Ab initio crystal structure determination of Na2Si3O7 from conventional powder diffraction data

Solid State Sciences 4 (2002) 1285–1292 www.elsevier.com/locate/ssscie

Ab initio crystal structure determination of Na2 Si3 O7 from conventional powder diffraction data V. Kahlenberg a,∗ , B. Marler b , J.C. Muñoz Acevedo c , J. Patarin d a Fachbereich Geowissenschaften (Kristallographie), Universität Bremen, Klagenfurter Str., 28359 Bremen, Germany b Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany c Facultad de Ciencias Físicas y Matemáticas, Laboratorio de Cristalografia, Universidad de Chile, Blanco Encalada 2008, Santiago, Chile d Laboratoire de matériaux minéraux, ENSCMu., UMR CNRS 7016, 3, rue Alfred Werner, 68093 Mulhouse cedex, France

Received 11 July 2002; received in revised form 22 August 2002; accepted 23 August 2002

Abstract The crystal structure of Na2 Si3 O7 has been determined by direct methods using integrated intensities of conventional X-ray powder diffraction data and subsequently refined with the Rietveld technique. The title compound was prepared from Na2 Si3 O7 × H2 O by careful thermal decomposition at 440◦ C. Sodium trisilicate adopts monoclinic symmetry, space group P 21 /c with unit cell parameters a = 7.1924(5) Å, b = 10.6039(8) Å, c = 9.8049(7) Å, β = 120.2478(4)◦ , V = 646.0(9) Å3 and Z = 4. It belongs to the group of interrupted framework silicates of four- and three-connected [SiO4 ]-tetrahedra with a ratio of Q3 :Q4 = 2:1. Within the framework the sodium atoms are coordinated by 4 to 6 oxygen ligands. The porous character of the new phase is reflected in a framework density FD = 18.6 T-atoms/1000 Å3 , a value which is comparable to those observed in zeolitic materials. The topology of the tetrahedral network is identical to the one observed in the hydrous sodium silicate Na2 Si3 O7 × H2 O. Differences can be attributed to tetrahedral rotations. In agreement with the structure analysis the 29 Si MAS NMR spectrum of Na2 Si3 O7 shows two peaks at −91.9 ppm and −102.5 ppm with an intensity ratio of 2:1 which are assigned to three- and four-connected [SiO4 ]-tetrahedra, respectively.  2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Sodium trisilicate; Na2 Si3 O7 ; Interrupted framework structure

1. Introduction The present paper is part of a systematic investigation on the crystal structures in the system Na2 O–SiO2 . The double oxides belonging to this system are of interest in both materials science and earth science and their phase relations have been studied in several investigations [1–3]. Especially the crystal structures of the numerous polymorphic forms of sodium disilicate have been investigated frequently (see [4] and the references cited in there). Further structurally characterized sodium silicates prepared at ambient pressure are Na4 SiO4 [5] as well as Na2 SiO3 [6]. The existence of a phase with composition Na2 Si3 O7 has been first reported by Williamson and Glasser [2]. Crystalline material was obtained by these authors from the devitrification of glasses containing > 67 mol.% SiO2 at tem* Correspondence and reprints.

E-mail address: [email protected] (V. Kahlenberg).

peratures of about 650◦ C. Single crystal photographs indicated an orthorhombic C-centred lattice with a = 20.6, b = 6.50 and c = 4.90 Å. However, the poor quality of the crystals rendered a detailed structure analysis impossible (Glasser, personal communication). Furthermore, a highpressure phase with the same composition containing tetrahedrally and octahedrally coordinated silicon has been prepared by Fleet and Henderson [7]. Recently, Matijasic et al. [8] described the synthesis of a water containing sodium trisilicate Na2 Si3 O7 × H2 O prepared by using a quasi nonaqueous route in the presence of ethylene glycol as solvent. A single crystal structure determination showed that this phase (named Mu-11) is a representative of the crystallochemically rare group of interrupted framework silicates. By heating this material above 360◦ C the water molecules can be removed from the structure and a new compound with a different X-ray powder pattern is formed which is stable up to about 580◦ C. The present paper reports the crystal structure of the previously unknown anhydrous Na2 Si3 O7 phase with a special emphasis on the relations with the sodium

1293-2558/02/$ – see front matter  2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 1 2 9 3 - 2 5 5 8 ( 0 2 ) 0 0 0 0 6 - 7

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trisilicate monohydrate and other interrupted silicate frameworks.

