Luminescence of Yb-doped YAG: Divalent ytterbium ions

Luminescence of Yb-doped YAG: Divalent ytterbium ions

Journal of Luminescence 169 (2016) 151–155 Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/loca...

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Journal of Luminescence 169 (2016) 151–155

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Luminescence of Yb-doped YAG: Divalent ytterbium ions Vladimir Solomonov, Vladimir Osipov, Alfiya Spirina n Institute of Electrophysics, UB RAS, Amundsen street 106, 620016 Yekaterinburg, Russia

art ic l e i nf o

a b s t r a c t

Article history: Received 2 March 2015 Received in revised form 1 September 2015 Accepted 1 September 2015 Available online 21 September 2015

This study reports the presence of Yb2 þ ions with 4f136s electron configuration of the ground state in addition to Yb3 þ ions in yttrium aluminium garnet ceramics activated by ytterbium. Pulsed cathodoluminescence and transmission spectra of divalent ytterbium ions appear as narrow lines of s2s transitions at 913–1050 nm range. These ions in association with oxygen vacancies generate Re–F centres which are able to absorb at 280, 365, 630 nm and emit at 325, 520, 590  720 nm in d2s transitions of Yb2 þ ions; the emission band at 590–720 nm consists of eight narrow lines. The Re–F centeres are manifested most clearly in the ceramic samples obtained directly after vacuum sintering at 1800 °C. & 2015 Elsevier B.V. All rights reserved.

Keywords: Yb:YAG Ceramics Absorption Transmission Pulsed cathodoluminescence Divalent ytterbium ions

1. Introduction Yttrium aluminium garnet (YAG) has been recognized as an attractive laser host material due to its excellent optical, mechanical, chemical, and thermal properties. YAGs activated by rare-earth ions, e.g. Nd:YAG and Yb:YAG, have been extensively studied [1–5]. The quantum efficiency of the Yb:YAG system (90.97% at λgen ¼1029.3 nm, pumping to 2F5/2 (10679 cm  1)) is known to be higher compared to the Nd:YAG system (81.87% at λgen ¼ 1061 nm, pumping to 4F3/2 (11512 cm  1)); however, the practical efficiency of the Yb:YAG is frequently lower than that of the Nd:YAG system. This phenomenon is commonly attributed to the 2F7/2 ground state thermal population of Yb ions [6]. An additional explanation for the reduced generation efficiency of the Yb:YAG system seems to be the presence of divalent ytterbium ions capable of capturing a part of the excitation energy. The optical absorption and luminescence of divalent ytterbium ions were studied earlier in alkali-halogen [7], fluorite type [8–9], sulphate [10], borate [11], phosphate [12], aluminate, and silicate [13] crystals. The ground state of Yb2 þ in these crystals was shown to be 4f14(1S0) with optical absorption occurring during transitions between the ground and the excited state of 4f135d electron configuration. The quantum-mechanical calculations of the 4f135d states of Yb2 þ in the cubic crystal field were successfully carried out in Ref. [14]. It was noted that the levels of the 4f135d electron configuration separate into four zones corresponding to orbital n

Corresponding author. Tel.: þ 7 343 267 87 79. E-mail address: alfi[email protected] (A. Spirina).

http://dx.doi.org/10.1016/j.jlumin.2015.09.006 0022-2313/& 2015 Elsevier B.V. All rights reserved.

