Anomalous Red and Infrared Luminescence of Ce3+ Ions in SrS:Ce

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Anomalous Red and Infrared Luminescence of Ce3+ ions in SrS:Ce Sintered Ceramics Dagmara Kulesza, Joanna Cybinska, Luis Seijo, Zoila Barandiaran, and Eugeniusz Zych J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06921 • Publication Date (Web): 13 Nov 2015 Downloaded from http://pubs.acs.org on November 19, 2015

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The Journal of Physical Chemistry

Anomalous Red and Infrared Luminescence of Ce3+ ions in SrS:Ce Sintered Ceramics Dagmara Kulesza,1,* Joanna Cybińska,1,2 Luis Seijo3, Zoila Barandiarán3, and Eugeniusz Zych1 1

Faculty of Chemistry, University of Wroclaw, 14F. Joliot-Curie Street, 50-383 Wroclaw, Poland Wroclaw Research Centre EIT+, 147 Stablowicka Street, 54-066 Wroclaw, Poland 3 Departamento de Química, Universidad Autónoma de Madrid, 28049 Madrid, Spain 2

ABSTRACT: A blue-green luminescence of SrS:Ce has been known for years. High-density sintered pellets of SrS:Ce were prepared for the first time and their unique spectroscopic properties are reported. A new luminescence band appearing in red and infrared part of spectrum and having all the characteristics of Ce3+ emission was obtained after sintering SrS:Ce at 1700 °C. The excitation features of the regular (blue-green) and the new, anomalous emissions are very much different. Together, both excitations spectra cover the whole range of UV and blue part of spectrum. Such characteristics allow for site-selective excitation of both centers. Experiments in the 25-435 K range of temperatures show that the new, anomalous Ce3+ emitting center hardly interacts with the regular one. Only excitation around 425 nm leads to superimposed emissions of both centers, especially at higher temperatures. Decay traces show that the regular center decay time is pretty constant between 25 K and 435 K (shortens from 25 ns to 23 ns). However, the anomalous center shows monoexponential decay only at 25 K with time constant of 79 ns, and at room temperature its trace is twoexponential with components of 5 ns (20 %) and 50 ns (80 %). Aggregation of the dopant ions and its excessive charge compensating defects were postulated to lead to the [Ce3+-S2--Ce3+] or [Ce3+-VSrʹʹ-Ce3+] clusters. The spectroscopic data on the anomalous luminescence were also

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shown to adhere with the possibility of Ce3+/Ce4+ intervalence charge transfer luminescence and semiquantitative scheme for relevant processes was presented. INTRODUCTION SrS:Ce is a known luminescent material with a high-brightness blue-green emission located in the 450-650 nm wavelength region and peaking around 480 nm.1,2 The SrS host has quite a narrow bandgap energy of about 4.3 eV (~280 nm).3,4 This allows to excite a dopant with all standard light sources either directly or through the host fundamental absorption followed by a subsequent transfer or the acquired energy to the activator. In the case of Ce3+ impurity both mechanisms can be easily executed, which, among others, opened research on SrS:Ce films for electroluminescent devices.5 Yet, applications of SrS:Ce are problematic and limited by its hygroscopic nature. However, Smet et al.2 published a comprehensive, in-depth review on sulphides stressing that this drawback, a property of many sulphides, should not be overemphasized and does not totally eliminate this group of phosphors from practical use. Indeed, just recently preparation of colloidal solutions of CaS and SrS activated with Ce3+ or Eu2+ were claimed to be potentially useful for practical purposes6 and not long ago CaS- and SrS-based phosphors were qualified for white LEDs.7-10 It was previously reported that luminescence of SrS:Ce and CaS:Ce gets red-shifted by 20-40 nm for higher Ce contents. The effect was assigned to formation of Ce3+ pairs and/or clusters.11,12 The interactions between Ce3+ ions were also reported by Hüttl et al. upon EPR data in samples having more than 0.2% of Ce.13 Compared to powders, a 25-30 nm red-shift of the SrS:Ce luminescence band was also reported for thin films.11 Similarly in nanocrystalline CaS:Ce and SrS:Ce6 a small red shift of the emission band was showed to occur for the Ce content of 1 % and got significant (~50 nm) for 10% concentration. Nevertheless, all the previously published literature presented barely a red-shift of the regular Ce3+ emission by 20-40 nm and not a totally new luminescent feature. Thus, excitation spectra