2. Experimental details The Na2 Si3 O7 material was prepared by heating the hydrous precursor material, Na2 Si3 O7 × H2 O [8] in an air stream at temperatures of 350◦ C, 400◦ C, 440◦ C, 540◦ C and 580◦ C for 2 hours. The material obtained at 440◦C was used for the structure analysis. Data collection was performed on a Siemens D5000 diffractometer in a modified Debye– Scherrer geometry with a capillary sample holder. The diffractometer is equipped with a curved Ge monochromator in the primary beam yielding a strictly monochromatic Cu Kα1 -radiation. The powder sample was loaded in a glass capillary of 0.3 mm diameter and a data set was recorded in the range between 12 and 90◦ 2θ . Details of the data collection, basic crystallographic data and definitions are given in Table 1. A first inspection of the X-ray diffraction pattern using the program TREOR [9] revealed that the peaks could be indexed on the basis of the monoclinic cell listed in Table 1. Weak peaks belonging to an impurity phase (see below) were excluded from the refinement. The figures of merit M20 [10] and F30 [11] for assessing the quality of the solution are M20 = 66 and F30 = 124 (0.0046, 35). An analysis of the systematic absences pointed to space group P 121 /c1. Extraction of the integral intensities as well as the structure solution by direct methods was accomplished using the EXPO program suite [12]. After subtraction of the background scattering the integral intensities of 194 reflections up to 2θ = 61◦ were obtained with Pearson VII profile function using a modified LeBail method. The final profile residual of this procedure converged to Rwp = 2.84%. The phase set with the maximum combined figure of merit resulted in an E-map, the most intense peaks of which could be interpreted as the atomic positions of a partial structure of a tetrahedral framework. The missing two oxygen atoms were located by difference Fourier calculations performed with the program SHELXL93 [13]. Although the resulting principal structural features seemed to be reasonable, several interatomic Si–O distances were beyond the spread usually observed in silicate structures. Therefore, the structural model was optimized by distance least squares calculations (program DLS-76, [14]) before the structure refinements of the atomic parameters were initiated with the program FullProf.2k [15]. For the Rietveld analysis (see Fig. 1) the background was determined by linear interpolation between consecutive breakpoints in the pattern. Intensities within 12 times the full width at half maximum of a peak were considered to contribute to the central reflection. The pseudo-Voigt function was chosen for the simulation of the peak shape, with two parameters defining the Lorentzian and the Gaussian character as a function of 2θ . The angular variation of the line width was accounted for by using the Cagliotti function [16].

Table 1 Data collection and Rietveld analysis X-ray diffraction experiment Radiation type, source Generator settings Discriminator Detector Data collection temperature Range in 2θ Step size Counting time per step Basic crystallographic data Unit cell parameters

X-ray, Cu Kα1 45 kV, 35 mA primary beam, curved Ge (111) monochromator Linear PSD room temperature 12–90◦ 0.007768◦ 28 sec

Space group Unit cell content

a = 7.1924(5) Å b = 10.6039(8) Å c = 9.8049(7) Å β = 120.2478(4)◦ P 121 /c1 Na2 Si3 O7 ; Z = 4

Rietveld analysis No. of steps No. of contributing reflections No. of structural parameters No. of profile parameters No. of structural restraints

10942 653 39 17 35

Residuals RI R wp R exp Chi2 Weighting scheme

0.050 0.095 0.067 2.01 1/σ (I)

