 LUMIN15992

To appear in: Journal of Luminescence Received date: 20 April 2018 Revised date: 30 August 2018 Accepted date: 8 October 2018 Cite this article as: Xiu Wang, Pengfei Li, Mikhail G. Brik, Xiaoman Li, Liyi Li and Mingying Peng, Thermal quenching of Mn4+ luminescence in SrAl12O19:Mn4+, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.10.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Thermal quenching of Mn4+ luminescence in SrAl12O19:Mn4+ Xiu Wanga1, Pengfei Lia1, Mikhail G. Brikbcde, Xiaoman Lia, Liyi Lia, Mingying Peng*a a

The China-Germany Research Center for Photonic Materials and Devices, The State Key Laboratory

of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, P.R. China b

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

d

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 e

Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland * E-mail: [email protected]

Abstract Thermal quenching is a common phenomenon hindering commercial applications of phosphors, and the relationship between matrix structure and thermal quenching is not yet clear especially for the red phosphor SrAl12O19:Mn4+. Here, we determined the energy levels of three types of Mn4+ sites in SrAl12O19 crystal structure based on the exchange-charge-model crystal field calculations and analyzed the Mn4+ ions preferential occupation combined with chemical bond covalence. The phosphor suffers from serious thermal quenching when the luminescence monitoring from 8 to 500 K was recorded. The combination of theoretical calculations and experimental verification also reveals that the symmetry and spatial distribution of octahedral sites for Mn4+ are crucial to better resistance to thermal quenching. This can be helpful to search for novel Mn4+ doped red phosphor with better resistance to thermal impact in the future. Keywords Red phosphor; Mn4+ site occupancy; Thermal quenching; Crystal field calculations

1

Equal contribution to this work.

1. Introduction In recent years, white light emitting diodes (WLEDs) have drawn great attention owing to their excellent properties in terms of high luminous efficiency, small size and environmental friendliness. They have been promoted in widespread applications, such as automobiles, telecommunications, display, agriculture and medicine, etc. [1-5]. Up to now, various strategies have been developed to produce WLEDs and the mainstream technology is based upon the combination of a LED chip with one or more conversion phosphors [3-6]. The commercial and most popular approach is the conjunction of an InGaN blue LED chip with Y3Al5O12:Ce3+ (YAG:Ce3+) yellow phosphors [1-5]. However, this design leads to poor color rendering index and high color temperature due to the lack of a red component in the white light spectrum. To address this issue, one of the best strategies is to supplement a red phosphor especially with strong blue absorption and efficient red emission that helps generate white light with the warm perception similar to incandescent light [2-7]. Thus, it becomes important to search desirable red phosphors and such idea has stimulated a series of findings, e.g. activation by rare earths, transition metals, divalent bismuth etc. [7-14]. Mn4+ doped phosphors were identified as one of the promising candidates because Mn4+ usually exhibits broad and strong absorption between 250 and 500 nm in hosts and emits light in the spectra of 600 nm to 760 nm once it is introduced into an octahedral site [2, 3, 15-20]. The luminescent properties remain quite similar from host to host because the lowest excited state energy, 2

E(t23), that barely changes upon variation of the crystal field [15, 16]. When

comparing the hosts in which Mn4+ can reside [2, 3, 9, 15-18, 21-26], we can notice only alkaline (earth) aluminate is lower-cost and benign to environment. At the same time, some of them comprising octahedral sites which are preferred to be substituted by Mn4+ in view of the good size match (RAl3+ = 0.535 Å and RMn4+ = 0.53 Å), are excellent host materials because of high luminescent efficiency, chemical stability and durability [3]. For example, Mn4+ doped

