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Composition dependent spectral shift of Mn4+ luminescence in silicate garnet hosts CaY2M2Al2SiO12 (M = Al, Ga, Sc)
Journal of Luminescence 198 (2018) 314–319
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Composition dependent spectral shift of Mn4+ luminescence in silicate garnet hosts CaY2M2Al2SiO12 (M = Al, Ga, Sc) T. Jansena, T. Jüstela, M. Kirmb, S. Vielhauerb, N.M. Khaidukovc, V.N. Makhovd,
T
⁎
a
Münster University of Applied Sciences, Stegerwaldstraße 39, 48565 Steinfurt, Germany Institute of Physics, University of Tartu, W. Ostwald Str. 1, 50411 Tartu, Estonia c N. S. Kurnakov Institute of General and Inorganic Chemistry, 31 Leninskiy Prospekt, 119991 Moscow, Russia d P. N. Lebedev Physical Institute, 53 Leninskiy Prospekt, 119991 Moscow, Russia b
A R T I C L E I N F O
A B S T R A C T
Keywords: Garnets Photoluminescence Mn4+ phosphors White LED Thermal quenching
Multi-component silicate garnet ceramics CaY2M2Al2SiO12 having different cations M = Al, Ga or Sc at octahedral sites doped with Mn4+ ions have been synthesized and studied as possible red-emitting phosphors for warm white pc-LED applications. The short-wavelength shift of Mn4+ emission spectrum has been obtained with increasing the radius of the cation at octahedral sites as a result of decreasing the covalence of the “Mn4+ligand” bonding, which leads to an increase of the energy of the emitting Mn4+ 2Eg level. However, simultaneously, stronger thermal quenching of Mn4+ luminescence is observed with the increase of the cation radius because of decreasing the energy of the 4T2g level as well as the O2- – Mn4+ CT state, leading to the decrease of activation barrier for thermal quenching due to non-radiative relaxation from the emitting Mn4+ 2Eg level to the Mn4+ ground state. All studied phosphors demonstrate rather poor thermal stability of Mn4+ luminescence with thermal quenching temperatures T1/2 < 240 K.
1. Introduction Phosphor-converted light emitting diodes (pc-LEDs) have presently no alternatives as light sources for general lighting due to their superior performances compared to any other kinds of white light sources [1]. Most of commercially available white pc-LEDs are based on the simplest and cost efficient scheme consisting of a blue LED chip and a yellowemitting phosphor, such as the Ce3+ doped garnet Y3Al5O12:Ce3+ (YAG:Ce). However, the main drawbacks of such white-light sources are a low color-rendering index (CRI) and a high correlated color temperature (CCT) due to the lack of a sufficient red component in the spectrum, which makes white light from such sources bluish, or “cold”, i.e. uncomfortable for human eyes. Accordingly, nowadays one of the challenging tasks in general lighting is the development of pc-LEDs emitting “warm” white light. This problem can be optimally resolved by adding to the above scheme of white pc-LED a red-emitting phosphor efficiently excited by blue LED chip radiation. Thus, the development of red emitting phosphors is of tremendous importance for the further optimization of solid-state light sources for indoor illumination applications. To achieve the optimum balance between high CRI and efficacy in white pc-LED devices, it is needed to explore narrow-band red-emitting phosphors, which strongly absorb
⁎
blue light and efficiently emit in red spectral range of 610–650 nm. In contrast to commercial red-emitting phosphors, which typically possess a broad emission band from Eu2+ ions [2], Mn4+ doped phosphors can exhibit an emission spectrum consisting of narrow lines only in the red spectral range, without a useless tail to longer (NIR) wavelengths, where human eyes are not sensitive. Since Mn4+ can be stabilized only in octahedral coordination the energy levels of Mn4+ are given by the Tanabe-Sugano diagram for the d3 electron configuration in an octahedral crystal field [3]. Accordingly, Mn4+ ions in crystal hosts have broad absorption bands in the blue and UV spectral ranges due to relatively strong spin-allowed transitions 4A2 → 4T2 and 4A2 → 4T1, respectively, and narrow-line emission due to spin-forbidden transition 2 Eg → 4A2g. By now a rather large number of Mn4+ red phosphors have been developed [4–9], which are based mainly on fluoride hosts because of relatively simple and efficient fabrication process of such phosphors and due to the fact that Mn4+ emission in fluorides is located close to optimal wavelength for the red phosphor near 630 nm. However, the stability of fluoride phosphors is not as good as oxide phosphors which generally have a better resistance to external conditions such as high temperature, high humidity, and high excitation density provided by high-power LEDs, i.e. in many cases fluoride phosphors are not suitable
Corresponding author. E-mail address: [email protected] (V.N. Makhov).
https://doi.org/10.1016/j.jlumin.2018.02.054 Received 30 November 2017; Received in revised form 9 February 2018; Accepted 20 February 2018 Available online 21 February 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.
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for practical applications. Mn4+ activated oxides are much more stable but their emission wavelengths are too long compared to optimal red emission. So far, the shortest peak wavelengths of temperature stable Mn4+ emission in oxide materials have been obtained for Mg14Ge5O24:Mn4+ (659 nm), Y2Mg3Ge3O12:Mn4+ (658 nm), CaAl12O19:Mn4+ (656 nm), SrAl12O19:Mn4+ (655 nm), and Sr4Al14O25:Mn4+ (654 nm) [9–13]. Hence, the currently known Mn4+ activated oxide phosphors are unsuitable for general lighting applications. It is commonly accepted that the energy of the Mn4+ 2Eg emitting level does not depend on the strength of the crystal field (CF) and is determined by the effect of delocalization of the outer d orbitals of Mn4+ ions because of formation of chemical bonds with ligands (nephelauxetic effect). The Mn4+ emission energy is inversely correlated with the Mn4+ - ligand delocalization degree, i.e. it is higher for more ionic bonding and is lower for more covalent bonding. As a result, the host materials allowing longer Mn4+ - ligand distance can provide blue shifted Mn4+ emission. Thus, one can expect that by increasing the ionic radius of the host cation at the octahedral site in a series of isostructural multicomponent compounds, i.e. by increasing the interionic distance between Mn4+ ions and ligands it will be possible to tune the Mn4+ emission spectrum to shorter wavelengths, which is required for developing narrow-band red-emitting phosphors for new generation of white pc-LEDs. In the present work a series of multi-component silicate garnet ceramics with different cations in octahedral sites of CaY2M2Al2SiO12 (M = Al, Ga, Sc) doped with Mn4+ ions has been synthesized and studied as possible red phosphors for pc-LED applications.