Several small regions of the XRD pattern which showed only weak reflexions of the impurity phase were excluded from the refinement: 23.40–24.70◦ 2θ , 31.60–32.00◦ 2θ , 34.00–34.40◦ 2θ , 37.60–38.20◦ 2θ , 41.40–41.95◦ 2θ , 43.20– 43.50◦ 2θ . The d-values of the excluded reflections indicated that the impurity phase might be sodium carbonate (named phase II in the following). The refinement was done in consecutive steps with the atomic coordinates and thermal parameters held fixed in the initial calculations. They were allowed to vary after the scale factor, zero point, peak shape parameters and lattice constants were close to convergence to their optimum values. In order to avoid convergence problems soft constraints for the T–O and the O–O bonds were applied assuming a Si–O distance of 1.62 Å, and a O–O distance of 2.65 Å. A low weight was given to these constraints by applying sigma values of 0.01 and 0.02 to the prescribed Si–O and O–O distances, respectively. T–O–T angles have not been constrained in order to permit a more flexible orientation of the tetrahedra in the framework structure. The refined fractional atomic coordinates are listed in Table 2 and selected interatomic distances and angles in Table 3. For drawing of structural details the program ATOMS [17] was used.

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Table 2 Atomic coordinates, isotropic displacement factors (B iso ) and bond valence sums (BVS). All atoms occupy general positions. The isotropic temperature factors for the three different types of atoms were constrained to be equal. In order to calculate realistic esd’s the last cycle of the Rietveld refinements was performed with only every 16th data point resulting in a d-value of 1.56 according to the Durbon-Watson statistics Atom Si1 Si2 Si3 Na1 Na2 O1 O2 O3 O4 O5 O6 O7

x

y

z

B iso

BVS

0.6207(9) 0.7210(9) 0.1636(9) 0.2499(17) 0.2106(15) 0.6731(22) 0.1041(22) 0.3738(12) 0.2168(18) 0.6101(21) 0.6477(21) −0.0141(12)

0.1833(6) 0.3948(5) 0.4319(5) 0.5607(9) 0.1969(10) 0.2940(9) 0.3923(13) 0.1447(11) −0.0839(7) 0.5271(9) 0.3188(9) 0.1030(12)

0.0248(7) 0.2733(7) 0.3143(7) 0.0687(12) 0.1244(12) 0.3820(12) 0.1427(11) −0.0462(14) −0.1521(13) 0.2434(15) 0.1083(11) −0.1315(13)

1.2(1) 1.2(1) 1.2(1) 2.0(2) 2.0(2) 0.9(1) 0.9(1) 0.9(1) 0.9(1) 0.9(1) 0.9(1) 0.9(1)

4.11(7) 3.97(7) 3.93(7) 1.01(2) 0.95(2) 2.00(5) 1.88(4) 2.13(5) 2.05(4) 1.75(4) 2.12(4) 2.06(5)

Table 3 Selected bond distances (Å) and angles (deg) T–O distances Si1–O1 –O3 –O4 –O6 Mean

1.641(11) 1.600(7) 1.602(11) 1.616(11) 1.615

Si2–O1 –O5 –O6 –O7 Mean

1.663(11) 1.567(11) 1.639(11) 1.646(7) 1.629

Na–O distances Na1–O1 –O2 –O2 –O5 –O6 –O4

2.53(1) 2.36(1) 2.39(1) 2.30(1) 2.54(1) 2.88(1)

Na2–O2 –O3 –O5 –O7

2.25(2) 2.54(1) 2.21(1) 2.41(2)