α-Al2O3, α-LiAlO2, LiAl5O8, CaAl4O7, MAl12O19 (M = Ca, Sr, Ba), Sr4Al14O25, CaMg2Al16O27, SrMgAl10O17, Sr2MgAl22O36, and Ca14Al10Zn6O35 phosphors have been considered as new potential candidates for warm W-LEDs [3, 9, 17, 18, 21, 25-30]. However, there are still many problems in phosphors that hinder their commercialization. For example, thermal quenching is a common phenomenon affecting applications of phosphors, and the relationship between matrix structure and thermal quenching is not yet clear. In this work, we select SrAl12O19:Mn4+ as a target since it used to be recognized as a red phosphor with higher luminous efficiency, milder preparation conditions and priori in-depth investigation [18, 31, 32]. On the basis of the theoretical calculations of crystal field and chemical bond covalence in the crystal lattice, we come to the conclusion on which kind of lattice sites Mn4+ can preferentially reside at. Moreover, we have inspected its temperature dependent luminescence in a wide range from 8 K to 500 K and discussed the mechanism for the thermal quenching. As result, we found that the resistance of luminescence for Mn4+ to thermal impacts was related to the symmetry and spatial distribution of octahedral sites for Mn4+. 2. Experimental 2.1. Preparation of SrAl12O19: Mn4+ The phosphors were synthesized by typical high temperature solid state reaction with analytical reagents SrCO3, Al(OH)3 and MnCO3 as raw materials. Individual batches of 5 g were weighed according to the chemical composition of SrAl12O19:0.1%Mn4+. The samples were mixed homogeneously in an agate mortar, and moved to alumina crucibles for subsequent processes. They were heated up to 973 K with a rate of 3 K/min and held for 5 h to thermally decompose the starting reagents, and then cooled down to room temperature naturally. After intermediate grinding to improve the mixing homogeneity, they were heated at a rate of 3 K/min to 1673 K and sintered for 5 h and 7 h with an intermediate grinding again to improve the crystallinity, and finally cooled down to room temperature. All the operations were performed in air. 2.2. Characterization

X-ray diffraction (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 were measured by an Edinburgh FLS 920 instrument equipped with a red-sensitive photomultiplier (Hamamatsu R928P) in a Peltier-cooled housing in the single photon counting mode. The instrument was equipped with a closed cycle helium cryostat for the low temperature (8 to 300 K) and a high-temperature cell for the high temperature (300 to 500 K) measurements. A microsecond pulsed xenon flash lamp μF900 with an average power of 60 W and a 450 W ozone-free xenon lamp were used, respectively, for the emission decay curves and the steady-state measurements. The morphology of the samples was characterized using a LEO 1530 VP (Germany) field emission scanning electron microscope (SEM). Absolute photoluminescence quantum yield (QY) was recorded by the absolute PL quantum yield spectrometer (C11347-11, Hamamatsu Photonics K. K., Japan). Measurements were performed at room temperature for general case unless otherwise stated. 3. Results and discussion 3.1. Structural analysis Fig. 1 depicts XRD pattern (upper line) of SrAl12O19:0.1%Mn4+ sample prepared at 1673 K temperature in air. The diffraction peaks can be well indexed with the reference pattern (bottom line) of SrAl12O19 (ICSD#69020). No other crystalline phases were detected within the detection limit, indicating that the as-prepared phosphors are highly pure, single-phase and the doped 0.1% Mn4+ ions have not caused any observable change in the host structure. The compound SrAl12O19 belongs to hexagonal magnetoplumbite-type structure [33, 34]. As shown in upper left of Fig. 1, the unit cell consists of spinel blocks with Al3+ ions in both tetrahedral and octahedral sites and bound with intermediate mirror layers containing large cations, in which Sr2+ ion is 12-coordinated and five different Al3+ ions sites occupy three kinds of coordination: one AlO4 tetrahedron, an unusual AlO5 trigonal bipyramid and three distinct AlO6 octahedrons [31, 34].