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of the red-shifted emissions were very similar if not identical to the spectrum of the regular Ce3+ luminescence.6,11 As we shall see, in the case of our sintered ceramics a Ce3+ luminescence of an entirely different origin is generated and its peak appears around 620 nm, which situates this material within just a few showing such a long-wavelength Ce3+ emission. We will prove that in SrS:Ce sintered bodies there exist two different luminescent centers having completely different local symmetries but both connected with Ce. This effect will be shown to appear even in strongly diluted compositions containing only 0.05 mol% of Ce but necessarily prepared at temperatures as high as 1700 °C. In literature one may find a rather limited number of compositions in which the Ce3+ emission is yellow or even less energetic.1419

. Jarý et al. researched a group of ternary sulfides with general formula of ALnS2, (A=alkali

metal) in which Ce3+ enters an octahedral position and produces yellow or even red emission.20-23 Comprehensive listings can be found in papers published by Dorenbos.24,25 The starting point of the research and its driving force was to reduce the harmful influence of the environment on SrS:Ce by sintering its powders into bulk pellets with significantly reduced, potentially eliminated, porosity. We shall show that effective sintering of SrS:Ce is indeed possible, and – surprisingly - it leads to a significant change of the spectroscopic properties of the product compared to any form of SrS:Ce phosphors reported in literature. To the best of our knowledge, this is the first report on sintered sulphides and their luminescent properties. EXPERIMENTAL SrS:Ce ceramics were prepared from SrSO4:Ce powders precipitated by mixing water solutions of Sr(NO3)2 (Merck, 99%), Ce(NO3)3·6H2O and (NH4)2SO4 (POCH, 98.5%). The later was used with ~10 % excess. The Ce concentration was 0.05 mol % with respect to Sr. The precipitated SrSO4:Ce was dried at 150 °C for 1 h, ground in an agate mortar and heated at 500 °C for 1h. Such SrSO4:Ce powders were uniaxially pressed under a load of 4 tons into



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a pellet 7 mm in diameter and reduced in a graphite crucibles and a presence of graphite powder at 1700 °C for 3h. During the reduction processes the graphite crucible was kept in an alumina crucible covered with a lid. For comparison SrS:Ce powder was also prepared reducing the SrSO4:Ce powder at the temperature of 1000 °C. Having noticed that sintered SrS:Ce has drastically different luminescent properties compared to its powders made at lower temperatures, we decided to fabricate at analogous conditions also Eu-activated powder (1000 °C) and ceramics (1700 °C) of SrS:Eu2+ (0.05%). Eu(NO3)3·6H2O (Sigma-Aldrich, 99.99%) served as a source of Eu. The XRD diffraction patterns of SrS:Ce were recorded with a D8 Advance X-ray Diffractometer from Bruker utilizing a Ni-filtered Cu Kα1 radiation (λ = 1.540596 Å). The microstructure of the sintered ceramics and powders was verified with a Hitachi S-3400N scanning electron microscope (SEM). Photoluminescence (PL), photoluminescence excitation (PLE) spectra and photoluminescence decay traces were measured in the 25-435 K temperature range with FLS980 fluorescence spectrometer from Edinburgh Instruments Ltd. A 450 W continuous Xe arc lamp was used as an excitation source for PL and PLE spectra, while decay kinetics traces were recorded with the Supercontinuum Fianium WhiteLase UV laser. Emission spectra were corrected for the recording system spectral efficiency and excitation spectra were corrected for the incident light intensity. The SrS:Ce samples were mounted on a closed-cycle helium cryostat holder and the temperature was controlled with Temperature Controller Model 336 supplied by Lake Shore. RESULTS In Figure 1 we present X-ray diffraction patterns of the powder made at 1000 °C and the ceramics sintered at 1700 °C. They are fully consistent with the literature data.26,27 Both the SrS:Ce powder and the sintered body possess the typical cubic structure of SrS, in which the


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Sr2+ ions have Oh local symmetry. Thus, if Ce3+ dopant accommodates the symmetry of Sr2+, its 5d orbitals should split into only two levels – the energetically lower t2g and the higher eg.

Figure 1. X-ray powder diffraction patterns of SrS:Ce sintered at 1700 °C ceramics and SrS:Ce powder after heat-treatment at 1000 °C. Simulated XRD pattern of cubic SrS according to ICSD #28900.26,27 While the sintering did not affect the crystal structure of SrS:Ce, the morphology of the powder and its sintered ceramic counterpart differed greatly (see Figure 2). The powder is composed of uniform ~3-5 µm grains and shows only insignificant agglomeration whereas the body sintered at 1700 °C is made up of tightly packed grains of 30-50 µm, and contains a limited number of relatively small voids. Thus, the sintering caused a great change in microstructure/morphology. A very significant enlargement of the grain sizes resulting from a vigorous, immense mass transfer upon sintering at 1700 °C was accomplished, and the densification effectiveness is estimated at 95% of the theoretical one. Despite the presence of the voids, the SEM images of the SrS:Ce sintered at 1700 °C prove that this composition can be efficiently densified. There is every reason to anticipate that the sintering process can be further optimized. The great transformation of the microstructure of SrS:Ce induced by sintering brought also a tremendous change in its luminescent properties.