O–T–O angles O1–Si1–O3 O1–Si1–O4 O1–Si1–O6 O3–Si1–O4 O3–Si1–O6 O4–Si1–O6 Mean

110.4(6) 111.7(5) 106.3(6) 112.6(6) 105.8(6) 109.6(6) 109.4

O1–Si2–O5 O1–Si2–O6 O1–Si2–O7 O5–Si2–O6 O5–Si2–O7 O6–Si2–O7 Mean

116.4(5) 104.4(6) 101.5(6) 112.1(7) 115.4(6) 105.8(5) 109.3

T–O–T angles Si1–O1–Si2 Si1–O3–Si3 Si1–O4–Si3 Si1–O6–Si2 Si2–O7–Si3 Mean

131.6(11) 146.9(6) 143.4(7) 146.5(7) 133.0(8) 140.3

For 29 Si MAS NMR spectroscopy the material synthesized at 400◦ C was used because it contained only a very small amount of impurities. The experiments were performed using a Bruker ASX 400 spectrometer with Tetramethylsilane (TMS) as a chemical shift standard at a resonance frequency of 79.49 Mhz. The pulse length was set to 6.5 µsec and the recycle time to 180 sec. The sample was rotated with a spinning rate of 3600 Hz, and 1200 scans were averaged.

Si3–O2 –O3 –O4 –O7 Mean

1.570(11) 1.653(12) 1.651(9) 1.654(6) 1.632

O2–Si3–O3 O2–Si3–O4 O2–Si3–O7 O3–Si3–O4 O3–Si3–O7 O4–Si3–O7 Mean

113.4(7) 112.7(6) 116.5(6) 106.9(6) 99.0(5) 107.1(6) 109.3

3. Results The title compound, Na2 Si3 O7 , could be prepared in a temperature range of 360◦ C to 540◦ C. The crystallinity of the Na2 Si3 O7 material differs with respect to the preparation temperature. At 360◦ C pure Na2 Si3 O7 was formed, but the crystals were of inferior quality as indicated by the presence of broad X-ray diffraction peaks. Crystals of very good quality resulted by heating the precursor material at 440◦ C. At 540◦ C the X-ray reflections became slightly broader

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Fig. 1. Observed (circles) and calculated (solid line) step intensities and their differences (line at the bottom of figure) of Na2 Si3 O7 . Peak positions permitted by the unit cell metric and the symmetry are indicated by tick marks (middle portion).

again. However, at temperatures of
400◦ C small amounts of an impurity phase (phase II) were present as indicated by additional weak reflections in the powder XRD diagram. The d-values of the weak reflections suggested that phase II might be sodium carbonate (see experimental section). At 580◦ C the title compound, Na2 Si3 O7 , was completely transformed to another material, phase III, which is currently under investigation. Finally, cristobalite is formed when the sample is heated to 600◦ C or above. Na2 Si3 O7 has an interrupted framework of [SiO4 ]-tetrahedra consisting of Q3 and Q4 groups in the ratio 2:1. Individual Si–O bond distances show a considerable scatter. However, the observed values are in the normal range for silicate structures [18]. For the two Q3 -type tetrahedra around Si2 and Si3 the Si–O bond distances to the non-bridging (nbr.) oxygen atoms O5 and O2 are significantly shorter (1.57 Å) than the bridging (br.) Si–O bonds, which average at about 1.650 Å and 1.653 Å, respectively. The shortening of the terminal bond lengths compared with the bridging bond lengths results from the stronger attraction between O and Si than between O and the Na cations in the structure, and is a feature frequently observed for silicates. Contrary to Si2 and Si3, the Q4 -type Si1-tetrahedron has fairly uniform Si–O distances which average at about 1.615 Å. The values of the O–O separations within the tetrahedra range from

2.52 to 2.74 Å. The O–Si–O bond angles range from 99 to 117◦ . These values are, again, rather typical of silicate structures [18]. According to [19] the distortion of the tetrahedra can be expressed numerically by means of the angle variance σ 2 . This parameter has values of 8.7, 35.5 and 40.7 for the three tetrahedra about Si1, Si2 and Si3, respectively. According to these values the Q4 -tetrahedron about Si1 shows the least degree of distortion. The connectivity of the T-atoms can be characterized by their coordination sequences which have the following values: Si1: 4-7-15-27-40-58-76-100129-154; Si2: 3-8-14-24-41-58-75-98-129-156; Si3: 3-7-1426-42-53-76-106-125-149. The vertex symbols [20] for the three tetrahedral centers are 4.8.6.8.82.105 (for Si1), 6.8.8. (for Si2) and 4.6.83 (for Si3). Charge balance in the structure is achieved by the incorporation of sodium atoms. Within the structure the two crystallographically independent sodium cations Na1 and Na2, respectively, are coordinated by four and five oxygen neighbors between 2.21 and 2.54 Å. Extending the limit for coordinating anions up to 3.0 Å, Na2 has an additional ligand at about 2.88 Å. Since typical Na–O bond lengths average at about 2.44 Å [21], this latter bond would be very weak. The coordination polyhedra around Na1 and Na2 can be approximately described as severely distorted tetrahedra and trigonal antiprisms, respectively. Bond valence calculations

