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ISSN 1063 7834, Physics of the Solid State, 2014, Vol. 56, No. 3, pp. 449­455. © Pleiades Publishing, Ltd., 2014. Original Russian Text © I.A. Sluchinskaya, A.I. Lebedev, A. Erko, 2014, published in Fizika Tverdogo Tela, 2014, Vol. 56, No. 3, pp. 442­447.

DIELECTRICS

Structural Position and Charge State of Nickel in SrTiO
I. A. Sluchinskaya *, A. I. Lebedev , and A. Erko
a b a, a b

3

Moscow State University, Moscow, 119991 Russia * e mail: irinasluch@nm.ru Helmholtz Zentrum, BESSY GmbH, Albert Einstein Strasse 15, 12489 Berlin, Germany
Received August 9, 2013

Abstract--The properties of nickel doped strontium titanate are studied using X ray diffraction and XAFS spectroscopy. It is shown that, independently of preparation conditions, the most stable phases in the samples are single phase SrTi1­ xNixO3 solid solution and NiTiO3 which can coexist. According to the EXAFS data, in the single phase SrTi0.97Ni0.03O3 sample the nickel atoms substitute the titanium atoms and are on center ones. In this case, no distortions of the oxygen octahedron which would appear in the presence of oxygen vacancies in the nickel environment were detected. An analysis of the XANES spectra shows that the nickel charge state in NiTiO3 is 2+, whereas in the SrTi1­ xNixO3 solid solution it is close to 4+. It is shown that the strongest light absorption in doped samples is associated with the presence of tetravalent Ni in the SrTi1 xNixO3 solid solution. This doping seems to be the most promising for solar energy converters based on the bulk photovoltaic effect. DOI: 10.1134/S1063783414030329

1. INTRODUCTION The bulk photovoltaic effect consists in the emer gence of a photocurrent and very high photovoltages when illuminating homogeneous crystals having no inversion center [1]. The idea of practical applications of this effect in ferroelectrics for solar energy conver sion was discussed as early as in the 1970s [2]. How ever, the quantum yield of this effect is usually small because of the short lifetime of photoexcited carriers, and this idea was regarded as unproductive. Recently, an interest in ferroelectric oxides with the perovskite structure revived again as new ideas how to increase the efficiency of solar energy converters based on the bulk photovoltaic effect have been proposed [3, 4]. The main disadvantage of the ferroelectric oxides is their relatively large bandgap, because of which they absorb only a small fraction of the solar energy. Recent theoretical studies showed that the substitution of Ti atoms at the B site of the perovskite structure with a divalent impurity with d8 electronic configuration (Ni, Pd, Pt), compensated by an oxygen vacancy, decreases the band gap and the resulting perovskites are polar semiconductor oxides [5, 6]. Additional interest to the study of the nickel impu rity is associated with the results obtained in recent experimental and theoretical studies of new materials, i.e., recently synthesized PbNiO3 with a very high cal culated spontaneous polarization [7­9] and BiNiO3 with unexpected charge states of nickel and bismuth atoms [10]. Furthermore, the search for new magnetic off cen ter impurities, which can cause simultaneous ferro

electric and magnetic ordering in incipient ferroelec trics and the appearance of magnetoelectric coupling, is still important. Materials with such properties belong to multiferroics. An example of such a material can be SrTiO3 doped with Mn impurity at the A site, in which a new type of magnetoelectric coupling was recently discovered [11, 12]. Nickel doped samples could be an another example. Since the doping impurity can enter several differ ent sites of the perovskite structure and stay in them in different charge states, the aim of this work was to study the structural position and charge state of the Ni impurity in SrTiO3 prepared under different condi tions using XAFS spectroscopy. We planned to check the possibility of obtaining samples with divalent Ni at the B site, the possibility of incorporating the Ni impurity into the A site, and to determine the correla tion between optical properties of samples, on the one hand, and the structural position and charge state of the impurity, on the other hand. The choice of SrTiO3 was dictated by the fact that we have previously deter mined the structural positions and charge states of a number of 3d impurities (Mn [13, 14], Co [15], Fe [16]) in this material; their comparative analysis can make it possible to find new promising impurities for solar energy converters. 2. SAMPLES AND EXPERIMENTAL TECHNIQUE Nickel doped SrTiO3 samples with the impurity concentration of 2­3% and various deviation from