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In Figure 3 we present photographs of the SrS:0.5%Ce powder after preparation at 1000 °C and ceramics made of this powder at 1600 °C and 1700 °C (top row). Variation of the color connected with increasing Ce concentration after sintering at 1700 °C is given in the bottom row of Figure 3. A very sever darkening with increasing temperature of sintering and with increasing Ce content is obvious. Finally, the sample sintered at 1700 °C and containing 5 % of Ce is almost black. Clearly, sintering at higher temperatures and especially when Ce concentration is high leads to development of strong, presumably broad absorption bands in visible part of spectrum and the effect upsurges with increasing sintering temperature and Ce concentration. These changes will have their emanations in luminescent properties, as we shall see shortly. While in this paper we shall basically deal with the luminescent properties of ceramics with low Ce content (0.05 %) the reader can find results of extensive spectroscopic studies of the concentration-dependence of spectroscopic properties of SrS:Ce sintered ceramics in a just published paper.28 Here, let us just state that the evolution of luminescent properties with increasing sintering temperature is rather complex though in general continuous increase of the red-IR luminescence is observed.


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Figure 2. SEM images of SrS:Ce a) powder heated at 1000 °C; b) ceramics sintered at 1700 °C.

Figure 3. Photographs presenting changes in the appearance of the SrS:0.5%Ce powder upon its sintering at 1600 and 1700 °C (top raw) and the ceramics containing different Ce concentration sintered at 1700 °C (bottom).



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In Figure 4 we compare the luminescence spectra of SrS:Ce powders made at 1000 °C and the ceramics prepared at 1700 °C upon excitation at 425 nm thus into the lowest (emitting) 5d level of the regular Ce3+ center in SrS.6 The powder shows only the well-recognized luminescence with maximum around 480 nm, covering the 450-650 nm range of wavelengths and showing the characteristic doublet split by 2280 cm-1.1,7,29

Figure 4. Normalized luminescence spectra of SrS:Ce powder heated at 1000 °C and ceramics sintered at 1700 °C. In both cases excitation at 425 nm was applied. However, the ceramics produces emission which is much broader, with the regular luminescence complemented by a new, structured, equally intense emission band located within about 550 and 800 nm and peaking around 620 nm. To learn more about the new red-IR luminescence of the ceramics sintered at 1700 °C, its PLE spectrum was measured and compared to that of the regular (~480 nm) luminescence. These data are presented in Figure 5 together with emissions excited at characteristic wavelengths of both centers. It is evident that the red-IR luminescence has its own PLE spectrum entirely different from the spectrum of the regular emission. Excitation at 380 nm, thus omitting the regular center, produces an intense emission band peaking around 620 nm and evidently composed of two overlapping components split by 2040 cm-1, which is


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characteristic for the spin-orbit splitting of the Ce3+ ground term into the 2F5/2 and 2F7/2 states.30-32

Figure 5. RT normalized excitation spectra of 480 nm and 650 nm emissions (solid lines) and emission spectra excited at 380 nm and 425nm wavelength (dotted lines) of SrS:Ce ceramics. Perfectly the same emission is generated upon excitation at 520 nm, into the lowest-energy excitation band of the red-IR luminescence. Clearly, the shape of the red-IR luminescence band seen in Figure 5 resembles very much a typical Ce3+ d-f emission, though its position in such a long-wavelength region is not common for this dopant. In the following section we shall present results which will further prove that the red-IR luminescence has all the characteristics of Ce3+ emission.