triplets (t2g) and doublets (eg) which split due to the spin–orbit interaction. In addition, the electric dipole transition from the 1S0 ground term is allowed for 18 out of 20 levels of this configuration. As a result, up to four broad intense bands of absorption were observed in cubic crystals at λ o 400 nm. Moreover, in Refs. [7–14] a broad luminescence band in the visible spectral range was observed for all the crystals. The spectra of absorption and luminescence of an Yb2 þ ion in the yttrium aluminium garnet single crystals and optical ceramics activated by ytterbium were shown to be significantly different from those considered above. This difference was first observed in work [15] where, apart from the UV absorption band at 380 nm, a strong long wavelength band at 670 nm was detected in γ-irradiated Yb:YAG single crystals. Similar absorption bands at 380 and 640 nm in Yb:YAG single crystals with Si4 þ ions introduced for stabilization of divalent ytterbium ions were later observed in Ref. [16]. Three broad absorption bands at 280, 400, and 660 nm in Yb:YAG single crystals were registered in Ref. [17]. Two broad absorption bands at 390 and 650 nm were detected in epitaxial Yb:YAG films [18]. The authors of all the abovementioned papers related the observed absorption bands to the 4f–5d optical transitions of Yb2 þ ions. However, the ytterbium Lα-line valence shift measurements in Yb:YAG crystals with a changed Yb-concentration and colouration degree in Ref. [19] showed no ytterbium ions with a filled 4f14 shell. The absorption bands at 280, 375, and 625 nm in the transmission spectrum of the transparent Yb:YAG ceramics were manifested in Refs. [20–22]. It was suggested in Ref. [20] that these bands emerge due to absorption by colour centres containing both Yb2 þ ions and oxygen vacancies arisen from the vacuum sintering

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of ceramics. This assumption is based on the fact that these bands disappear from the transmission spectrum of the same ceramics samples annealed in air for 35 h at 1400 °C. In Ref. [22] the ground state electron configuration of an Yb2 þ ion belonging to a colour centre was assumed to be 4f136s instead of 4f14, similar to other crystal matrices [7–14]. Little information on YAG luminescence relating to an Yb2 þ ion was offered in Refs. [15–17,20,21]. Two bands in the visible spectral range of X-ray luminescence in epitaxial Yb:YAG films were discovered in Ref. [18]. Two bands were observed in the films grown on a lead oxide substrate with a ytterbium content of less than 40 at%: one with a maximum at 580 nm and another in the range of 650–750 nm. Two bands were observed in the films grown on a bismuth oxide substrate; however, their maxima were located at 480 nm and 580 nm. The authors [18] supposed the location bands of 480 nm and 580 nm is result from the intracenter 5d–4f transfers of Yb2 þ ions. The bands at 330 nm and 500 nm in samples with ytterbium content of 40 at% were observed in Ref. [18]. The authors of Ref. [18] attributed these bands to the radiative transitions of Yb3 þ ions to the 2F5/2,7/2 levels with the charge transfer. In the visible spectral range of the pulsed cathodoluminescence [22], the narrow well-structured lines exhibiting the maxima in the range of 591–711 nm were observed, with the exception of two broad bands at 325 nm and 520 nm, which are believed to be the result of Yb2 þ radiation. Despite the abovementioned extensive research efforts, a clear understanding of the nature of absorption and luminescence bands of Yb:YAG single crystals and ceramics is still lacking. The purpose of this paper is to specify the nature of the absorption and luminescence centres in Yb:YAG.

2. Methods The current paper involved analysing the optical and electron spin resonance (ESR) spectra of Yb:YAG ceramics. The methods of optical spectroscopy were selected in order to register absorptive and luminescent bands in these ceramics. The ESR method frequently used to identify paramagnetic centres was employed to determine the Yb ion valence. Yb:YAG ceramics were prepared from a mixture of Al2O3, Y2O3 and Yb2O3 nanopowders. The concentration of Yb2O3 amounted to 10 mol% with the percentage of Al2O3 and (Y2O3 þYb2O3) corresponding to the garnet stoichiometry. All nanopowders were obtained by laser evaporation of the target followed by condensation in the air stream [23]. The nanopowder mixtures were homogenized by wet mixing for 48 h with grinding media. Afterwards dried precursors were isostatically pressed into compacts. The compacts of 15 mm in diameter and 3 mm in height, with a density of about 0.5 of the theoretical density of the garnet structure were prepared. Subsequent sintering of compacts was performed in a vacuum furnace at 1750–1800 °C for 20 h (GERO model, W-heaters, P ¼10  5 mbar) to yield transparent green ceramics. The ceramics lost their coloration after additional annealing in air at 1350 °C for 3 h. The as-obtained ceramic samples were characterized by a crystal structure typical for yttrium–aluminium garnet (ρ ¼4.757 70.001 g/cm3, a ¼1.2000 70.0001 nm), according to the X-ray diffraction analysis performed with a D8 Discover diffractometer. Pulsed cathodoluminescence (PCL) was excited using a CLAVI setup [24]. The samples without any special treatment were irradiated in air at room temperature by an electron beam with the average energy of 180 keV, a current density of 160 A/cm2, , and a duration of 2 ns. The time-integrated luminescence spectrum was registered by three multichannel photodetectors; the spectral