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450 Intensity, arb. units 3000 2500 2000 1500 1000 500 0 5 4 3 2 1 20 30 40 50 2, deg TiO2 RP RP

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RP

60

70

Fig. 1. X ray diffraction patterns of (1) NiTiO3, (2) SrTi0.97Ni0.03O3 annealed at 1500°C, (3) Sr0.98Ni0.02TiO3 annealed at 1100°C, (4) Sr0.98Ni0.02TiO3 annealed at 1500°C, and (5) SrTi0.97Ni0.03O3 annealed at 1100°C. Arrows indicate the reflections of TiO2 and Ruddlesden­ Popper (RP) phases.

stoichiometry were prepared by the solid state reac tion method. The starting components were SrCO3, nanocrystalline TiO2 prepared by the hydrolysis of tet rapropyl orthotitanate and dried at 500°C, and Ni(CH3COO)2 · 4H2O. The components were weighed in required proportions, ground in acetone, and annealed in air in alumina crucibles at 1100°C for 8 h. The obtained powders were ground again and repeatedly annealed under the same conditions. Some samples were additionally annealed in air at 1500°C for 2 h. To incorporate nickel into the A and B sites of the perovskite structure, the composition of the sam ples was intentionally deviated from the stoichiometry toward excess of titanium or strontium. The reference samples NiO, NiTiO3, and BaNiO3­ used to determine the Ni impurity charge state in SrTiO3 were prepared as follows. The NiO sample was obtained by thermal decomposition of Ni(CH3COO)2 · 4H2O. Two other samples were prepared by the solid state reaction method in air: the NiTiO3 sample was obtained from Ni(CH3COO)2 · 4H2O and TiO2 at 1100°C, the BaNiO3­ sample was prepared from BaO2 and NiO at 650°C. The phase composition of the obtained samples was checked by the X ray dif fraction. The spectra of the extended X ray absorption fine structure (EXAFS) and X ray absorption near edge structure (XANES) were measured at KMC 2 station of the BESSY synchrotron radiation source (the beam energy is 1.7 GeV, the maximum beam current is 290 mA) at the Ni K edge (8.34 keV) at 300 K. The radiation was monochromatized by a Si1­ xGex(111)

double crystal monochromator. The EXAFS spectra were measured in fluorescence mode. The radiation intensity incident on the sample (I0) was measured using an ionization chamber, the X ray fluorescence intensity (If) was measured by a silicon energy disper sive RæNTEC X flash detector with an active area of 10 mm2. The EXAFS oscillating function (k) was extracted from the fluorescence excitation spectra (E) = If/I0 (here E is the X ray photon energy) by the conven tional method [17, 18]. After subtracting the pre edge background, the monotonic atomic part of the spec trum 0(E) was extracted using splines, and the depen dence of = ( ­ 0)/0 was calculated as a function of photoelectron wave vector k = (2m(E ­ E0)/ 2)1/2. The energy corresponding to the inflection point at the absorption edge was taken as the energy origin E0. For each sample three spectra were recorded, they were then independently processed, and then the obtained curves (k) were averaged. Information about the first three shells was isolated from the obtained curves (k) using the direct and inverse Fourier transforms with a modified Hanning window. The distances Rj and Debye­Waller factors j for the jth shell ( j = 1­3) were determined by min imizing the root mean square deviation between experimental and calculated k2(k) curves. In addition 2 to the parameters Rj and j , the energy origin correc tion dE0 was simultaneously varied. The coordination numbers were considered fixed at the values given by a structural model. The number of adjustable parame ters (8) was approximately a half of the number of independent data points, Nind = 2kR/ 16. The scattering amplitudes and phase shifts, the phase shift of the central atom, and the photoelectron mean free path as a function of k, required to calculate the theoretical curves (k), were computed using the FEFF6 software [19]. The EXAFS spectra were also processed with the widely used IFEFFIT software package [20]. The experimental EXAFS function was extracted using the ATHENA program; its fitting to the theoretical curve calculated for a given structural model was performed using the ARTEMIS program. In this approach, the amplitudes and phases shifts for all single and multi ple scattering paths were also calculated using the FEFF6 software. The results obtained with two differ ent processing methods were in good agreement. 3. RESULTS 3.1. X Ray Data The diffraction patterns of all studied samples are shown in Fig. 1. It is seen that the SrTi0.97Ni0.03O3 sam ple annealed at 1500°C is the only single phase sam
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STRUCTURAL POSITION AND CHARGE STATE XANES signal 1.6 1.2 0.8 0.4 0 8320 1 2 3 4 5 6 7 8340 Energy, eV
Fig. 2. XANES spectra for SrTiO3(Ni) samples and refer ence compounds of nickel: (1) NiO, (2) NiTiO3, (3) Sr0.98Ni0.02TiO3 annealed at 1500°C, (4) BaNiO3­ , (5) SrTi0.97Ni0.03O3 annealed at 1100°C, (6) Sr0.98Ni0.02TiO3 annealed at 1100°C, and (7) SrTi0.97Ni0.03O3 annealed at 1500°C.