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Figure 6. Normalized low temperature (25 K) emission and excitation spectra revealing existence of two different Ce3+ luminescence centers in SrS:Ce ceramics sintered at 1700 °C. To better characterize the new luminescence feature and relate it to the regular one a set of experiments at different temperatures was performed. Let us first discuss excitation and luminescence spectra of the characteristic emissions recorded at 25 K, which are presented in Figure 6. Both excitation and emission spectra appear very similar to those recorded at room temperature (Figure 5), with the main difference being a much better splitting of the luminescent components. Excitation at 425 nm, essentially into the emitting level of the regular Ce3+ center, produces four well resolved bands covering the 450-800 nm spectral range. However, excitation at 380 nm results only in the red-IR luminescence comprising two components, which coincides with the two long-wavelength constituents of the emission excited at 425 nm. It is thus clear that excitation at 425 nm gives rise to emissions from both centers, while upon 380 nm only the anomalous one gets excited and produces its characteristic red-IR luminescence. From now on we shall denote the two emitting centers as a Ce1 center giving the classic luminescence in the blue-green region and a Ce2 entity producing emission in the red-IR part of spectrum. Let us note, that the measurements at 25 K confirm that the Ce2 red-IR emitting center has its lowest-energy excitation (absorption) level positioned perfectly within the emission range of the regular Ce3+ center (Ce1), somewhere around 470-560 nm. This is a potentially perfect situation for efficient energy transfer from the Ce1 to the Ce2 center by the resonant mechanism described by Förster.33 Yet, as we shall shortly see in kinetics measurements, the large average Ce-Ce distance actually limits the Ce1→Ce2 nonradiative energy transfer. In spite of that, examining the excitation spectrum of the 650 nm (Ce2) emission, see Figure 6, such a transfer of energy from the regular, Ce1 center to the anomalous, Ce2 one might be at first granted. Namely, it contains a well visible bump around 425 nm, which coincides


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perfectly with the strong excitation band of the regular 480 nm (Ce1) emission. A very similar, though slightly less resolved, component was seen in an analogous spectrum taken at room temperature (Figure 5). These data might suggest that some energy leak from the regular, Ce1 center to the red-IR, Ce2 one indeed occurs. Yet, we should be aware that also some radiative energy transfer may occur in these circumstances, which would give similar effects in excitation spectra. Although there is no evident excitation feature around 280 nm when the Ce2 center emission is monitored we shall see later that this excitation also produces some Ce2 luminescence. This may result from the significant overlapping of the f-d transition of the regular Ce1 center in that range of wavelengths with the host lattice fundamental absorption. The excitation spectrum of the regular luminescence of the Ce1 center is composed of basically two bands located around 425 nm and 280 nm (which coincides and strongly overlaps with the fundamental absorption of the SrS host), see Figures 5 and 6. This is in agreement with the high (Oh) local symmetry of Sr2+ in SrS. If this symmetry is adopted by the Ce3+ ion, the 5d orbitals should split into two energy levels, t2g and eg.6,34 However, the excitation spectrum of the Ce2 emitting center appears much more complex. A very broad structured band in the range of ~280-420 nm with three components appearing around 385 nm, 330 nm and 310 nm is accompanied by an asymmetric band around 520 nm. Indeed, fitting (not presented in Figure 5) the latter revealed two strongly overlapping constituents. It is thus obvious that the new emitting center, Ce2, though also made up of Ce3+ ion(s), encounters much lower local symmetry compared to the regular one, Ce1. As we already concluded from excitation spectra presented in Figures 5 and 6, the direct excitation into the lowest 5d level of the regular Ce3+ (Ce1) (~425 nm) leads to both the bluegreen and the anomalous red-IR emissions suggesting at first that some Ce1→Ce2 energy



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transfer occurs. This supposition seemed to get further confirmation from luminescence spectra presented in Figure 7a.

Figure 7. Temperature dependence of (a) luminescence spectra excited at 425 nm and normalized at 480 nm; (b) luminescence spectra excited at 280 nm; decay time traces excited at (c) 425 nm and monitored at 485 nm; (d) 520 nm and monitored at 650 nm. As the temperature increases from 25 K to 435 K excitation at 425 nm (Ce1) produces progressively more of the anomalous luminescence from the Ce2 ions. Finally, at 435K the anomalous emission even dominates the regular one under such excitation. Yet, at the same time the main component of the decay time of the regular, Ce1 center, see Figure 7c, gets reduced only slightly from ~25 ns to 23 ns between 25 K and 435 K, which obviously does not account for the great increase of the Ce2 center emission intensity. Consequently, the anticipated nonradiative Ce1→Ce2 Förster-type energy transfer33 cannot be the main mechanism standing behind the increasing intensity of the Ce2 luminescence upon 425 nm excitation. Note, that when the excitation goes mainly to the host lattice (~280 nm) the ratio of the Ce2 and Ce1 emission intensities appears pretty independent on the sample temperature (Figure