range of the first photodetector – 220–400 nm with the wavelength measurement error of Δλ ¼ 70.2 nm, the spectral range of the second photodetector – 350–850 nm with Δλ ¼ 70.5 precision, and the spectral range of the third photodetector – 750– 1040 nm with Δλ ¼ 71.0 nm precision. The spectral information averaged over 64 pulses, with the stability of the amplitude parameters of the PCL ceramics spectrum exceeding 90%. The transmission spectra were measured using a Shimatzu UV – 1700 spectrophotometer in the range from 200 nm to 1100 nm with the wavelength measurement error of 7 0.1 nm. The electron spin resonance spectra were measured in the static magnetic field ranging from 200 to 6000 G with a Bruker ELEXSYS–580 device with the resonant frequency of 9.27 GHz. The ceramic samples were fractured for this analysis. All measurements of ESR spectra were carried out at room temperature.

3. Results Transmission and pulsed cathodoluminescence spectra were registered to scrutinize the nature of optically active centres in Yb: YAG. Fig. 1 shows typical transmission spectra of unannealed (curve 1) and annealed (curve 2) Yb:YAG samples. The ultraviolet and visible spectrum ranges of an unannealed sample contain three broad strong absorption bands at 280 nm, 365 nm, 630 nm, which is in line with Refs. [20–22]. The strong absorption bands at 365 nm and 630 nm provide a green coloration of this sample. It can be seen that these bands disappear after annealing, while a band at 454 nm (Fig. 1, curve 2) is clearly displayed. A long-wave region at 860–1070 nm in the transmission spectra contains an identically structured absorption band mainly formed by Yb3 þ ions (Fig. 1). This band is typical for the single crystals [25–27] and the optical ceramics [20–22] of Yb:YAG. However, it should be noted that a part of the absorption lines at 913.9, 930.1, 957.9, 978.7, 1000, 1004.7 and 1047.5 nm in this band (Fig. 1) is absent in the energy structure of Yb3 þ ion in according to Refs. [25–27]. Fig. 2 demonstrates a pulsed cathodoluminescence spectrum of the same unannealed ceramic sample. The luminescent spectrum remains qualitatively permanent after a single annealing step; however, the intensity of emission bands in the 200–850 nm range

Fig. 1. Transmission spectra of unannealed (1) and annealed (2) ceramics.

V. Solomonov et al. / Journal of Luminescence 169 (2016) 151–155

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Fig. 2. PCL spectrum of unannealed ceramics.

Fig. 3. ESR spectrum of Yb:YAG.

almost doubly decreases. These bands disappear completely upon 4–5 annealing steps. Two broad bands at 325 nm and 520 nm are observed in the PCL spectra, this being consistent with the X-ray luminescence spectrum [18,22]. Moreover, this spectrum also registers eight narrow lines at 591.3, 609.2, 631.2, 649.1, 675.2, 689.6, 696.8, 711.3 nm, observed in Ref. [22]. Five bands at 936, 941, 968, 1025 and 1030 nm in the 900–1060 nm range corresponding to the energy structure of Yb3 þ ions are presented, which is in line with Ref. [25]. The former two bands are weakly intense, while the latter are relatively strong. However, several additional weaker bands observed at 912, 979, 986, 1000 and 1005 nm do not satisfy the energy level diagram structure of Yb3 þ . Therefore, it has been observed that both transmission and luminescence spectra of Yb:YAG contain the bands which do not correspond the Yb3 þ ion energy structure of YAG. We suppose that these bands may be emitted by divalent ytterbium ions. To confirm the presence of Yb2 þ ions, electron spin resonance spactra of Yb:YAG were recorded. A typical ESR spectrum of Yb:YAG is demonstrated in Fig. 3. It contains seven structured series of responses.