These three types of octahedral aluminum sites are ideal for accommodation of Mn4+ ions. The SEM image (upper right of Fig. 1) of SrAl12O19:Mn4+ appears as a parallel stacked orderly-layered structure with thickness about 110 nm, and some small fragments distribute in interlaminations. 3.2. Spectra characteristic analysis The as-prepared Mn4+ doped SrAl12O19 phosphor emits bright red light under either UV or blue lights. Fig. 2(a) shows the excitation and emission spectra of SrAl12O19:0.1%Mn4+, which present typical Mn4+ red luminescence due to substitution for octahedral Al sites [16]. The excitation spectrum exhibits a broad band in the ultraviolet and blue spectral ranges from 20000 to 40000 cm-1, but not the green spectral region. Therefore, the phosphor will not emit red light by consuming the green emission part of the WLED. Three peaks of the excitation spectra at ~30488(328 nm), ~25316 (395 nm), and ~21277(470 nm)cm-1 are assigned to the transitions of spin-allowed 4A2→4T1 (partly Mn4+-O2- charge transfer transition), 4

A2→2T2 and 4A2→4T2, respectively [3, 9, 16, 17, 23]. Under 328 nm excitation, the

emission spectrum presents narrow band between 13699 and 15873 cm-1 with the strongest peak at ~15198 cm-1 (658 nm) and two shoulder peaks at ~15552 cm-1(643nm) and ~14815 cm-1(675 nm) owing to the 2E→4A2 transition of Mn4+. The emission decay curve of sample as shown in Fig. 2(b) on a logarithmic scale, could well fit to the monoexponential decay equation and the fit leads to the lifetime τ = 1000.39 s. These spectral characteristics are similar to other Mn4+ doped phosphors [3, 9, 17, 29]. 3.3. Crystal field calculations of Mn4+ energy levels and analysis of the Mn4+ site occupancy The energy levels of three types of Mn4+ sites in SrAl12O19 crystal structure can be evaluated by the exchange-charge-model crystal field calculations [35, 36]. The relevant calculation process has been described in detail in the previous work and will not be repeated here. A small number of fitting parameters is one of the strongest exchange charge model (ECM) features. In addition, the ECM gives an opportunity to calculate the

crystal field parameters (CFPs) and energy levels of impurity ions in crystals without making any a priori assumptions about the impurity center symmetry. Up to now, the ECM has been successfully applied to the calculations of the energy levels of both transition metal and rare earth ions in crystals [37-41]. The crystal structure data of SrAl12O19 were taken from ref. [41]. According to this reference, SrAl12O19 crystallizes in the P63/mmc space group (No. 194), with the lattice constants a, b, c (in Å): 5.5666, 5.5666, and 22.0018, respectively; angle γ = 120º. There are two formula units in one unit cell. After doping, the Mn4+ ions can substitute for the Al3+ ions; some charge compensation is needed to achieve the electric neutrality of the samples. There are five inequivalent Al sites in one unit cell. However, one of them is 4-fold coordinated by the oxygen ions, and another one is 5-fold coordinated by the oxygen ions. They cannot be occupied by the Mn4+ ions, which exhibit the preference to enter the octahedral sites. The three types of aluminum sites, labels Al1, Al4, and Al5 are used [41], are in principle suitable for occupation by Mn4+. The CFPs were calculated using a large cluster consisting of 37833 ions around an impurity site, to ensure proper convergence of the crystal lattice sums needed to calculate the CFPs. All non-zero CFPs are given in Table 1. The Racah parameter B = 810 cm−1 and greater than Sr4Al14O25:Mn4+ of 790 cm-1, while Racah parameter C and non-dimensional ECM parameter G for the Mn4+ ions at the Al1, Al4, and Al5 sites in SrAl12O19 are all lower than Sr4Al14O25:Mn4+ (C = 3192 cm−1 and G = 7.0) [36]. The Mn4+-O2 overlap integrals required for the calculations of the CFPs were taken from ref.[42]. With these CFPs and the Racah parameters also given in the table the following energy levels of Mn4+ were obtained in Table 2. The structure of the crystal field Hamiltonian and the pattern of the energy levels splitting (orbital doublets remain to be doublets, orbital triplets are split into a singlet and a double states) suggests trigonal symmetry of the Al1 and Al4 sites, whereas the Al5 site has the lowest symmetry among all considered, since all orbital triplets are split into the singlet states.