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Fig. 2. Connectivity between the Si-atoms in the structures of (a) Na2 Si3 O7 and (b) Na2 Si3 O7 × H2 O in a projection parallel [001]. Q4 - and Q3 -atoms are represented by black and white spheres, respectively. The oxygen atoms have been omitted.

were performed using the parameters given by Brown and Altermatt [22] for the Si–O and Na–O pairs, and the bond distances of the first coordination sphere given in Table 3. The bond valence sums (BVS) for all atoms are given in Table 2. The values were close to the formal atomic valences except for the terminal oxygen atom O5 which has a BVS of only 1.75(4). This might indicate that the structure still contains a very small amount of (undetected) protons which contribute to the BVS of O5. There exist different possibilities to subdivide the interrupted framework structure of Na2 Si3 O7 into tetrahedral layers which build up the whole network by corner sharing. Fig. 2a shows the connectivity between the Si-atoms of a single tetrahedral layer perpendicular to [001] in the sodium trisilicate. Within the corrugated sheet, chains can be isolated which consist of four-membered (S4R) and sixmembered (S6R) rings (cf. Fig. 3a). At the interface between neighboring chains (S8R) are formed. Each S4R consists of two Q3 and two Q4 groups, whereas a single S6R contains four Q3 and two Q4 . A projection of Na2 Si3 O7 parallel to [100] is given in Fig. 4. As can be seen the framework contains channels running along a with pore openings formed by ten-membered rings. The Na2 atoms are located within the central regions of these tunnels, whereas the Na1 ions occupy positions closer to the boundary of the channels. Up to this point the oxygen atoms have been omitted from the graphical representations since the visualization of the tetrahedra is not very helpful in the highly corru-

gated layers mentioned above. However, a concise description can be obtained considering 6 Å wide slabs cut from ¯ (cf. Fig. 5a). The slab conthe framework parallel (102) tains a layer with tetrahedra arranged in four- and twelvemembered rings. With regard to this slab, the sodium atoms ¯ close too are approximately located in layers parallel (102) to the apical oxygens. The framework density [23] of the porous crystal structure of Na2 Si3 O7 is 18.6 T-atoms/1000 Å3 , a value which is comparable to those observed in zeolitic materials. The 29 Si MAS NMR spectrum of Na2 Si3 O7 shows only two peaks at −91.9 ppm and −102.5 ppm (Fig. 6) which are assigned to three- and four-connected [SiO4 ]-tetrahedra, respectively. The intensity ratio between the signals of Q3 - and Q4 -units is 2:1. This is in agreement with the structure analysis which proved that 8 three-connected [SiO4 ]-tetrahedra and 4 four-connected [SiO4 ]-tetrahedra are present per unit cell. Since the 8 three-connected [SiO4 ]-tetrahedra, Si1 and Si2, occupy two symmetrically independent sites in the structure two signals in the 29 Si MAS NMR spectrum were expected but these signals could not be resolved. The chemical shift values of the signals are at the very limit of the ranges typically observed in silicates: −90 to −102 ppm for Q3 -units and −106 to −120 ppm for Q4 -units [24]. We interpret this to be due to unusually small Si–O–Si angles (mean value = 140.4◦) observed in the Na2 Si3 O7 structure (see Table 3) which differ from those of a relaxed silica framework having mean Si–O–Si angles of ca. 144◦ [18]. In the case

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(a)

(b)

Fig. 3. Single chain consisting of four- and six-membered rings in (a) Na2 Si3 O7 and (b) Na2 Si3 O7 × H2 O.