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7b). This is reasonable if we remember that this radiation operates only on the surface of the material due to the exceptionally high absorption cross section of the band-to-band transitions. Thus, all the at first puzzling data appear to be comprehensible. Contrary to the regular Ce1 emission at 480 nm the luminescence of the Ce2 center excited at 520 nm shows temperature quenching already at RT (Figure 7d). While significant afterglow strongly obscures details of decay traces at higher temperatures (RT and 435 K), some qualitative conclusions can be drawn. Clearly, much shorter and nonexponential luminescence decay is characteristic for the Ce2 center at room and higher temperatures. At RT the average decay time35 constant of the 650 nm emission is on the order of 45-50 ns, with two components, 5 ns (20 %) and 55 ns (80 %) given by the fit. Analogous traces were collected upon the 425 nm excitation, when the incident energy goes mainly to the Ce1 regular center. We shall get back to this observation later discussing the possibility of intervalence charge transfer luminescence. The origin of the slow component in the decay of the Ce2 luminescence at higher temperatures is not clear at present. All we can say now about this effect is that the relatively low-energy excitation (520 nm) does not seem to favor a photoionization of this center. On the other hand, the rather harsh fabrication conditions may lead to a formation of a whole variety of defects, which may serve as active energy-trapping sites. A broader research would be needed to explain this observation. It is noteworthy that excitation spectra, whether at room temperature or at 25 K, prove that combined absorption of the two emitting centers covers all the 250-550 nm wavelength range, with only a small gap around 465 nm. Thus, the high-energy part of Earth-reaching Sun-light spectrum36,37 could be efficiently absorbed by the sintered SrS:Ce plates and get converted into emission within 450-800 nm, which the silicon solar cells can efficiently use.36 Since all the absorption bands results from the allowed 4f→5d transitions a quite thin layer would be



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sufficient to fully absorb this part of radiation and convert it to the more useful less energetic photons. This is in particular evident for slightly higher concentrations of Ce, see Fig. 3. Unfortunately, as we shall see shortly, the luminescence of the anomalous Ce2 center is partially quenched at RT, and this makes such application not likely unless the problem of thermal quenching is solved. DISCUSSION The least we may conclude from the structure of the excitation spectrum of the anomalous red emission is that the Ce2 center has very low site symmetry which differentiates the energies of all its 5d orbitals. This raises the question about the mechanism leading to this effect. At first we have to remember that Ce3+ replacing Sr2+ in the host needs charge compensation. This may be achieved by two mechanisms: (i) interstitial S2- ion might compensate the excessive charge of two Ce3+ ions or (ii) a Sr2+ vacant site would compensate the higher charge of also two ions of Ce3+. Let us note, however, that either of these effects should also occur when the synthesis is performed at lower temperatures, around 1000 °C. Yet, only hightemperature sintering leads to the abnormal luminescence. Thus only at the high temperatures the Ce2 center is formed. Thus, we postulate that, whichever is the mechanism of the charge compensation, the high-temperature sintering allows the defects – Ce3+ ions and the charge compensating entities – to cluster into spatially correlated point defects. The schemes presenting possible arrangements of the defects within the postulated clusters are shown in Figure 8. Let us note that either of the two postulated above mechanisms of the excessive Ce3+ charge compensation leads to a significant distortion of the dopant local symmetry. Yet, the clustering is very much probable as during the high-temperature sintering the ions mobility is significant and thus the defects may attain thermodynamically preferred positions. This is much less probable at lower temperatures when mobility of the various entities is insignificant.38,39,40 Let us yet note that the clustering towards either [Ce3+-S2--Ce3+] or [Ce3+-


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VSrʹʹ-Ce3+] is reasonable also due to the electrostatic attraction of the constituent point defects. An important, plausible if not persuasive, support for the above conclusion on formation of either [Ce3+-S2--Ce3+] or [Ce3+- VSrʹʹ-Ce3+] clusters comes from experimental results on europium-activated sintered SrS:Eu2+ ceramics. Let us note that Eu2+ has ionic radius almost identical with Sr2+ (1.25 Å vs. 1.26 Å, respectively) and obviously the same charge. Thus, the electrostatic attraction of the point defects as the driving force of formation of their clusters is not present in this case.

Figure 8. Schematic presentation of the postulated clustered point defects leading to the generation of the Ce2 emitting center. The charge compensation occurs either by (top) interstitial S2- ion or (bottom) Sr2+ vacant site. As shown in Figure 9, in this composition, whether it is powder made at 1000 °C or ceramic sintered at 1700 °C, the emission is firmly positioned around 620 nm. Just a slight blue-shift of the emission band for ceramics can be seen. Excitation spectra in both cases (not shown) are very similar and agree with those reported in literature.3,6 Thus, in the case of Eu2+ activator sintering does not lead to any prominent variations in the phosphor properties.