100 (420)
Ga Al
36,5
37,0
(840)
(800)
(444)
(611)
(640) (642)
36,0
(721)
(510)
(321)
20
35,5
(521)
40
(422)
(400)
60
(211)
Relative Intensity (arb. units)
Sc 80
0 10
20
30
40
50
60
70
2θ (degree) Fig. 1. XRD pattern of CaY2Sc2Al2SiO12:Mn4+; numbers on the graph represent the Miller indices, (hkl). In the insertion the peak positions of XRD (422) reflexes for studied garnets CaY2Al2Al2SiO12:Mn4+ (1), CaY2Ga2Al2SiO12:Mn4+ (2) and CaY2Sc2Al2SiO12:Mn4+ (3) are shown in an extended scale.
0.1 nm. The recorded PL spectra were corrected by applying a spectral sensitivity function obtained from a tungsten incandescent lamp certified by the National Physics Laboratory, UK. Temperature dependent PL measurements from 80 to 500 K were performed using an Oxford Instruments cryostat MicrostatN2, where liquid nitrogen was applied as the cooling agent. Typical temperature stabilization time was 60 s and accuracy of maintaining temperature was ± 3 K. PL measurements at 3 K were performed in a closed cycle cryocooler Optistat AC-V12 from Oxford Instruments, where helium was used as the cooling agent.
2. Experimental Ceramic samples of CaY2M2Al2SiO12 (M = Al, Ga, Sc) doped with Mn4+ were obtained under thermal treatment in ambient air atmosphere by using precursors synthesized under hydrothermal conditions. For hydrothermal synthesis of undoped samples copper-insert lined autoclaves with a volume of ~40 cm3 were utilized. Grinded mixtures of oxides for appropriate stoichiometries were subjected to hydrothermal treatment during 100 h in a 0.1 mol% NaOH aqueous solution under autogenous pressure at ~ 450 °C, with a filling degree of 50%. The resulting precursors were washed with distilled water and ethanol and thereafter oven-dried at 1000 °C for 4 h in air. The hydrothermal garnet powders were mixed with KMnO4 (1.5–0.1 mol%) and the compensative oxides (e.g. MgO) by taking into account that Mn4+ ions occupy the octahedral site in the garnet structure, namely M3+ + M3+ → Mg2+ + Mn4+, and K+ ions will be removed as a result of the following multi-stage thermal treatment as described below. The blends were uniaxially pressed into pellets with a diameter of 10 mm and a thickness of 3 – 5 mm to promote intimate contact and the pellets were heated at 500 °C for 24 h in air. Subsequently, the samples were thoroughly ground, re-pelletized and sintered at 1150 °C for 8 h in air. Finally, the samples were again ground into finely dispersed powders, re-pelletized and sintered at 1300 °C for 8 h in air. Eventually, pellets of synthesized ceramic phosphors were polished for later characterization. The structure type and phase purity of the synthesized samples were characterized with conventional powder X-ray diffraction (XRD) technique and powder XRD patterns were obtained by using a Bruker D8 Advance X-Ray powder diffractometer with Cu Kα radiation. Identification of synthesized compounds, indexing of X-ray powder diffraction patterns and refinement of unit cell parameters were performed with the Diffrac. Suite. EVA software (Bruker). Unit cell parameters were determined with an accuracy of around 0.001 Å. Photoluminescence (PL) as well as PL excitation (PLE) spectra were recorded on an Edinburgh Instruments FLS920 spectrometer equipped with a Xenon arc lamp (450 W) and a cooled (− 20 °C) single-photon counting photomultiplier (Hamamatsu R2658P). Spectral resolution for the PL and PLE spectra measurements was typically set to value of
3. Results and discussion The XRD patterns have proved that synthesized samples have the garnet crystal structure without traces of any impurity phases (Fig. 1). With increasing ionic radius of the cation at the octahedral site the diffraction reflexes shift to smaller angles, and the unit cell volume as well as the interionic distance in the octahedral site increase, as expected (see Table 1). Synthesized Mn4+ doped garnet phosphors possess typical Mn4+ PL spectrum in the red spectral range, which is dominated by the Stokes vibronic sidebands of the 2Eg → 4A2 g transition of the Mn4+ ion (Fig. 2). The peak wavelength of Mn4+ PL spectrum shifts to shorter wavelengths with the increase of the octahedral cation size, as predicted (Table 2), and at 300 K the shortest peak wavelength obtained for Sc-based compound is 658 nm, still in rather deep red region. Simultaneously, the long-wavelength shift of excitation bands due to spin-allowed Mn4+ transitions as well as owing to the O2- – Mn4+ charge transfer (CT) transition occurs. At low temperature, the PL spectrum of the CaY2Al4SiO12:Mn4+ phosphor shows well-resolved fine structure (Fig. 3a) with a zerophonon line (ZPL) corresponding to pure electronic transition 2Eg → 4 A2g and a set of Stokes vibronic sidebands. Also the bands in the PLE spectrum (Fig. 3b) corresponding to the second spin-allowed Mn4+ transition 4A2 → 4T1 and the O2- – Mn4+ CT transition are much better resolved. Table 1 Lattice parameters of garnets CaY2M2Al2SiO12 (M = Al, Ga, Sc).
315
Compound
a (Ǻ)
V (Ǻ3)
RM3+ (Ǻ) (CN = 6)
R(M3+-O2-) (Ǻ)
CaY2Al2Al2SiO12 CaY2Ga2Al2SiO12 CaY2Sc2Al2SiO12
11.959 12.053 12.302
1710.3 1751.0 1861.8
0.535 0.620 0.745
1.922 1.939 1.975
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T. Jansen et al.
1,0
Sc
Al
Sc
Relative Intensity (arb. units)
CaY 2 M 2 Al2 SiO 12:Mn 0,8
M = Al, Ga, Sc
2
4
E g - A 2g
T = 300 K
0,6
4
0,4
CT 0,2
Al
4+
2-
4
A 2g - T 2g Sc
Al
O - Mn
4+
0,0 300
400
500
600
700
800
W avelength (nm) Fig. 2. Normalized PL and PLE spectra of CaY2M2Al2SiO12:Mn4+(1.5 mol-%) (M = Al, Ga, Sc) at 300 K.