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k2(k) 2 1 0 -1 -2 8380 2 4 6 k, å
-1

8360

8

10

12

Fig. 3. EXAFS spectrum obtained at the Ni K edge at 300 K for the SrTi0.97Ni0.03O3 sample annealed at 1500°C (thin line) and its best theoretical fit (thick line).

ple with the cubic perovskite structure; the diffraction patterns of other samples exhibit additional reflec tions. For the SrTi0.97Ni0.03O3 and Sr0.98Ni0.02TiO3 samples annealed at 1100°C, along with the reflec tions characteristic of the perovskite phase, additional reflections were observed, which indicate the presence of a small amount of TiO2 and, presumably, NiTiO3 in these samples. The identification of the possible NiO phase was complicated by the closeness of the position of its reflections to the position of NiTiO3 reflections. In addition, stronger lines of the Sr3Ti2O7 and Sr4Ti3O10 Ruddlesden­Popper phases were observed in the SrTi0.97Ni0.03O3 sample annealed at 1100°C. In the Sr0.98Ni0.02TiO3 sample annealed at 1500°C, only one additional NiTiO3 phase was detected. Since barium nickelate BaNiO3­ is a defect phase with the parameter depending on preparation condi tions, the actual composition of our BaNiO3­ sample was determined using the dependence of the hexago nal lattice parameters of this phase on [21]. In our sample, the hexagonal lattice parameters were a = 5.568(1) å and c = 4.838(1) å, which correspond to 0.4. 3.2. Analysis of XANES Spectra To determine the charge state of the Ni impurity in SrTiO3, the absorption edge position in the XANES spectra of the samples under study was compared with the edge positions in the reference compounds. The XANES spectra of all studied samples and three refer ence compounds are shown in Fig. 2.
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A comparison of the spectra of the Sr0.98Ni0.02TiO3 sample annealed at 1500°C with the spectra of cubic NiO and rhombohedral NiTiO3 (with the ilmenite structure) shows that their absorption edges are very close. It can be concluded that the Ni impurity in this sample is in the 2+ charge state. The absorption edges for the SrTi0.97Ni0.03O3 and Sr0.98Ni0.02TiO3 samples annealed at 1100°C almost coincide and are close to the absorption edge in the BaNiO3­ reference com pound. In the single phase SrTi0.97Ni0.03O3 sample annealed at 1500°C, the absorption edge is shifted to higher energies (by 2.5, 2.9, and 1.3 eV in comparison with NiO, NiTiO3, and BaNiO3­ , respectively). 3.3. Analysis of EXAFS Spectra The structural position of the Ni impurity was determined from the analysis of the EXAFS spectra. The typical EXAFS spectrum k2(k) for the single phase SrTi0.97Ni0.03O3 sample annealed at 1500°C and its best theoretical fit which takes into account the multiple scattering effects are shown in Fig. 3. The best agreement between the calculated and experi mental spectra is achieved in the model in which Ni atoms substitute Ti atoms in SrTiO3. The interatomic distances and Debye­Waller factors for three nearest shells in this sample are given in the table. The small Debye­Waller factors for the first and second shells, which are typical for the thermal vibrations in perovs kites at 300 K, allow to draw the following conclu sions: (i) they exclude the possibility of off centering of Ni atoms and (ii) they indicate the absence of the oxygen vacancies near the impurity atoms. An analysis of the EXAFS spectra allowed us to accurately determine the composition of the second phase precipitating in the SrTi0.98Ni0.02O3 sample