The calculated energy levels are superimposed onto the experimental excitation and emission spectra in Fig. 2(a). Although the calculated Mn4+ energy levels for these Al1, Al4, and Al5 sites are located closely to three each other. Careful differentiation shows that the sites of Al1 and Al5 should be more preferable. Al5 site is even most preferable may be due to the lowest symmetry, which can be obtained from the coordination of the individual atoms in Table 3. 3.4. Luminescence thermal quenching behavior of SrAl12O19:Mn4+ For higher doped concentrations, energy transfer and reabsorption of the emission can occur and concentration quenching becomes more pronounced at elevated temperatures, while for low concentration, interaction between the luminescence ions and reabsorption will be negligible [43]. Hence, low Mn4+ concentrations (0.1%) were adopted to form isolated luminescence centers and insight into the intrinsic thermal quenching for Mn4+ in SrAl12O19 could, therefore, be possible. As shown in Fig. 3, the luminescent properties of the phosphor of SrAl12O19:Mn4+ possess clear temperature dependence. At 8 K, the excitation spectrum does not appear fine structured, and it comprises three peaks at 31250 cm-1, 25316 cm-1 and 21598 cm-1. These peaks redshift obviously as the temperature increases (Fig. 3(a)). For instance, the strongest peak lies at ~31250 cm-1 at 8 K, and it goes to ~30395 cm-1, ~30303 cm-1, ~29586 cm-1 at 300 K, 400 K, and 500 K, respectively. The red shift might be due to the strengthened electron-phonon interaction with increasing temperature. As increasing the temperature, the lattice vibrations become stronger, the crystal field strength becomes weaken, and the lowest excited state energy level becomes lower, that is, the red shift phenomenon occurs. Therefore, this redshift phenomenon to some extent reflects the enhanced interaction between Mn4+ and local field with increasing temperature. As shown in Fig. 3(b), the emission spectra also exhibit redshift. At low temperature of 8-100 K, zero phonon line (ZPL) is observed at 15314 cm-1 and a little higher than the calculated values of 2Eg in Table 2, and a sharp peak is observed at 15244 cm-1. When the temperature reaches 150 K, a new peak of 15625 cm-1 appears. This implies that the three peaks are possible associated with three different Mn4+

emission centers and corresponding to three kinds of octahedral aluminum site. Chemical bond covalence fc of Al-O bond are calculated by using the Born-Haber thermochemical cycle improved by Zhang et al.[44-46] and the results are presented in Table 3, the values are 0.1853, 0.1851 and 0.1523 for Al1, Al5, and Al4, respectively. The low energy peak of 15244 cm-1 should be due to the Al1 site with higher covalency according to the criterion developed by Brik et al. [16]. It is worth noting that the Al1 and Al5 sites with higher covalence are occupied preferentially as mentioned in part 3.2. Emission decay behavior depends strongly on temperature (Fig. 3(c)). At lower temperature, decay curve fits well single exponential decay equation, and the fitting deteriorates as the temperature increases. The lifetime reduces significantly from 1833μs@8K to 15μs@500K for instance upon excitation of 328 nm. Fig. 3(d) summarizes the dependences of emission lifetime and intensity on temperature. And they show similar trend for excitation schemes of either UV or blue. The intensity at 300 K decreases to 60%, 61% and 82% of the value at 8 K under 328, 390 and 470 nm excitation, respectively. However, the thermal quenching temperature T50% (the temperature at which the intensity decreases to half of the value at room temperature (300 K)) is about 350 K, while the value for CaAl4O7:Mn4+ is about 340 K [9]. Although SrAl12O19:Mn4+ has less practically valuable in the application of pc-WLEDs from the view of thermal quenching, it has slightly better resistance to thermal impact than CaAl4O7:Mn4+. At 500 K, a weak luminescence of Mn4+ could still be recorded. Significant lifetime shortening gives evidence for both cases of excitation that there is intrinsic severe quenching in SrAl12O19:Mn4+. The decrease in lifetime is accompanied by the decrease of emission intensity. Furthermore, Quantum Yield (QY) is one of the important optical properties of fluorescent materials. It is defined as the number of emitted photons divided by the number of absorbed photons by a sample. We noticed that the QY of the sample has not been reported. Hence, the absolute fluorescence QY of the SrAl12O19:Mn4+ was recorded under the excitation of 300 nm light by the