the same transmission and luminescence spectral range. These additional bands appear in the Yb:YAG optical ceramics and single crystals. In Ref. [26] the bands were interpreted as phonon replicas of the band at 968 nm originated upon the optical transition between the lowest Stark components of the 2F5/2 and 2F7/2 levels of Yb3 þ ions. However, this explanation raises a question as to why such an intense vibronic structure in the optical spectra of yttrium aluminium garnet doped by trivalent rare-earth ions is observed exceptionally for the ions of Yb3 þ . We suppose that these additional narrow bands may be emitted and absorbed by divalent ytterbium ions. It should be noted that features describing the energy structure of an impurity Yb2 þ ion in YAG have not yet been sufficiently studied [15–21]. The ionic radius of Yb2 þ (0.116 nm [28]) is much bigger than that of Al3 þ (0.053 nm); therefore, Yb2 þ ions can replace only Y3 þ ions, located in dodecahedral positions of the YAG crystal. The wavenumber of the gravity centre of 4f135d-levels of a free Yb2 þ ion [14] is close to the bandgap of YAG (Eg ¼44,000 cm  1). In the CaF2 crystals [8,14], the minimum value of the wavenumber of the lowest level of the 4f135d electron configuration in relation to the 4f14 (1S0) ground state of an Yb2 þ ion is 27 450 cm  1. In the cubic position of CaF2 the crystal field strength, Dq, amounts to the same order as in the dodecahedral positions of YAG (Dq ¼  1600 cm  1). Therefore, the absorption of an Yb2 þ ion with the configuration of the ground state of 4f14 (1S0) in YAG is to be expected in the range of wavelengths shorter than 370 nm. However, long-wavelength absorption is present in the spectrum. To elucidate this fact it seems reasonable to address an energy level diagram with 4f135d configuration of an Yb2 þ ion in the cubic field [14]. According to this diagram, the 4f135d levels are split into the orbital doublet eg and triplet t2g and, further, into two energy zones of 2F7/2eg, 2F5/2eg and 2F7/2t2g, 2F5/2t2g levels due to the spin–orbit interaction. The energy of this splitting, ESO, approximates to 10327 cm  1 and equals the splitting energy of the 2 F7/2,5/2 doublet level of an Yb3 þ ion [22,23]. In Ref. [22] a long-wavelength shift in the YAG crystal structure is explained by the electronic configuration of the ground state of an Yb2 þ ion being 4f136s (3F4,3,2, 1F3) rather than 4f14 (1S0) as is true for the CaF2 crystals. This assumption is proved by the fact

4. Discussion Scanning of the transmission and PCL spectra supported a wellestablished fact of the Yb3 þ ion presence in the Yb:YAG ceramics. These ions constitute a simple energy structure consisting of two levels. The upper 2F5/2 level and the lower 2F7/2 level are split in the crystal field into three and four Stark components respectively [25–27]. Optical transitions from the lower Stark component of the 2 F7/2 level to the three Stark components of the 2F5/2 level are manifested in the absorption spectra in the form of narrow bands at 936, 941, and 968 nm. Emitting bands on the reverse transitions at the same wavelengths, as well as bands on the transitions from the lower Stark component of 2F5/2 level to the upper Stark components of 2F7/2 at 1025, 1030 and 1048 nm appear. All the bands except for the emission band at 1048 nm are observed in the transmission and luminescence spectra of both unannealed and annealed ceramic samples (Figs. 1 and 2). Apart from them, narrow bands lacking in the energy structure of Yb3 þ ion are observed in