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Structural parameters obtained from the EXAFS data anal ysis for the SrTi0.97Ni0.03O3 sample annealed at 1500°C (Ri is the distance to the ith shell, i is the Debye­Waller factor for this shell) Shell 1 2 3 Ri, å 1.914(4) 3.342(6) 3.877(4) i , å2 0.0035(6) 0.0084(7) 0.0053(5)
2 2

ples annealed at 1100°C were dark brown; the single phase SrTi0.97Ni0.03O3 sample annealed at 1500°C was almost black, and the NiTiO3 sample was bright yel low. Thus, the sample color is determined by the rela tive amount of black SrTi1­ xNixO3 and yellow NiTiO3. 4. DISCUSSION A combined analysis of X ray and EXAFS data shows that the SrTi0.97Ni0.03O3 sample annealed at 1500°C is a single phase solid solution in which the Ni atoms substitute the Ti atoms at the B sites and are on center. This means that the nickel solubility at the B sites in SrTiO3 exceeds 3% at 1500°C. In the sample with a nominal composition of SrTi0.97Ni0.03O3 annealed at 1100°C, the appearance of the reflections of NiTiO3 and of the Ruddlesden­Popper phases indi cates that some of the Ni atoms are involved in the NiTiO3 formation, while the others remain at the B sites. In this case, the "untapped" Sr atoms form the SrO planes which are embedded into the perovskite structure to form Sr3Ti2O7 and Sr4Ti3O10 phases. In attempting to incorporate Ni atoms into the A sites of strontium titanate upon annealing at 1500°C (the samples with a nominal composition of Sr0.98Ni0.02TiO3), the NiTiO3 second phase is precipi tated. In this case, the nickel concentration in the solid solution phase is low as it follows from the EXAFS data and the sample color. On the contrary, upon annealing of the Sr0.98Ni0.02TiO3 sample at a lower temperature (1100°C), the nickel in the sample is in a mixture of NiTiO3 and SrTi1­ xNixO3 solid solution as it follows from the XANES, EXAFS data, and the sample color. The incorporation of Ni into SrTiO3 is confirmed by precipitation of a small amount of the TiO2 phase (its appearance is a consequence of the removing of a part of Ti from the B sites when doping strontium titanate with nickel). As for the appearance of the TiO2 phase in the SrTi0.97Ni0.03O3 sample upon its annealing at 1100°C, we suppose that the appearance of this phase is possi ble for kinetic reasons. The solid solution is apparently formed as a result of a chain of chemical reactions in which the nickel titanate phase is formed first and its subsequent dissolving during the reaction with SrO is complicated. As a result, a mixture of the solid solu tion, nickel titanate, and TiO2 is formed in the samples annealed at 1100°C. At a higher annealing tempera ture, the kinetic processes are faster, the reaction is completed, and the sample composition is fully con trolled by the deviation from stoichiometry. Thus, the stable phases in the studied samples are the single phase SrTi1­ xNixO3 solid solution and NiTiO3; their ratio depends on the deviation from sto ichiometry and the annealing temperature. It is impossible to incorporate nickel into the A site of strontium titanate. The low stability of Ni at the A site
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Atom O Sr Ti

annealed at 1500°C. Although the distances to the nearest atoms and the coordination numbers for the first shell of nickel in NiO and NiTiO3 are close, the coordination numbers for the second shell in these compounds differ by a factor of 3. A comparison of the Fourier transforms of the EXAFS spectra for the sam ple under consideration and NiTiO3 and NiO refer ence compounds shows a better agreement of its spec trum with the spectrum of NiTiO3 (Fig. 4). This means that among two possible phases, NiTiO3 and NiO, the NiTiO3 phase is precipitated in our sample. As for the SrTi0.97Ni0.03O3 and Sr0.98Ni0.02TiO3 two phase samples annealed at 1100°C, the comparison of their EXAFS spectra with that of the single phase solid solution and that of NiTiO3 shows that the spec tra of the samples under consideration can be regarded as a superposition of the spectra of NiTiO3 and of the solid solution in a ratio close to 1:1. The optical properties of the samples under study agree with the data obtained above. The Sr0.98Ni0.02TiO3 sample annealed at 1500°C was pale brown; the SrTi0.97Ni0.03O3 and Sr0.98Ni0.02TiO3 sam
3 Amplitude of FT, arb. units 1 2 3

2

1

0 0 1 2 3 R, å 4 5 6

Fig. 4. Comparison of the Fourier transforms of the EXAFS k2(k) spectra obtained for (1) Sr0.98Ni0.02TiO3 sample annealed at 1500°C and reference (2) NiTiO3 and (3) NiO compounds.