absolute photoluminescence quantum yield spectrometer at room temperature, the QY of SrAl12O19:Mn4+ can reach up to 40% under the 300 nm excitation. One of the mechanisms for Mn4+ luminescence quenching might be quenching by thermally activated crossover from a high vibrational level in the first excited state of 2E to the ground state 4A2. For evaluating activation energy (Ea) for thermal quenching, we fit the temperature dependence of emission lifetime rather than intensity to a modified Arrhenius equation [6]: (T)=0/(1+0Ce-Ea/kT)

(5)

Where (T) is the emission decay time at temperature T (K), 0 is the emission decay time at 0 K, C is a rate constant for the process. This is because emission intensity when compared to lifetime is very susceptible to various factors (e.g., temperature dependence of the absorption strength, energy migration and reabsorption) [36]. Unlike the intensities, the temperature dependence of emission lifetime appears the same tendency irrespective of the excitation wavelengths (see Fig. 3(d)). Best fitting leads to Ea = 0.198 eV. It is much lower than 0.6 eV of SrSi2O2N2:Eu2+ and slightly higher than 0.196 eV of CaAl4O7:Mn4+, which can well explain why luminescence quenching occurs in SrAl12O19:Mn4+ and CaAl4O7:Mn4+ but not SrSi2O2N2:Eu2+ at 500 K [6, 9]. As a comparison, we also studied the thermal luminescence quenching behavior of Sr4Al14O25:Mn4+[36]. With the same Mn4+ doped concentrations (0.1%), T50% was found as 423 K for the compound, and it is much higher than either CaAl4O7:Mn4+ [9] or SrAl12O19:Mn4+. On the basis of crystallographic data of these compounds, we can see clearly the differences of lattice sites from compound to compound in which Mn4+ could eventually occupy after high temperature reaction, and we believe these could be the reason. The differences are the site symmetry, the site spatial distribution and the possible defects created along the successful substitution of Mn4+ for lattice ions. In CaAl4O7:Mn4+, Mn4+ ions are located in a very distorted octahedral sites due to the substitution for the seven coordinated divalent sites. So, the symmetry of Mn4+ sites is lowest in the three compounds, and the mismatch of charge is largest between the dopant and the substituted lattice cation. More defects have to be created to balance

the charge mismatch, and they might become the luminescence killers. These factors could be the reason why T50% is lowest in CaAl4O7:Mn4+. The difference of SrAl12O19:Mn4+ and Sr4Al14O25:Mn4+ lies in the spatial distribution of octahedral sites. In SrAl12O19:Mn4+, all the octahedra are connected to each other (see upper left of Fig. 1) while Sr4Al14O25:Mn4+ shows different picture. The octahedra in Sr4Al14O25 share common faces and form into isolated regularly AlO6 layers (see Fig. 1(c) of ref. [3]). When Mn4+ is incorporated into AlO6 layers, the interactions with other Mn4+ ions can be well blocked especially in the direction of axis a. Therefore, better thermal stability is resulted. In addition, it was found that the thermal quenched luminescence of Mn4+ has a good sensitivity. Normalized temperature-dependent spectra of SrAl12O19: 0.1%Mn4+ collected from 50 to 350 K is illustrated in Fig. 4(a). It is clear that the integrated intensity ratio of range 1(between 15385 and 16103 cm-1) to range 2(between 15151 and 15385 cm-1) increases monotonously with the elevation in temperature. Fig. 4(b) presents the monolog plot of the fluorescence intensity ratio (FIR) value as a function of inverse absolute temperature in the range of 50 to 350 K. The experimental data are fitted to a straight line with the slope of about -289.44. Fig. 4(c) illustrates the temperature dependence of the FIR value in the range of 50 to 350 K for SrAl12O19: 0.1%Mn4+. The SR and SA of SrAl12O19: 0.1%Mn4+ are investigated and their fitting results as a function of temperature from 50 to 350 K are described in Fig. 4(d). The calculated SR value is 289.44 K-1. In addition, the SA value increases with the temperature and is up to the maximum of 2.97×10-3 K-1at 138 K. Therefore, SrAl12O19:0.1%Mn4+ phosphor has high temperature sensitivity, and this characteristic can make it a potential application in temperature sensing, which provides an application direction for materials with thermal quenching properties. 4. Conclusions The theoretical and experimental results indicate the doped Mn4+ ions occupy preferentially the Al1 and Al5 higher covalent sites rather than the Al4 site in the SrAl12O19 crystal lattice. The lower quenching temperature and