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that measuring the valency shift of the Lα-line of Yb in the Yb:YAG single crystals did not detect ytterbium ions with a filled 4f14 electron shell [19]. The 4f135d levels of the Yb2 þ ion electronic configuration are located at about 14,000 cm  1 higher than the 4f136s (3F4) level [28]. For the 4f136s electron configuration, intermediate coupling, rather than LS, is typical. In that case, the 4f136s (3F4,3,2, 1F3) levels, designated using LS-approach, are split into two groups (3F4, 3F3) and (3F2, 1F3), which are separated from each other by the same order energy value as the energy of the spin–orbit interaction ESO with the lowest level being 3F4. The optical s2s transitions in the form of narrow absorption and luminescence bands in the range of wavelength 1 μm should be observed between these groups of levels. Indeed, the bands in this spectral range of the transmission and the PCL spectra are present; moreover, they do not correspond to the energy structure of Yb3 þ . The ESR spectrum (Fig. 3) contains seven structured series of responses, which might not be associated with the Yb3 þ ions. This conclusion is based on the fact that ESR occurs between Zeeman components when the projection of the total moment changes by unit, ΔMJ ¼ 71 (J – quantum number of total moment). In a magnetic field each Stark level of the lower state 2F7/2 of Yb3 þ is split into two Zeeman components with 7MJ. The difference of the projections of the total moment between these components is |ΔMJ| ¼|2MJ| 41. Therefore, an optical transition between the components is forbidden, and the electron spin resonance cannot be observed. Because of the high value of MJ for an Yb2 þ ion optical transitions between Zeeman components with the same sign of MJ of neighbouring Stark sublevels of the 3F4 and 3F3 lower states are allowed. In this case electron spin resonances in the range of magnetic induction B ¼200–6000 G, are likely to appear at ΔESt E νr ¼ 0.31 cm  1 (fr ¼9.27 GHz). In case of ΔESt 4 νr and ΔESt o νr transitions are possible between the Zeeman components with the positive projections of the total components (þ MJ) and negative projections of the total components (  MJ), respectively. Moreover, if ΔESt of neighbouring levels with Δ|MJ| ¼1 are not identical, the ESR spectrum should contain seven responses corresponding to the seven Stark sublevels that were experimentally proven (Fig. 3). The position of responses at the B scale indicates that the Stark splitting of 3F4,3 levels of an Yb2 þ ion is small with the order of νr. The fine structure of each series is due to the interaction with a nuclear spin. Naturally occurring ytterbium contains 69.6, 14.3, and 16.1% of isotopes with nuclear spins of 0, 1/2, and 5/2, respectively [29]. In accordance with the level diagram (Fig. 4) absorption and luminescence might be expected to occur at the 2F7/2eg-3F4,3 transitions in the 600 nm region and at the 2F7/2eg-3F2,1F3 transitions in the 1.5–2.7 μm region. The 1.5–2.7 μm region of the luminescence spectrum is out of the spectral range of our photodetectors. In the 600 nm region a structured band at 590–720 nm is observed, the full width of which is equal to 2850 cm  1 being in good agreement with the width of the level zone of 4f135d (2F7/2eg) in a cubic crystal field [14]. The band consists of eight narrow luminescence bands at λ ¼591.3, 609.2, 631.2, 649.1, 675.2, 689.6, 696.8, and 711.3 nm, which may be attributed to the optical transition between the levels of the orbital doublet of 4f135d (2F7/2eg) and the ground state of 4f136s (3F4,3). A broad absorption band at 630 nm corresponds to the reverse transition. Broad absorption bands at 280, 365 nm and luminescence bands at 325, 520 nm (Figs. 1 and 2) correspond to the optical transitions 2F5/2eg23F4,3 and 2F7/2t2g23F4,3, respectively. The difference in the wavenumbers between the band centres 365 and 630 nm is equal to 11520 cm  1 which is in good agreement with the energy of spin–orbit interaction ESO. The difference between the band centres 280 and 630 nm is 19840 cm  1. This value is

Fig. 4. Energy-level diagram of an Yb2 þ ion in the cubic field.