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Consequently, this is the Ce3+ charge incompatibility with Sr2+ combined with a great masstransfer (ions mobility)

Figure 9. Normalized luminescence spectra of SrS:Eu powder heated at 1000 °C and ceramics sintered at 1700 °C. In both cases excitation at 435 nm was applied. at the high temperature of sintering (see Figure 2) which leads to the terrific transformation of SrS:Ce luminescent properties. Altogether, the data and our conclusions appear to form a consistent picture. Below we present yet another possible explanation of the peculiarities in luminescence of SrS:Ce ceramics. It is in fact not an alternative proposition but rather extension of what was already said. This is about the so called intervalence charge transfer (IVCT) transitions.41,42 This would require the presence of some Ce4+ ions, which at first does not seem likely due to the reducing atmosphere of the fabrication. On the other hand, the fraction of the oxidized cerium would not need to be high to make this postulate a rational course. Therefore, it seems also reasonable to consider if, assuming a presence of some Ce4+, the IVCT might be a conceivable spectroscopic process responsible for the long-wavelength luminescence of Ce2 in SrS:Ce sintered ceramics.


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The initial reason for this hypothesis is the following: On one hand, the almost black color of the ceramic samples (Figure 3) with higher Ce concentrations is typical of mixed valence compounds, where it is due to a broad IVCT absorption which decays to the ground state43, so that it can be taken as an indication of the possible presence of Ce4+. On the other hand, the anomalous emission of Ce3+ in elpasolites has recently been interpreted as an IVCT emission between Ce3+ and Ce4+ centers. Such anomalous emission is positioned at much lower energy and is much broader than the regular one. These are all features common with the present redIR emission in SrS:Ce sintered ceramics. Here we explore the above hypothesis, by making a semi-quantitative configuration coordinate diagram of the IVCT states of Ce3+/Ce4+ pairs in the SrS:Ce ceramics. We use a simple model for IVCT configuration coordinate diagrams44 and explored reasonable empirical data for the differences between Ce3+-S and Ce4+-S distances (a fraction of around 90% of the ionic radii difference between Ce3+ and Ce4+), for vibrational frequencies of the Ce3+ and Ce4+ defects (values between 200 and 250 cm−1)45, and for the energy levels of the Ce3+ centers (from the regular emission). The constructed diagram we present in Figure 10.



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Figure 10. Energy levels of postulated IVCT-related spectroscopic processes of the Ce3+/Ce4+ pair. f1d1 stands for Ce3+1 4f-5d electron excitation, f1d2 for Ce3+1 4f – Ce4+2 5d electron excitation, d2f1 for Ce3+2 5d – Ce4+1 4f electron de-excitation, and so on. The states of a Ce3+1–Ce4+2 and a Ce4+1–Ce3+2 represent configurations of a Ce3+/Ce4+ mixed-valence pair. In the first configuration Ce3+ is in the highest symmetry site 1 and Ce4+ is in the lowest symmetry site 2; in the second configuration the two ions switch sites. Obviously, an electron transfer from Ce3+ to Ce4+ from any of the two configurations leads to the other. The lowest, black lines correspond to the 2F5/2 and 2F7/2 states of Ce3+ in both pairs: Ce3+1 (2F5/2 )-Ce4+2 and Ce3+1 (2F7/2)-Ce4+2, which have their minimum in the left side, and Ce4+1–Ce3+2 (2F5/2) and Ce4+1-Ce3+2 (2F5/2), which have their minimum in the right side. The blue lines correspond to the three states of the 5d12g configuration of Ce3+ split by low symmetry fields in each pair; the Ce4+1-Ce3+2(5dt12g) states have a larger splitting than the Ce3+1(5dt12g)-Ce4+2 states. Finally, the green lines correspond to the split of 5de1g states of Ce3+. We have assumed that the ground state has a higher energy in the Ce4+1-Ce3+2 than in the Ce3+1-Ce4+2 configuration of the pair. Besides the regular transitions in Ce3+, the diagram allows for intervalence charge transfer transitions between Ce3+ and Ce4+. The arrows in the diagram represent the possible and in fact experimentally observed excitation/absorption and emission transitions. The broken lines represent non-radiative relaxations of the excited electron while the dotted vertical lines stand for radiative transitions with low intensity. For the next discussion, we have also assumed that photoinduced electron transfer between 4f orbitals of Ce3+1 and 5dt2g orbitals of Ce4+2 have a much higher probability than between 4f orbitals of Ce3+2 and 5dt2g orbitals of Ce4+1, which would have be related with the structures of sites 1 and 2. Now let us describe how the set of states presented in Fig. 10 can explain the experimental observations. 1) The 425 nm absorption excites only the regular emission (480 and 540 nm) in