Fig. 4. Normalized PL spectra of CaY2Al4SiO12:Mn4+ (1), CaY2Ga2Al2SiO12:Mn4+ (2), CaY2Sc2Al2SiO12:Mn4+ (3) and Ca2YSc2Ga2Si2O12:Mn4+ (4) measured at 3 K.
Table 2 Peak wavelengths of emission and excitation bands in the PL and PLE spectra of Mn4+ in garnets CaY2M2Al2SiO12 (M = Al, Ga, Sc) at 300 K. Compound
RM3+ (Ǻ) (CN = 6)
λem (nm)
λex (4T2g) (nm)
λex (CT) (nm)
CaY2Al2Al2SiO12 CaY2Ga2Al2SiO12 CaY2Sc2Al2SiO12
0.535 0.620 0.745
674 668 658
481 500 502
341 343 353
cationic composition (whose spectrum is also shown in Fig. 4) the revealed broadening is so large that the emission spectrum has a Gaussian-like shape. This inhomogeneous broadening can be explained by structural disorders caused by statistical distribution of different cations at the dodecahedral, octahedral and tetrahedral crystallographic sites in the garnet structure. Due to this feature both local phonon frequencies and energies of Mn4+ electronic states vary from site to site resulting in broadening of the emission spectra. The low temperature emission spectra show that although ZPL in Mn4+ emission spectrum is at the highest energy for the Sc3+ based compound (as expected), the energy of the ZPL is actually larger for the host with M = Al than M = Ga. This feature can be due to the fact that Sc3+ ions show a strong tendency to occupy only the octahedral sites in the garnet lattice, whereas Ga3+ ions can substitute for Al3+ ions not only in octahedral but also in tetrahedral sites. The different cationic substitution in the tetrahedral site neighbor to the Mn4+ ion entering the octahedral site can differently affect the distortion of the crystalfield symmetry around Mn4+, which is known influences strongly the energy of the 2Eg state (see, e.g. [16]). Similar feature of the spectral shift of emission spectra was observed in [17] for Ce3+ ions doped into the same series of garnet hosts. The concentration dependence of Mn4+ PL spectra studied for the CaY2Al4SiO12:Mn4+ phosphor has shown that there are no significant
Two ZPLs are clearly observed in the PL spectrum of CaY2Al4SiO12:Mn4+ with the energy gap between ZPLs about 16 cm−1. This value is comparable to e.g. YAG:Mn4+ with a gap of ~ 19 cm−1 obtained in [14], where this feature was interpreted as being due to luminescence of Mn4+ ions entering the ‘regular’ and ‘distorted’ octahedral sites in the garnet structure. However, for the clarification of the real nature of this fine structure the careful analysis is required [15], which cannot be done only on the basis of this observation. On the other hand, our investigation shows that Mn4+ PL spectra from Ga- and Sc-containing garnets possess a broadened structure even at low temperature (Fig. 4) although the approximate position of respective ZPLs can be well identified in the spectra. For the multi-component garnet Ca2YScGa2Si2O12:Mn4+ with even more complicated
Fig. 3. (a) Normalized PL spectra of CaY2Al4SiO12:Mn4+(1.5 mol.%) at 300 K (excitation at 341 nm) and 3 K (excitation at 327 nm); in the insertion the fine structure of zero-phonon line is shown in an enlarged scale; (b) Normalized PLE spectra of Mn4+ luminescence in CaY2Al4SiO12:Mn4+(1.5 mol.%) at 300 K (monitored at 675 nm) and 3 K (monitored at 662 nm).
316
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Fig. 7. Temperature dependences of integral luminescence intensities for CaY2M2Al2SiO12:Mn4+(1.5 mol.%) (M = Sc, Ga, Al) phosphors. Dots are experimental data. Curves are Fermi - Dirac fits.
Fig. 5. Dependence of integral luminescence intensity of CaY2Al4SiO12:Mn4+ phosphors on the concentration of doping Mn4+ ions. Excitation wavelength is 330 nm. T = 300 K.
versus temperature (Fig. 7) in accordance with the well-known Fermi-
(
changes of the shapes of PL and PLE spectra with the doping concentration. The optimal Mn4+ concentration, for which the maximum intensity of Mn4+ luminescence is observed, is about 0.2 mol-% (Fig. 5). The concentration dependence of Mn4+ luminescence intensity, with maximal light output for 0.2 mol-% Mn4+, looks unusual. Typically, Mn4+ luminescence is not quenched up to concentration of few percent [18]. The possible explanation of such specific dependence is the increase of concentration of quenching centers with increasing doping level. The quenching centers can be both crystal structure defects and Mn ions in different valence states. Accordingly, optimization of synthesis conditions is very important for developing efficient Mn4+ phosphors. All studied phosphors demonstrate rather poor thermal stability of Mn4+ luminescence with thermal quenching temperature, determined as the temperature at which the emission intensity is decreased by 50% of its maximal value, T1/2 < 240 K. At room temperature the emission intensity is already strongly quenched as can be seen in Fig. 6 where PL spectra measured at different temperatures are presented for the most thermally stable CaY2Al4SiO12:Mn4+ phosphor. The values of the activation energies for thermal quenching have been obtained from fitting of experimental dependencies of the integral luminescence intensity
Dirac model: I (T ) = I0/ 1 + A∙exp
( ) ), where I Ea kB T
0
is luminescence
intensity at T = 0 K, A characterizes the rate of thermal quenching, kB is the Boltzmann constant, and Ea is the energy barrier (activation energy) for thermal quenching. It is clearly seen (see Table 3) that activation energies Ea and thermal quenching temperatures T1/2 decrease with the increase of ionic radius of the host cation M3+ in the octahedral site (or with the increase of the respective inter-ionic distance). Generally, two mechanisms of thermal quenching of Mn4+ luminescence have been proposed so far. The first one is connected with a crossover of the emitting Mn4+ 2Eg state to the Franck-Condon shifted Mn4+ 4T2g level (Fig. 8a) as has been studied e.g. for YAG:Mn4+ [6]. The second one is related to a crossover of the emitting Mn4+ 2Eg state to the Franck-Condon shifted and low-lying O2- – Mn4+ CT state (Fig. 8b), which has been considered as a quenching mechanism in some Mn4+ doped stannates [19]. The observed decrease of T1/2 with the ionic radius of the cation M3+ correlates with both experimental findings: lowering the energy of the Mn4+ 4T2g state being obviously caused by the decrease of the CF strength with the increase of inter-ionic distance, and the decrease of the energy of the O2- - Mn4+ CT state. Since each of these excited states can serve as a quencher for Mn4+ luminescence it is not possible to establish the dominant mechanism of thermal quenching in the studied phosphors from such simple qualitative consideration. Generally speaking, some other processes can be also considered as possible mechanisms of thermal quenching, in particular, multi-phonon relaxation from the excited 2E state to the 4A2 ground state. However, for this process the energy gap ~ 15300 cm−1 between these states should be bridged with emission of several tens phonons (with typical energy of several hundreds of cm−1). It looks unreal that such a process can compete with radiative transition between these states. Also, our experimental results demonstrate that thermal quenching occurs at different temperatures in different hosts (with M = Sc, Ga, Al) with almost equal energies of the 2E state and obviously rather similar Table 3 Activation energies of thermal quenching Ea and thermal quenching temperatures T1/2 of Mn4+ luminescence from garnets CaY2M2Al2SiO12:Mn4+ (M = Al, Ga, Sc).