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of strontium titanate is probably a consequence of the significant difference of the Ni2+ and Sr2+ ionic radii (the twelvefold configuration is not typical of nickel; at a coordination number of 6, the Ni2+ ionic radius (0.69 å) is much smaller than the Sr2+ ionic radius (1.18 å) [22]). The XANES data used to determine the nickel charge state are in complete agreement with X ray and EXAFS data. In the sample with a nominal composi tion of Sr0.98Ni0.02TiO3 annealed at 1500°C, in which nickel is in the NiTiO3 phase, the Ni charge state is identical to its charge state in NiO and NiTiO3 and is 2+. In the single phase SrTi0.97Ni0.03O3 sample annealed at 1500°C, the absorption edge shift with respect to NiTiO3 is maximum and is about twice the shift between NiTiO3 and BaNiO3­ reference com pounds. If, following [21], we proceed from the num ber of ions, their nominal charges, and the value 0.4 determined from the lattice parameters, the aver age nickel charge in BaNiO3­ is (4 ­ 2) = 3.2. Then, according to the shift of the absorption edge in the SrTi0.97Ni0.03O3 sample annealed at 1500°C, the nickel charge state in this sample should be close to 4+. For the SrTi0.97Ni0.03O3 and Sr0.98Ni0.02TiO3 samples annealed at 1100°C, which we consider as a mixture of two nickel containing phases in the ratio close to 1 : 1, the edge position is intermediate between the edge positions in two stable phases and is close to the edge position in BaNiO3­ (~3+). It should be noted that the problem of the Ni charge state in SrTi0.97Ni0.03O3 is not so simple. The Ni charge state in BaNiO3­ is still discussed; in particu lar, the doubts were expressed [23] on the adequacy of its formal determination based on the number of ions and their nominal charges. In [23], the MÆssbauer spectroscopy data for BaNiO3 indicate the Ni charge state close to 4+, whereas the photoelectron spectros copy data give the value close to 3+. The authors of [23] proposed a model in which Ni is trivalent, and its charge is compensated by a hole bound by one or two negatively charged oxygen ions. If this is the case, the Ni charge state determined from the absorption edge shift in the SrTi0.97Ni0.03O3 solid solution should be closer to 3+. However, this explanation contradicts the fact that no distortion of the oxygen octahedron was observed in our EXAFS spectra, whereas the hole localization at one or two oxygen ions should cause its distortion. At the same time, the experimental studies of XANES spectra of the LixNiO2 compound used in lithium batteries showed that a change in the degree of intercalation x changes the Ni charge state from 2+ to 4+ and shifts the absorption edge in the XANES spec tra by ~3.5 eV [24]. This shift is consistent with the shift of 2.9 eV we observed between NiTiO3 and the SrTi0.97Ni0.03O3 sample annealed at 1500°C. Further more, the Ni­O interatomic distance (1.914 å) we
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determined from the EXAFS measurements appears smaller than the sum of the Ni3+ and O2­ ionic radii (0.56 + 1.4 = 1.96 å) and is closer to the sum of the Ni4+ and O2­ ionic radii (0.48 + 1.4 = 1.88 å). One more argument in favor of tetravalent nickel can be the 2 small Debye­Waller factors 1 indicating the absence of the Ni displacements from the B sites in the SrTi0.97Ni0.03O3 solid solution. If the nickel atoms were incorporated into the crystal in the Ni3+ charge state, this would require its charge compensation by an oxy gen vacancy located nearby the impurity atom (such axial Ni3+­VO centers with the Ni displacement up to ~0.3 å from the octahedron center were observed in the electron paramagnetic resonance (EPR) spectra in [25, 26]). However, our EXAFS measurements detected no noticeable distortions of oxygen octahe dra surrounding the nickel atoms, and the Ni coordi nation number for the first shell remains close to 6. Therefore, we come to the conclusion that the nickel charge state at the B site of strontium titanate is close to 4+; this conclusion is not consistent with the assumption made previously [5, 6] that in the related ferroelectric PbTiO3 nickel is in the 2+ charge state. Although the question about the actual Ni charge state in this material is to be studied experimentally, the obtained data indicate that the strong light absorption in nickel doped SrTiO3 should be associated with the tetravalent nickel. From the viewpoint of possible applications of doped perovskites in solar energy conversion, it is of interest to compare the properties of strontium titan ate doped with nickel and with other 3d elements [13­ 16]. The absorption spectra of these samples are sys tematically shifted to the infrared region as the atomic number of the element increases from Mn to Ni: man ganese doped samples are greenish brown; iron, cobalt, and nickel impart brown, dark brown, and almost black color, respectively, to the samples. Thus, to create samples that strongly absorb light in the entire visible region, the nickel doping seems to be the most promising. It is interesting that the small value of the Debye­ 2 Waller factor for the first shell ( 1 0.0035 å2) excludes the possibility of Jahn­Teller instability of the Ni4+ ion, which is possible for the d6 octahedral configuration. Indeed, the Jahn­Teller instability of Ni4+ in LixNiO2 manifests itself as the oxygen octahe dron distortions with the Ni­O bond lengths of 1.88 and 2.08 å. In the EXAFS spectra of this compound, the oxygen atoms should look like one shell with an average interatomic distance of 1.947 å and a static Debye­Waller factor of 0.009 å2. The latter value is 2 much more than our experimental value of 1 . The interpretation of our data on the nickel charge state in strontium titanate differs much from the con