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powders but both the regular and the anomalous emission (605 and 700 nm) in ceramics. The Ce4+ centers created at the high temperature used for ceramics are not present in powders; hence, only the regular emission is excited in powders. In ceramics, the IVCT diagram applies: the f1d1 absorption excites the regular d1f1 emission, but the high-energy component of the f2d2 absorption also excites the broad, intense d2f1 red-IR IVCT emission. 2) The 380 nm absorption only excites the anomalous emission. The f1d2 IVCT absorption decays mostly to Ce4+1-Ce3+2(5dt12g) and gives the broad, intense d2f1 red-IR IVCT emission with a double peak. Branching back to Ce3+1(5dt12g)-Ce4+2 during the nonradiative decay would take some time and it would probably be hindered by the fast decay. So only the red-IR emission would be excited. Thermal equilibrium would transfer some population to such state and some amounts of the regular emission would also show at high temperatures: this would explain the shoulders in the 425-550 nm on the red IR emission at RT, which disappear at 25K. 3) The excitation spectrum of the regular emission (480 nm), see Figures 5 and 6, has a peak at 425 nm, due to the regular 4f − 5dt2g absorption, and another at 280 nm, in same region of host absorption, perhaps due to regular 4f − 5deg absorption. This can be due to Ce3+1 centers far enough from any Ce4+. However, it also shows some week features around 350 nm. These can be due to a small fraction of branching to Ce3+1(5dt12g)-Ce4+2 after f1d2 IVCT excitation. 4) The excitation spectrum of the red-IR emission (650 nm) -see Figures 5 and 6- has main peaks at 385, 330 and 310nm. These can be IVCT excitations from Ce3+1 4f to the three components of Ce4+2 5dt2g in the f1d2 absorption. But it also shows a shoulder at 425 nm and a band at 520 nm. These could be due to lower and higher components of f2d2. 5). In Figure 7a it is shown that rising the temperature with 425nm excitation increases the red-IR emission vs. the regular blue emission. The barrier between Ce3+1(5dt12g)-Ce4+2 and Ce4+1-Ce3+2(5dt12g) can be small; the latter is populated by the 425 nm excitation and the second is the emitter of the red-IR luminescence; rising the temperature should increase the population transferred to



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the latter. 6) The red-IR luminescence excited with 520 nm lowers its intensity with increasing temperature. The lowest component of f2d2 (520 nm) populates Ce4+1Ce3+2(5dt12g), which is the red-IR emitter; rising temperature facilitates passing the barrier mentioned in 5), so that some population will give regular d1f1 emission instead of red-IR emission. Note that the above presented idea of the IVCT mechanism does not stay in contradiction but rather extends the discussed role of postulated [Ce3+-S2--Ce3+] or [Ce3+- VSrʹʹ-Ce3+] defects in the generation of the red luminescence in SrS:Ce sintered ceramics. The IVCT would, however, require that some of the Ce3+ ions in the clusters are oxidized to the Ce4+ state. CONCLUSIONS For the first time SrS:Ce3+ was prepared as sintered ceramic material at 1700 °C in reducing atmosphere of CO. An immense grain growth occurred during the processing and an efficient densification was achieved, although numerous pores and voids were still present in the final bodies. The spectroscopy of the SrS:Ce sintered ceramics greatly differed compared to the SrS:Ce powders made at 1000 ᵒC. The ceramics presented not only the regular blue-green luminescence peaking at 480 nm but also an intense and never reported emission located in red and infrared part of spectrum and peaking at 620 nm. The long-wavelength luminescence possessed all the characteristics of the Ce3+ ion luminescence. Its decay time at 25 K was 79 ns and the emission band was composed of two overlapping components separated by 2040 cm-1 – a characteristic value of the spin-orbit splitting of the 2F term into the 2F5/2 and 2F7/2 states. All these effects were observed for materials containing only 0.05% of Ce. The excitation spectrum of the long-wavelength luminescence differ entirely from the spectrum of the regular blue-green emission proving a greatly different, much lower, local symmetry of the new Ce2 center. While the classic blue-green emission was practically not quenched up to


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435 K, the red-IR luminescence experienced a noticeable thermal quenching already at room temperature. The anomalous luminescence was postulated to result from agglomerated point defects whose clustering is driven by high-temperature sintering and enormous mass transfer occurring then. In such a multifaceted center, [Ce3+-S2--Ce3+] or [Ce3+-VSrʹʹ-Ce3+], the Ce ions experience much lower symmetry of the surrounding than the regular center occupying Oh site. An intervalence charge transfer luminescence in such clusters was showed to be a conceivable spectroscopic process if only existence of some fraction of the activator as Ce4+ ion would be credible. A few years ago Smet et al.2 published a paper entitled “Luminescence in Sulfides: A Rich History and a Bright Future”. The luminescent properties of the SrS:Ce sintered ceramics we report here seem to nicely adhere with this projection. AUTHOR INFORMATION Corresponding Author *(D.K.) Telephone: +48 (71) 3757265. E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors wish to acknowledge Karolina Fiączyk for taking the SEM images. The research was supported by POIG.01.01.02-02-006/09 project co-funded by European Regional Development Fund within the Innovative Economy Program. Priority I, Activity 1.1. Subactivity 1.1.2, which is gratefully acknowledged.