Fig. 6. Temperature dependence of PL spectrum of CaY2Al4SiO12:Mn4+(1.5 mol.%). Excitation wavelength is 341 nm.
317
Compound
Ea (eV)
T1/2 (K)
CaY2Al2Al2SiO12 CaY2Ga2Al2SiO12 CaY2Sc2Al2SiO12
0.174 ± 0.011 0.111 ± 0.020 0.065 ± 0.004
239.7 ± 2.0 224.5 ± 6.8 159.4 ± 3.0
Journal of Luminescence 198 (2018) 314–319
T. Jansen et al.
Fig. 8. Two mechanisms of thermal quenching of Mn4+ luminescence.
Mn4+ CT transition. Accordingly, for the calculation of the Racah parameter B it is necessary to use the barycenter of all energy levels of the 4T1 state [24], which could result in other values of the Racah parameters. However, the consideration of this problem is out of the scope of the present paper.
phonon spectra, which is not in favor of multi-phonon relaxation mechanism. Moreover, the lowest thermal quenching temperature in the series of studied materials is observed for the phosphor CaY2Sc2Al2SiO12:Mn4+ with the highest energy of the emitting 2E state. Another possible process which can be responsible for thermal quenching is thermally stimulated ionization of a 3d electron from the Mn4+ 2E state to the host conduction band, the mechanism, which was widely discussed for thermal quenching of luminescence due to 5d – 4f transitions in rare earth ions (see, e.g. [20]), in particular for YAG:Ce3+ type phosphors [21]. However, unfortunately, there is no data available in the literature on the positions of Mn4+ energy levels with respect to the host energy bands. Accordingly, a possible influence of thermally stimulated ionization on thermal quenching of Mn4+ luminescence cannot be analyzed on the basis of obtained results and will be a future subject of studies. Using obtained experimental data on Mn4+ luminescence and the standard formulas [22], CF strength parameter Dq as well as Racah parameters B and C have been calculated for the Mn4+ ion in the studied silicate garnet phosphors (Table 4). For these calculations the peak energies of 4T2 and 4T1 bands in the excitation spectra have been used, which were determined by decomposition of excitation spectra measured at 3 K into multiple Gaussian functions in the linear energy scale. Also the nephelauxetic parameter β1 = (B / B0)2 +(C / C0 )2 (B0 and C0 are the Racah parameters of the Mn4+ ion in a free state) has been calculated, the value which was introduced recently for the characterization of luminescence energy of transition metals in crystals [23]. As can be seen in Table 4, in the studied silicate garnets the Mn4+ ions show smaller values of parameter B and larger values of parameter C compared to those in many other hosts (see data collection on Mn4+ Racah parameters in different hosts in [23]). The possible reason of this feature can be splitting of the 4T1 state due to strong distortion of the octahedral environment of Mn4+ ions, the higher-energy component of this splitting being hidden under the broad band ascribed here to O2- -
4. Conclusions Monophase silicate garnet ceramics with the composition of CaY2M2Al2SiO12:Mn4+ (M = Al, Ga, Sc) have been successfully obtained with high-temperature solid-state reactions using precursors synthesized under hydrothermal conditions. It has been shown that it is possible to obtain a blue shift of the Mn4+ emission spectrum by increasing the radius of the cations on the octahedral sites in the garnet structure. Such a shift is a result of decreasing covalence of the “Mn4+ligand” bonding, which leads to an increase of the energy of the emitting Mn4+ 2Eg state. The shortest-wavelength emission of Mn4+ with a peak wavelength at 658 nm (at 300 K) has been obtained for the CaY2Sc2Al2SiO12 garnet host with the largest radius of the cation (Sc3+) at the octahedral site. However, the energies of the excited 4T2g state and the O2- – Mn4+ CT state are decreasing with the increase of the host cation radius, resulting in stronger thermal quenching of the Mn4+ luminescence. In this context, thermal quenching temperatures of Mn4+ luminescence (T½ < 240 K) in the studied phosphors CaY2M2Al2SiO12:Mn4+ (M = Al, Ga, Sc) are too low for potential pcLED application. High energy of both the Mn4+ 4T2g state and the O2- – Mn4+ CT state is crucial for obtaining a high thermal stability of the emission of Mn4+ phosphors. The inhomogeneous broadening of Mn4+ luminescence spectra observed in the studied multi-component garnet hosts can be explained by the structural disorder caused by statistical distribution of heterogeneous cations (Ca, Y, Al, Si, Ga, Sc) at the host lattice sites. Acknowledgements
Table 4 Crystal field strength (Dq), Racah (B and C) and β1 parameters of Mn4+ in garnets CaY2M2Al2SiO12 (M = Al, Ga, Sc). Compound
Dq (cm−1)
B (cm−1)
C (cm−1)
ZPL (2Eg) (cm−1)
β1
CaY2Al2Al2SiO12 CaY2Ga2Al2SiO12 CaY2Sc2Al2SiO12
2066 2012 1992
418 442 463
3969 3903 3855
15258 15221 15347
0.990 0.984 0.981
This research was performed within the ERA.Net RUS Plus Programme, project NANOLED # 361. Financial support from Russian Foundation for Basic Research (Grants 16-52-76027 ERA_а and 16-5376027 ERA_а), Estonian Research Council and BMBF (Germany) is gratefully acknowledged by the Moscow, Tartu and Münster groups. The Tartu group also acknowledges financial support from the ERDF funding in Estonia granted to the Center of Excellence TK141 (project No. 2014-2020.4.01.15-0011). 318
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Composition dependent spectral shift of Mn4+ luminescence in silicate garnet hosts CaY2M2Al2SiO12 (M = Al, Ga, Sc) T. Jansena, T. Jüstela, M. Kirmb, S. Vielhauerb, N.M. Khaidukovc, V.N. Makhovd,
T
⁎
a
Münster University of Applied Sciences, Stegerwaldstraße 39, 48565 Steinfurt, Germany Institute of Physics, University of Tartu, W. Ostwald Str. 1, 50411 Tartu, Estonia c N. S. Kurnakov Institute of General and Inorganic Chemistry, 31 Leninskiy Prospekt, 119991 Moscow, Russia d P. N. Lebedev Physical Institute, 53 Leninskiy Prospekt, 119991 Moscow, Russia b
A R T I C L E I N F O
A B S T R A C T