equal to the crystal field strength Dq E 1984 cm  1. Dq does not surpass the typical values of the crystal field strengths but is somewhat higher than that of Dq in undistorted dodecahedral positions of YAG. The field enhancement is probably due to the distortion of its local symmetry in the position of Yb2 þ in the presence of oxygen vacancies. The values of the Stokes shifts of luminescence band centres relative to the absorption bands (around 5000 and 8000 cm  1) are close to the widths of the upper level groups of the optical transitions [14]. In such a schema, the levels of the 2F5/2t2g orbital triplet of an Yb2 þ ion are located within the conduction band and do not appear in the optical spectra. Thus, divalent ytterbium ions with an electron configuration of the 4f136s ground state are present in the Yb:YAG optical ceramics. Their formation may be explained by the fact that an Yb3 þ ion captures an electron for its outer 6s shell. This 6s electron in addition to the internal 5s25p6 electrons enhances the shielding of the 4f-shell of an Yb2 þ ion that results in the small Stark splitting of the lower levels. The lifetime of this Yb2 þ centre may be limited; nevertheless, a stable equilibrium concentration n02 is established. This value is estimated from the ratio of the absorption coefficients of the strongest bands of Yb2 þ ions (λ ¼914 nm) and Yb3 þ (λ ¼968 nm) and is equal to n02 E3  10  3n03(f3/f2), where n03 is the concentration of Yb3 þ ions, f3 and f2 are the oscillator strengths of the transitions into Yb3 þ and Yb2 þ ions, respectively. To determine this parameter the absorption spectrum is split on the Gaussians in the wavenumber scale. This data is available in Ref. [26]. According to this data the equilibrium concentration of divalent ytterbium ions n02 is approximately the same, where the absorption spectrum was registered at the cryogenic temperature of the samples. It is worth noting that the oxygen vacancies also play a sufficient role in the formation of luminescence and absorption bands. The analysis of the absorption coefficients of αλ,b(a) in the narrow bands centred at a wavelength λ in the range of 1 μm, aλ;bðaÞ l ¼ a0;bðaÞ l  ln T bðaÞ shows that after annealing of the samples in air, i.e. after the elimination of oxygen vacancies, a stable increase trend of the relative absorption coefficient kλ;a ¼ ðaλ;a  aλ;b Þ=aλ;b in the bands assigned to an Yb2 þ ion is observed within the limits of the

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measurement error; this tendency is less noticeable for the bands of an Yb3 þ ion. In particular, for the strongest absorption band of an Yb2 þ ion at 914 nm the mean value is k914 E0.008, while for the Yb3 þ band at 968 nm the mean value is k968 E 0.001. Here, the subscripts b and a mean the measurements performed before (b) and after (a) annealing of the sample, l – thickness of the sample, T – ceramic transmittance coefficient in the absorption band maximum at λ, measured with the accuracy of ΔT/T ¼0.01, α0 is the ceramics scattering coefficient identified at 1070 nm. This analysis shows that after oxygen vacancies have been annealed the ytterbium ions in the ceramics, previously associated with these vacancies basically remain in the divalent state. Moreover, these ions demonstrate optical activity in the infra-red region in s2s transitions. The presence of an oxygen vacancy near an Yb2 þ ion leads to a redistribution of the oscillator strengths of an Yb2 þ ion in favour of d2s transitions. This effect is manifested in the enhancement of the absorption and luminescence bands in the UV and visible region of the spectrum and is interpreted as the formation of a new Re–F – centre. The occurrence of this centre is attributed to the change in the strength and symmetry of the crystal field in the position of an Yb2 þ ion in the presence of an oxygen vacancy.

5. Conclusions We have identified that a divalent ytterbium ion with the electron configuration of the ground state of 4f136s is present in the optical Yb:YAG ceramic, beside a trivalent ytterbium ion. The emission and absorption of this ion appears in s2s transitions in the form of narrow bands in the infrared spectrum. In association with an oxygen vacancy it forms a Re–F – colour centre known to exhibit broad absorption bands at 280 nm, 385 nm, 640 nm and luminescence bands at 325 nm, 520 nm, 590–720 nm, with the former possessing a very developed structure. Energy levels of Yb2 þ and Yb3 þ ions do not form resonances, but the optical transition bands at 1024.8 and 1047.5 nm (Yb2 þ ) in YAG: Yb overlap with the bands at 1024.4 nm and 1048.0 nm (Yb3 þ ). This seems a likely reason for the efficiency of the lasing of an Yb3 þ ion at these wavelengths to be less than at 1029.3 nm. Future work will focus on the construction of the energy diagram of Yb2 þ into the YAG.

Acknowledgements The authors express their gratitude to V.A. Shitov and K.E. Lukyashin for the synthesis of ceramics, to S.F. Konev and S.O.

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Cholakh for conducting the ESR analysis and to A.N. Orlov for the registration of the transmission spectra. This work has been performed within State Task No.0389-20140003, has been supported in part by the President Grant of the Russian Federation №MK-3669.2015.2, RFBR Grant No. 14-0800181.

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