activation energy demonstrate that SrAl12O19:Mn4+ suffers from serious thermal quenching. Moreover, the temperature sensitivity of the sample was analyzed. The dominating quenching mechanism may be the thermally activated crossover from the first excited state to the ground state. Low temperature luminescence shows in one hand that there are multiple Mn4+ centers in the compound, and in the other that anti-Stokes sideband of Mn4+ disappears at temperature lower than 100 K and the temperature dependent process becomes enhanced as the temperature increases. Combination of theoretical calculations and experimental verification reveal that the most desirable compound with better resistance of luminescence for Mn4+ to thermal impact should comprise the octahedral sites with higher symmetry and well isolated sites for Mn4+ from each other. In particular, the valence of the substituted host cations should match the Mn4+ ions as better as possible, which can avoid the defect formation during substitution due to charge compensation. We believe this work could be helpful for designing higher efficient novel Mn4+ doped red phosphors in future.

Acknowledgements We acknowledge financial support from National Natural Science Foundation of China (Grant No. 51672085), Program for Innovative Research Team in University of Ministry of Education of China (Grant No. IRT_17R38), the Joint Fund of Ministry of Education of China, National Key Research and Development Plan (Grant No. 2017YFF0104504) , Local Innovative Research Team Project of “Pearl River Talent Plan” (Grant No. 2017BT01X137), Guangdong Natural Science Foundation (Grant No. 2018B030308009) and Fundamental Research Funds for the Central Universities. References [1] P. Pust, P.J. Schmidt, W. Schnick, A revolution in lighting, Nat. Mater., 14 (2015) 454-458. [2] H. Zhu, C.C. Lin, W. Luo, S. Shu, Z. Liu, Y. Liu, J. Kong, E. Ma, Y. Cao, R. Liu, X. Chen, Highly efficient non-rare-earth red emitting phosphor for warm white light-emitting diodes, Nat. Commun., 5 (2014).

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Fig. 1. XRD pattern of SrAl12O19:0.1%Mn4+, un-doped SrAl12O19 sample and the reference pattern of SrAl12O19 (ICSD#69020). The insets show crystal structure (upper left) of SrAl12O19 drawn on the basis of the corresponding crystallographic data (ICSD#69020) and SEM image (upper right) of the sample with 1 μm scale bar.

Fig. 2. (a) Exemplary excitation (blue line, λem = 658 nm) and emission (red line, λex = 328 nm) spectra of SrAl12O19:0.1%Mn4+, the calculated energy levels of Mn4+ are shown by the vertical lines. (b) Emission decay curve (black line) of SrAl12O19:0.1%Mn4+ with the best-fit curve (solid red line) by a single exponential decay equation [y = 3.4 + 2817 exp(-x/1.00039)], R2 = 99.07%.