Keywords: Garnets Photoluminescence Mn4+ phosphors White LED Thermal quenching
Multi-component silicate garnet ceramics CaY2M2Al2SiO12 having different cations M = Al, Ga or Sc at octahedral sites doped with Mn4+ ions have been synthesized and studied as possible red-emitting phosphors for warm white pc-LED applications. The short-wavelength shift of Mn4+ emission spectrum has been obtained with increasing the radius of the cation at octahedral sites as a result of decreasing the covalence of the “Mn4+ligand” bonding, which leads to an increase of the energy of the emitting Mn4+ 2Eg level. However, simultaneously, stronger thermal quenching of Mn4+ luminescence is observed with the increase of the cation radius because of decreasing the energy of the 4T2g level as well as the O2- – Mn4+ CT state, leading to the decrease of activation barrier for thermal quenching due to non-radiative relaxation from the emitting Mn4+ 2Eg level to the Mn4+ ground state. All studied phosphors demonstrate rather poor thermal stability of Mn4+ luminescence with thermal quenching temperatures T1/2 < 240 K.
1. Introduction Phosphor-converted light emitting diodes (pc-LEDs) have presently no alternatives as light sources for general lighting due to their superior performances compared to any other kinds of white light sources [1]. Most of commercially available white pc-LEDs are based on the simplest and cost efficient scheme consisting of a blue LED chip and a yellowemitting phosphor, such as the Ce3+ doped garnet Y3Al5O12:Ce3+ (YAG:Ce). However, the main drawbacks of such white-light sources are a low color-rendering index (CRI) and a high correlated color temperature (CCT) due to the lack of a sufficient red component in the spectrum, which makes white light from such sources bluish, or “cold”, i.e. uncomfortable for human eyes. Accordingly, nowadays one of the challenging tasks in general lighting is the development of pc-LEDs emitting “warm” white light. This problem can be optimally resolved by adding to the above scheme of white pc-LED a red-emitting phosphor efficiently excited by blue LED chip radiation. Thus, the development of red emitting phosphors is of tremendous importance for the further optimization of solid-state light sources for indoor illumination applications. To achieve the optimum balance between high CRI and efficacy in white pc-LED devices, it is needed to explore narrow-band red-emitting phosphors, which strongly absorb
⁎
blue light and efficiently emit in red spectral range of 610–650 nm. In contrast to commercial red-emitting phosphors, which typically possess a broad emission band from Eu2+ ions [2], Mn4+ doped phosphors can exhibit an emission spectrum consisting of narrow lines only in the red spectral range, without a useless tail to longer (NIR) wavelengths, where human eyes are not sensitive. Since Mn4+ can be stabilized only in octahedral coordination the energy levels of Mn4+ are given by the Tanabe-Sugano diagram for the d3 electron configuration in an octahedral crystal field [3]. Accordingly, Mn4+ ions in crystal hosts have broad absorption bands in the blue and UV spectral ranges due to relatively strong spin-allowed transitions 4A2 → 4T2 and 4A2 → 4T1, respectively, and narrow-line emission due to spin-forbidden transition 2 Eg → 4A2g. By now a rather large number of Mn4+ red phosphors have been developed [4–9], which are based mainly on fluoride hosts because of relatively simple and efficient fabrication process of such phosphors and due to the fact that Mn4+ emission in fluorides is located close to optimal wavelength for the red phosphor near 630 nm. However, the stability of fluoride phosphors is not as good as oxide phosphors which generally have a better resistance to external conditions such as high temperature, high humidity, and high excitation density provided by high-power LEDs, i.e. in many cases fluoride phosphors are not suitable
Corresponding author. E-mail address: [email protected] (V.N. Makhov).
https://doi.org/10.1016/j.jlumin.2018.02.054 Received 30 November 2017; Received in revised form 9 February 2018; Accepted 20 February 2018 Available online 21 February 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.
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for practical applications. Mn4+ activated oxides are much more stable but their emission wavelengths are too long compared to optimal red emission. So far, the shortest peak wavelengths of temperature stable Mn4+ emission in oxide materials have been obtained for Mg14Ge5O24:Mn4+ (659 nm), Y2Mg3Ge3O12:Mn4+ (658 nm), CaAl12O19:Mn4+ (656 nm), SrAl12O19:Mn4+ (655 nm), and Sr4Al14O25:Mn4+ (654 nm) [9–13]. Hence, the currently known Mn4+ activated oxide phosphors are unsuitable for general lighting applications. It is commonly accepted that the energy of the Mn4+ 2Eg emitting level does not depend on the strength of the crystal field (CF) and is determined by the effect of delocalization of the outer d orbitals of Mn4+ ions because of formation of chemical bonds with ligands (nephelauxetic effect). The Mn4+ emission energy is inversely correlated with the Mn4+ - ligand delocalization degree, i.e. it is higher for more ionic bonding and is lower for more covalent bonding. As a result, the host materials allowing longer Mn4+ - ligand distance can provide blue shifted Mn4+ emission. Thus, one can expect that by increasing the ionic radius of the host cation at the octahedral site in a series of isostructural multicomponent compounds, i.e. by increasing the interionic distance between Mn4+ ions and ligands it will be possible to tune the Mn4+ emission spectrum to shorter wavelengths, which is required for developing narrow-band red-emitting phosphors for new generation of white pc-LEDs. In the present work a series of multi-component silicate garnet ceramics with different cations in octahedral sites of CaY2M2Al2SiO12 (M = Al, Ga, Sc) doped with Mn4+ ions has been synthesized and studied as possible red phosphors for pc-LED applications.