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clusions of earlier studies. A comparison with the data of [27] in which SrTiO3(Ni) samples were also studied by the XAFS spectroscopy shows that the XANES and EXAFS spectra obtained in our work and in [27] are qualitatively different. For example, the absorption edge shift in SrTiO3(Ni) with respect to NiO was only 1.1 eV in [27], whereas it was 2.5 eV in this work. In addition, even the sample color is different (beige in [27] and almost black in the present work). We believe that these differences in the sample properties result from the different methods of their preparation (solid state reaction method in our case and hydrothermal synthesis at 150°C in [27]). In [25, 26], single crystals of SrTiO3(Ni) were studied by the EPR method. In these studies, the EPR spectra related to Ni3+­VO axial complexes and the spectra attributed to Ni2+ and Ni3+ "cubic" centers were systematically observed [25]. Unfortunately, no arguments on the basis which the conclusion about the charge states of the Ni impu rity were made were presented in [25], and a possible interpretation of the EPR spectra as those of Ni4+ was not discussed. 5. CONCLUSIONS The study of nickel doped strontium titanate using X ray diffraction and XAFS spectroscopy allowed us to draw the following conclusions. (i) The preparation conditions of single phase nickel doped SrTiO3 samples with the impurity con centration up to 3% were found. In attempting to incorporate the impurity into the A site, the NiTiO3 second phase is precipitated. Independently of the preparation conditions, the most stable phases in the samples are the single phase SrTi1­ xNixO3 solid solu tion and NiTiO3 phase which can coexist. (ii) According to the EXAFS data, in the single phase SrTi0.97Ni0.03O3 sample, the Ni atoms substitute the Ti atoms and are on center. No distortions of oxy gen octahedra which could appear as a result of the presence of the oxygen vacancies in the Ni environ ment were detected. (iii) An analysis of the XANES spectra showed that the nickel charge state in NiTiO3 is 2+; in the SrTi1 xNixO3 solid solution it is close to 4+. (iv) It was shown that the strongest light absorption in doped samples is associated with the presence of tetravalent nickel in the SrTi1­ xNixO3 solid solution. This doping seems to be the most promising for devel oping of solar energy converters that exploit the bulk photovoltaic effect. In the future, we plan to perform similar experi ments on ferroelectric BaTiO3 doped with 3d elements to test the possibility of obtaining the polar materials with similar optical and physical properties, in which the bulk photovoltaic effect could be observed for pho tons in the entire spectrum of solar radiation.

ACKNOWLEDGMENTS I.A.S. and A.I.L. acknowledge the Russian­Ger man laboratory for financial support and hospitality during their stay at BESSY. This work was supported by the Russian Founda tion for Basic Research (project no. 13 02 00724). REFERENCES
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Translated by A. Kazantsev

PHYSICS OF THE SOLID STATE

Vol. 56

No. 3

2014