Fig. 3. (a) Excitation spectra (λem = 658nm), (b) emission spectra (λex = 328 nm) and (c) decay curves (λem = 658 nm, λex = 328 nm) of SrAl12O19:0.1%Mn4+ at different temperatures as indicated. (d) The temperature dependence of the integrated emission intensities and lifetimes (em = 658 nm) at different excitation wavelengths as indicated (“red ball” for ex = 328 nm; “green star” for ex = 390 nm; “blue snowflake” for ex = 470 nm). The black line through the data points is fit to (T) =0/(1+0Ce-Ea/kT) with 0 = 1833 μs, C = 1.27, and Ea = 0.198 eV. The goodness of fitting is 97.8%.

Fig. 4 (a) Eission spectra (λex = 328 nm)normalized at 15314 cm-1 at different temperatures from 50 to 350 K. (b) Monolog plot of the FIR as a function of inverse absolute temperature. (c) Temperature dependence of FIR. (d) The relative sensitivity SR and the absolute sensitivity SA at different temperatures from 50 to 350 K.

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Table 1 CFPs (Stevens normalization, all in cm-1), Racah parameters B, C (in cm-1) and non-dimensional ECM parameter G for the Mn4+ ions at the Al1, Al4, and Al5 sites in SrAl12O19. CFPs B2-2 B2-1 B2 0 B2 1 B2 2 B4-4 B4-3 B4-2 B4-1 B4 0 B4 1 B4 2 B4 3 B4 4 B C G

Al1 site -2.4 -11.7 706.5 -6.8 1.3 0.0 0.0 0.0 0.0 -3925.5 0.0 0.0 102470.3 0.0 810 3069 5.68

Al4 site -1.6 -9.4 -1369.2 -5.5 0.9 0.0 0.0 0.0 0.9 -3520.8 0.0 0.0 -106582.5 0.0 810 3067 6.92

Al5 site 1659.8 -1854.0 -1783.4 1057.1 961.4 -787.9 0.0 -417.3 72.1 -3219.3 -2.8 -240.9 109726.2 454.9 810 3115 6.45

Table 2 Calculated energy levels (all in cm-1) for the Mn4+ions at the Al1, Al4, and Al5 sites in SrAl12O19. The orbital double states are denoted with an asterisk. Terms 4

A2g Eg 2 T1g 4 T2g 2 T2g 4 T1g … 4 T1g 2

Al1 site 0 15165* 15850*, 16054 21117, 21189* 22909, 23382* 28840, 29610* … 46121*, 46660

Calculated energy levels Al4 site Al5 site 0 0 15164* 15112, 15206 15737, 16013* 15657, 16101, 16207 * 21024, 21281 20992, 21358, 21364 * * 23006 , 23605 23225, 23346, 24123 * 29168 , 29815 28065, 29656, 30778 … … * 45406, 46848 45370, 47101, 47228

Table 3 Bond length d(Å) and chemical bond covalence fc of Al-O bond. Al Al(1) Al(2)

Al(3)

Al(4)

Al(5)

Bond Al(1)-O(4) × 6 Al(2)-O(3) × 3 Al(2)-O(1) × 1 Al(2)-O(1)' × 1 Average Al(3)-O(4) × 1 Al(3)-O(4)' × 2 Al(3)-O(2) × 1 Average Al(4)-O(5) × 2 Al(4)-O(5)' × 1 Al(4)-O(3) × 1 Al(4)-O(3)' × 2 Average Al(5)-O(5) × 2 Al(5)-O(1) × 1 Al(5)-O(2) × 1 Al(5)-O(4) × 2 Average

d(Å) 1.8760 1.7646 2.0264 2.4488 1.9538 1.7978 1.7983 1.8136 1.8020 1.8764 1.8769 1.9622 1.9626 1.9195 1.8129 1.8464 1.9844 1.9989 1.9091

fc 0.1853 0.1878 0.2681 0.2744 0.2212 0.4105 0.4105 0.4104 0.4105 0.1853 0.1853 0.1193 0.1193 0.1523 0.1863 0.1857 0.1841 0.1840 0.1851