100 (420)
Ga Al
36,5
37,0
(840)
(800)
(444)
(611)
(640) (642)
36,0
(721)
(510)
(321)
20
35,5
(521)
40
(422)
(400)
60
(211)
Relative Intensity (arb. units)
Sc 80
0 10
20
30
40
50
60
70
2θ (degree) Fig. 1. XRD pattern of CaY2Sc2Al2SiO12:Mn4+; numbers on the graph represent the Miller indices, (hkl). In the insertion the peak positions of XRD (422) reflexes for studied garnets CaY2Al2Al2SiO12:Mn4+ (1), CaY2Ga2Al2SiO12:Mn4+ (2) and CaY2Sc2Al2SiO12:Mn4+ (3) are shown in an extended scale.
0.1 nm. The recorded PL spectra were corrected by applying a spectral sensitivity function obtained from a tungsten incandescent lamp certified by the National Physics Laboratory, UK. Temperature dependent PL measurements from 80 to 500 K were performed using an Oxford Instruments cryostat MicrostatN2, where liquid nitrogen was applied as the cooling agent. Typical temperature stabilization time was 60 s and accuracy of maintaining temperature was ± 3 K. PL measurements at 3 K were performed in a closed cycle cryocooler Optistat AC-V12 from Oxford Instruments, where helium was used as the cooling agent.
2. Experimental Ceramic samples of CaY2M2Al2SiO12 (M = Al, Ga, Sc) doped with Mn4+ were obtained under thermal treatment in ambient air atmosphere by using precursors synthesized under hydrothermal conditions. For hydrothermal synthesis of undoped samples copper-insert lined autoclaves with a volume of ~40 cm3 were utilized. Grinded mixtures of oxides for appropriate stoichiometries were subjected to hydrothermal treatment during 100 h in a 0.1 mol% NaOH aqueous solution under autogenous pressure at ~ 450 °C, with a filling degree of 50%. The resulting precursors were washed with distilled water and ethanol and thereafter oven-dried at 1000 °C for 4 h in air. The hydrothermal garnet powders were mixed with KMnO4 (1.5–0.1 mol%) and the compensative oxides (e.g. MgO) by taking into account that Mn4+ ions occupy the octahedral site in the garnet structure, namely M3+ + M3+ → Mg2+ + Mn4+, and K+ ions will be removed as a result of the following multi-stage thermal treatment as described below. The blends were uniaxially pressed into pellets with a diameter of 10 mm and a thickness of 3 – 5 mm to promote intimate contact and the pellets were heated at 500 °C for 24 h in air. Subsequently, the samples were thoroughly ground, re-pelletized and sintered at 1150 °C for 8 h in air. Finally, the samples were again ground into finely dispersed powders, re-pelletized and sintered at 1300 °C for 8 h in air. Eventually, pellets of synthesized ceramic phosphors were polished for later characterization. The structure type and phase purity of the synthesized samples were characterized with conventional powder X-ray diffraction (XRD) technique and powder XRD patterns were obtained by using a Bruker D8 Advance X-Ray powder diffractometer with Cu Kα radiation. Identification of synthesized compounds, indexing of X-ray powder diffraction patterns and refinement of unit cell parameters were performed with the Diffrac. Suite. EVA software (Bruker). Unit cell parameters were determined with an accuracy of around 0.001 Å. Photoluminescence (PL) as well as PL excitation (PLE) spectra were recorded on an Edinburgh Instruments FLS920 spectrometer equipped with a Xenon arc lamp (450 W) and a cooled (− 20 °C) single-photon counting photomultiplier (Hamamatsu R2658P). Spectral resolution for the PL and PLE spectra measurements was typically set to value of
3. Results and discussion The XRD patterns have proved that synthesized samples have the garnet crystal structure without traces of any impurity phases (Fig. 1). With increasing ionic radius of the cation at the octahedral site the diffraction reflexes shift to smaller angles, and the unit cell volume as well as the interionic distance in the octahedral site increase, as expected (see Table 1). Synthesized Mn4+ doped garnet phosphors possess typical Mn4+ PL spectrum in the red spectral range, which is dominated by the Stokes vibronic sidebands of the 2Eg → 4A2 g transition of the Mn4+ ion (Fig. 2). The peak wavelength of Mn4+ PL spectrum shifts to shorter wavelengths with the increase of the octahedral cation size, as predicted (Table 2), and at 300 K the shortest peak wavelength obtained for Sc-based compound is 658 nm, still in rather deep red region. Simultaneously, the long-wavelength shift of excitation bands due to spin-allowed Mn4+ transitions as well as owing to the O2- – Mn4+ charge transfer (CT) transition occurs. At low temperature, the PL spectrum of the CaY2Al4SiO12:Mn4+ phosphor shows well-resolved fine structure (Fig. 3a) with a zerophonon line (ZPL) corresponding to pure electronic transition 2Eg → 4 A2g and a set of Stokes vibronic sidebands. Also the bands in the PLE spectrum (Fig. 3b) corresponding to the second spin-allowed Mn4+ transition 4A2 → 4T1 and the O2- – Mn4+ CT transition are much better resolved. Table 1 Lattice parameters of garnets CaY2M2Al2SiO12 (M = Al, Ga, Sc).
315
Compound
a (Ǻ)
V (Ǻ3)
RM3+ (Ǻ) (CN = 6)
R(M3+-O2-) (Ǻ)
CaY2Al2Al2SiO12 CaY2Ga2Al2SiO12 CaY2Sc2Al2SiO12
11.959 12.053 12.302
1710.3 1751.0 1861.8
0.535 0.620 0.745
1.922 1.939 1.975
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1,0
Sc
Al
Sc
Relative Intensity (arb. units)
CaY 2 M 2 Al2 SiO 12:Mn 0,8
M = Al, Ga, Sc
2
4
E g - A 2g
T = 300 K
0,6
4
0,4
CT 0,2
Al
4+
2-
4
A 2g - T 2g Sc
Al
O - Mn
4+
0,0 300
400
500
600
700
800
W avelength (nm) Fig. 2. Normalized PL and PLE spectra of CaY2M2Al2SiO12:Mn4+(1.5 mol-%) (M = Al, Ga, Sc) at 300 K.