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(39) Trojan-Piegza, J.; Zych, E.; Hölsä, J.; Niittykoski, J. Spectroscopic Properties of Persistent Luminescence Phosphors: Lu2O3:Tb3+,M2+(M = Ca, Sr, Ba). J. Phys. Chem. C 2009, 113, 20493-20498. (40) Kulesza, D.; Wiatrowska, A.; Trojan-Piegza, J.; Felbeck, T.; Geduhn, R.; Motzek, P.; Zych, E.; Kynast, U. The Bright Side Of Defects: Chemistry and Physics of Persistent and Storage Phosphors. J. Lumin. 2013, 133, 51-56. (41) Seijo, L.; Barandiarán, Z. Intervalence Charge Transfer Luminescence: The Anomalous Luminescence of Cerium-Doped Cs2LiLuCl6 Elpasolite. J. Chem. Phys. 2014, 141, 214706. (42) Barandiarán, Z., Seijo, L. Intervalence Charge Transfer Luminescence: Interplay Between Anomalous and 5d − 4f Emissions in Yb-Doped Fluorite-Type Crystals. J. Chem. Phys. 2014, 141, 234704. (43) Blasse, G. Optical Electron Transfer Between Metal Ions and Its Consequences. Struct. & Bond. 1991, 76, 153-187. (44) Barandiarán, Z.; Meijerink, A.; Seijo, L. Configuration Coordinate Energy Level Diagrams of Intervalence and Metal-to-Metal Charge Transfer States of Dopant Pairs in Solids. Phys. Chem. Chem. Phys. 2015, 17, 19874-19884. (45) Yamashita, N.; Harada, O.; Nakamura, K. Photoluminescence Spectra of Eu2+ Centers in Ca(S, Se):Eu and Sr(S, Se):Eu. Jpn. J. Appl. Phys. 1995, 34, 5539-5545.


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TOC



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X-ray powder diffraction patterns of SrS:Ce sintered at 1700 °C ceramics and SrS:Ce powder after heattreatment at 1000 °C. Simulated XRD pattern of cubic SrS according to ICSD #28900. 76x59mm (300 x 300 DPI)


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SEM images of SrS:Ce a) powder heated at 1000 °C; b) ceramics sintered at 1700 °C. 114x171mm (300 x 300 DPI)



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Photographs presenting changes in the appearance of the SrS:0.5%Ce powder upon its sintering at 1600 and 1700 °C (top raw) and the ceramics containing different Ce concentration sintered at 1700 °C (bottom). 82x69mm (300 x 300 DPI)


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Normalized luminescence spectra of SrS:Ce powder heated at 1000 °C and ceramics sintered at 1700 °C. In both cases excitation at 425 nm was applied. 76x58mm (300 x 300 DPI)



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RT normalized excitation spectra of 480 nm and 650 nm emissions (solid lines) and emission spectra excited at 380 nm and 425nm wavelength (dotted lines) of SrS:Ce ceramics. 76x61mm (300 x 300 DPI)


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Normalized low temperature (25 K) emission and excitation spectra revealing existence of two different Ce3+ luminescence centers in SrS:Ce ceramics sintered at 1700 °C. 76x61mm (300 x 300 DPI)



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Temperature dependence of (a) luminescence spectra excited at 425 nm and normalized at 480 nm; (b) luminescence spectra excited at 280 nm; decay time traces excited at (c) 425 nm and monitored at 485 nm; (d) 520 nm and monitored at 650 nm. 84x67mm (300 x 300 DPI)


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Schematic presentation of the postulated clustered point defects leading to the generation of the Ce2 emitting center. The charge compensation occurs either by (top) interstitial S2- ion or (bottom) Sr2+ vacant site. 56x89mm (300 x 300 DPI)



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Normalized luminescence spectra of SrS:Eu powder heated at 1000 °C and ceramics sintered at 1700 °C. In both cases excitation at 435 nm was applied. 76x57mm (300 x 300 DPI)


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Energy levels of postulated IVCT-related spectroscopic processes of the Ce3+/Ce4+ pair. f1d1 stands for Ce3+1 4f-5d electron excitation, f1d2 for Ce3+1 4f – Ce4+2 5d electron excitation, d2f1 for Ce3+2 5d – Ce4+1 4f electron de-excitation, and so on. 75x92mm (300 x 300 DPI)


Anomalous Red and Infrared Luminescence of Ce3+ Ions in SrS:Ce

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