Fig. 4. Normalized PL spectra of CaY2Al4SiO12:Mn4+ (1), CaY2Ga2Al2SiO12:Mn4+ (2), CaY2Sc2Al2SiO12:Mn4+ (3) and Ca2YSc2Ga2Si2O12:Mn4+ (4) measured at 3 K.
Table 2 Peak wavelengths of emission and excitation bands in the PL and PLE spectra of Mn4+ in garnets CaY2M2Al2SiO12 (M = Al, Ga, Sc) at 300 K. Compound
RM3+ (Ǻ) (CN = 6)
λem (nm)
λex (4T2g) (nm)
λex (CT) (nm)
CaY2Al2Al2SiO12 CaY2Ga2Al2SiO12 CaY2Sc2Al2SiO12
0.535 0.620 0.745
674 668 658
481 500 502
341 343 353
cationic composition (whose spectrum is also shown in Fig. 4) the revealed broadening is so large that the emission spectrum has a Gaussian-like shape. This inhomogeneous broadening can be explained by structural disorders caused by statistical distribution of different cations at the dodecahedral, octahedral and tetrahedral crystallographic sites in the garnet structure. Due to this feature both local phonon frequencies and energies of Mn4+ electronic states vary from site to site resulting in broadening of the emission spectra. The low temperature emission spectra show that although ZPL in Mn4+ emission spectrum is at the highest energy for the Sc3+ based compound (as expected), the energy of the ZPL is actually larger for the host with M = Al than M = Ga. This feature can be due to the fact that Sc3+ ions show a strong tendency to occupy only the octahedral sites in the garnet lattice, whereas Ga3+ ions can substitute for Al3+ ions not only in octahedral but also in tetrahedral sites. The different cationic substitution in the tetrahedral site neighbor to the Mn4+ ion entering the octahedral site can differently affect the distortion of the crystalfield symmetry around Mn4+, which is known influences strongly the energy of the 2Eg state (see, e.g. [16]). Similar feature of the spectral shift of emission spectra was observed in [17] for Ce3+ ions doped into the same series of garnet hosts. The concentration dependence of Mn4+ PL spectra studied for the CaY2Al4SiO12:Mn4+ phosphor has shown that there are no significant
Two ZPLs are clearly observed in the PL spectrum of CaY2Al4SiO12:Mn4+ with the energy gap between ZPLs about 16 cm−1. This value is comparable to e.g. YAG:Mn4+ with a gap of ~ 19 cm−1 obtained in [14], where this feature was interpreted as being due to luminescence of Mn4+ ions entering the ‘regular’ and ‘distorted’ octahedral sites in the garnet structure. However, for the clarification of the real nature of this fine structure the careful analysis is required [15], which cannot be done only on the basis of this observation. On the other hand, our investigation shows that Mn4+ PL spectra from Ga- and Sc-containing garnets possess a broadened structure even at low temperature (Fig. 4) although the approximate position of respective ZPLs can be well identified in the spectra. For the multi-component garnet Ca2YScGa2Si2O12:Mn4+ with even more complicated
Fig. 3. (a) Normalized PL spectra of CaY2Al4SiO12:Mn4+(1.5 mol.%) at 300 K (excitation at 341 nm) and 3 K (excitation at 327 nm); in the insertion the fine structure of zero-phonon line is shown in an enlarged scale; (b) Normalized PLE spectra of Mn4+ luminescence in CaY2Al4SiO12:Mn4+(1.5 mol.%) at 300 K (monitored at 675 nm) and 3 K (monitored at 662 nm).
316
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Fig. 7. Temperature dependences of integral luminescence intensities for CaY2M2Al2SiO12:Mn4+(1.5 mol.%) (M = Sc, Ga, Al) phosphors. Dots are experimental data. Curves are Fermi - Dirac fits.
Fig. 5. Dependence of integral luminescence intensity of CaY2Al4SiO12:Mn4+ phosphors on the concentration of doping Mn4+ ions. Excitation wavelength is 330 nm. T = 300 K.
versus temperature (Fig. 7) in accordance with the well-known Fermi-
(
changes of the shapes of PL and PLE spectra with the doping concentration. The optimal Mn4+ concentration, for which the maximum intensity of Mn4+ luminescence is observed, is about 0.2 mol-% (Fig. 5). The concentration dependence of Mn4+ luminescence intensity, with maximal light output for 0.2 mol-% Mn4+, looks unusual. Typically, Mn4+ luminescence is not quenched up to concentration of few percent [18]. The possible explanation of such specific dependence is the increase of concentration of quenching centers with increasing doping level. The quenching centers can be both crystal structure defects and Mn ions in different valence states. Accordingly, optimization of synthesis conditions is very important for developing efficient Mn4+ phosphors. All studied phosphors demonstrate rather poor thermal stability of Mn4+ luminescence with thermal quenching temperature, determined as the temperature at which the emission intensity is decreased by 50% of its maximal value, T1/2 < 240 K. At room temperature the emission intensity is already strongly quenched as can be seen in Fig. 6 where PL spectra measured at different temperatures are presented for the most thermally stable CaY2Al4SiO12:Mn4+ phosphor. The values of the activation energies for thermal quenching have been obtained from fitting of experimental dependencies of the integral luminescence intensity
Dirac model: I (T ) = I0/ 1 + A∙exp
( ) ), where I Ea kB T
0
is luminescence
intensity at T = 0 K, A characterizes the rate of thermal quenching, kB is the Boltzmann constant, and Ea is the energy barrier (activation energy) for thermal quenching. It is clearly seen (see Table 3) that activation energies Ea and thermal quenching temperatures T1/2 decrease with the increase of ionic radius of the host cation M3+ in the octahedral site (or with the increase of the respective inter-ionic distance). Generally, two mechanisms of thermal quenching of Mn4+ luminescence have been proposed so far. The first one is connected with a crossover of the emitting Mn4+ 2Eg state to the Franck-Condon shifted Mn4+ 4T2g level (Fig. 8a) as has been studied e.g. for YAG:Mn4+ [6]. The second one is related to a crossover of the emitting Mn4+ 2Eg state to the Franck-Condon shifted and low-lying O2- – Mn4+ CT state (Fig. 8b), which has been considered as a quenching mechanism in some Mn4+ doped stannates [19]. The observed decrease of T1/2 with the ionic radius of the cation M3+ correlates with both experimental findings: lowering the energy of the Mn4+ 4T2g state being obviously caused by the decrease of the CF strength with the increase of inter-ionic distance, and the decrease of the energy of the O2- - Mn4+ CT state. Since each of these excited states can serve as a quencher for Mn4+ luminescence it is not possible to establish the dominant mechanism of thermal quenching in the studied phosphors from such simple qualitative consideration. Generally speaking, some other processes can be also considered as possible mechanisms of thermal quenching, in particular, multi-phonon relaxation from the excited 2E state to the 4A2 ground state. However, for this process the energy gap ~ 15300 cm−1 between these states should be bridged with emission of several tens phonons (with typical energy of several hundreds of cm−1). It looks unreal that such a process can compete with radiative transition between these states. Also, our experimental results demonstrate that thermal quenching occurs at different temperatures in different hosts (with M = Sc, Ga, Al) with almost equal energies of the 2E state and obviously rather similar Table 3 Activation energies of thermal quenching Ea and thermal quenching temperatures T1/2 of Mn4+ luminescence from garnets CaY2M2Al2SiO12:Mn4+ (M = Al, Ga, Sc).
Fig. 6. Temperature dependence of PL spectrum of CaY2Al4SiO12:Mn4+(1.5 mol.%). Excitation wavelength is 341 nm.
317
Compound
Ea (eV)
T1/2 (K)
CaY2Al2Al2SiO12 CaY2Ga2Al2SiO12 CaY2Sc2Al2SiO12
0.174 ± 0.011 0.111 ± 0.020 0.065 ± 0.004
239.7 ± 2.0 224.5 ± 6.8 159.4 ± 3.0
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Fig. 8. Two mechanisms of thermal quenching of Mn4+ luminescence.
Mn4+ CT transition. Accordingly, for the calculation of the Racah parameter B it is necessary to use the barycenter of all energy levels of the 4T1 state [24], which could result in other values of the Racah parameters. However, the consideration of this problem is out of the scope of the present paper.
phonon spectra, which is not in favor of multi-phonon relaxation mechanism. Moreover, the lowest thermal quenching temperature in the series of studied materials is observed for the phosphor CaY2Sc2Al2SiO12:Mn4+ with the highest energy of the emitting 2E state. Another possible process which can be responsible for thermal quenching is thermally stimulated ionization of a 3d electron from the Mn4+ 2E state to the host conduction band, the mechanism, which was widely discussed for thermal quenching of luminescence due to 5d – 4f transitions in rare earth ions (see, e.g. [20]), in particular for YAG:Ce3+ type phosphors [21]. However, unfortunately, there is no data available in the literature on the positions of Mn4+ energy levels with respect to the host energy bands. Accordingly, a possible influence of thermally stimulated ionization on thermal quenching of Mn4+ luminescence cannot be analyzed on the basis of obtained results and will be a future subject of studies. Using obtained experimental data on Mn4+ luminescence and the standard formulas [22], CF strength parameter Dq as well as Racah parameters B and C have been calculated for the Mn4+ ion in the studied silicate garnet phosphors (Table 4). For these calculations the peak energies of 4T2 and 4T1 bands in the excitation spectra have been used, which were determined by decomposition of excitation spectra measured at 3 K into multiple Gaussian functions in the linear energy scale. Also the nephelauxetic parameter β1 = (B / B0)2 +(C / C0 )2 (B0 and C0 are the Racah parameters of the Mn4+ ion in a free state) has been calculated, the value which was introduced recently for the characterization of luminescence energy of transition metals in crystals [23]. As can be seen in Table 4, in the studied silicate garnets the Mn4+ ions show smaller values of parameter B and larger values of parameter C compared to those in many other hosts (see data collection on Mn4+ Racah parameters in different hosts in [23]). The possible reason of this feature can be splitting of the 4T1 state due to strong distortion of the octahedral environment of Mn4+ ions, the higher-energy component of this splitting being hidden under the broad band ascribed here to O2- -
4. Conclusions Monophase silicate garnet ceramics with the composition of CaY2M2Al2SiO12:Mn4+ (M = Al, Ga, Sc) have been successfully obtained with high-temperature solid-state reactions using precursors synthesized under hydrothermal conditions. It has been shown that it is possible to obtain a blue shift of the Mn4+ emission spectrum by increasing the radius of the cations on the octahedral sites in the garnet structure. Such a shift is a result of decreasing covalence of the “Mn4+ligand” bonding, which leads to an increase of the energy of the emitting Mn4+ 2Eg state. The shortest-wavelength emission of Mn4+ with a peak wavelength at 658 nm (at 300 K) has been obtained for the CaY2Sc2Al2SiO12 garnet host with the largest radius of the cation (Sc3+) at the octahedral site. However, the energies of the excited 4T2g state and the O2- – Mn4+ CT state are decreasing with the increase of the host cation radius, resulting in stronger thermal quenching of the Mn4+ luminescence. In this context, thermal quenching temperatures of Mn4+ luminescence (T½ < 240 K) in the studied phosphors CaY2M2Al2SiO12:Mn4+ (M = Al, Ga, Sc) are too low for potential pcLED application. High energy of both the Mn4+ 4T2g state and the O2- – Mn4+ CT state is crucial for obtaining a high thermal stability of the emission of Mn4+ phosphors. The inhomogeneous broadening of Mn4+ luminescence spectra observed in the studied multi-component garnet hosts can be explained by the structural disorder caused by statistical distribution of heterogeneous cations (Ca, Y, Al, Si, Ga, Sc) at the host lattice sites. Acknowledgements
Table 4 Crystal field strength (Dq), Racah (B and C) and β1 parameters of Mn4+ in garnets CaY2M2Al2SiO12 (M = Al, Ga, Sc). Compound
Dq (cm−1)
B (cm−1)
C (cm−1)
ZPL (2Eg) (cm−1)
β1
CaY2Al2Al2SiO12 CaY2Ga2Al2SiO12 CaY2Sc2Al2SiO12
2066 2012 1992
418 442 463
3969 3903 3855
15258 15221 15347
0.990 0.984 0.981
This research was performed within the ERA.Net RUS Plus Programme, project NANOLED # 361. Financial support from Russian Foundation for Basic Research (Grants 16-52-76027 ERA_а and 16-5376027 ERA_а), Estonian Research Council and BMBF (Germany) is gratefully acknowledged by the Moscow, Tartu and Münster groups. The Tartu group also acknowledges financial support from the ERDF funding in Estonia granted to the Center of Excellence TK141 (project No. 2014-2020.4.01.15-0011). 318
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