(a)

(b)

Fig. 4. Projection of the structure of (a) Na2 Si3 O7 and (b) Na2 Si3 O7 × H2 O along [100] (the oxygen atoms have been omitted). The two symmetrically independent sodium atoms are distinguished by different hatching patterns.

of Na2 Si3 O7 × H2 O the chemical shift values of the signals of the 29 Si MAS NMR spectrum were −87.5, −92.9 and −100.0 ppm and the mean value of the Si–O–Si angle was 138.7◦ [8]. Thus, the silicate framework of the anhydrous title structure is seemingly slightly more relaxed than the framework of the hydrous precursor.

4. Comparison with related structures and discussion The comparison between the basic crystallographic data of Na2 Si3 O7 × H2 O and Na2 Si3 O7 listed in Table 4 suggests a close structural relation. Indeed this hypothesis is supported from a comparison between the crystal structures shown in Figs. 2 and 4. The basic building units of both framework structures as well as the connectivities are identical. The shortening of the translation period parallel [010] in Na2 Si3 O7 can be primarily rationalized by a significant reduction of the pore size of the six-membered rings due to rotations of the tetrahedra. The four membered rings belong-

ing to the same layer are almost unchanged. On the other hand, the expansion of the c axis in the anhydrous phase is also explainable by rotations of the tetrahedra without any breaking of bonds. The structural modifications can be directly attributed to the dehydration process. The Na1 atom in Na2 Si3 O7 × H2 O, for example, is coordinated by four oxygen anions and one water molecule. Removing the H2 O group without any structural changes would result in an unusual and energetically unfavorable almost planar quadratic coordination environment of the remaining four oxygen ligands around Na1. Therefore, the structure relaxes and a more stable distorted tetrahedral coordination geometry is achieved. Although the crystal chemistry of natural and synthetic silicates has been studied in great detail, the occurrence of interrupted framework structures is rather an exception. To our best knowledge so far only eight materials are known among the group of interrupted framework compounds where the T-sites are exclusively occu-

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(a)

(b) ¯ containing four- and twelve-membered rings in (a) Na2 Si3 O7 and (b) Na2 Si3 O7 × H2 O. Fig. 5. Single tetrahedral layer parallel (1 0 2)

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Table 4 Comparison between basic crystallographic data in Na2 Si3 O7 × H2 O and Na2 Si3 O7 a (Å)

b (Å)

c (Å)

β (◦ )

V (Å3 )

Space group

Reference

7.3087 7.1924

12.7246 10.6039

9.0913 9.8049

119.01 120.2479

739.4 646.0

P 121 /c1 P 121 /c1

Matijasic et al. (2000) This work

Acknowledgement JCMA is a grateful recipient of a DAAD scholarship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] Fig. 6. 29 Si MAS NMR spectrum of Na2 Si3 O7 . Two sharp signals are visible which correspond to three- and four-connected [SiO4 ]-tetrahedra, respectively (Q3 - and Q4 -units). Spinning side bands are indicated by * .

pied by silicon: the title compound, Na2 Si3 O7 × H2 O, K3 NdSi7 O17 [25], K2 CeSi6 O15 [26], Rb6 Si10 O23 and isotypic Cs6 Si10 O23 [27], K2 Si2 O5 [28] and microporous RUB22, (N(CH3 )4 )4 [Si32 O60 (OH)12] (Marler et al., unpublished results). The proportion of the Q4 sites in these phases differ significantly: while the frameworks in K2 CeSi6 O15 and potassium disilicate are build from Q3 -groups exclusively, the other materials have a certain proportion of Q4 sites. In K3 NdSi7 O17 only one in seven tetrahedra has four bridging oxygen ligands, in Na2 Si3 O7 and Na2 Si3 O7 × H2 O one in three tetrahedra has four bridging oxygen ligands. In RUB-22 half of the tetrahedra correspond to Q4 sites and in Rb6 Si10 O23 /Cs6 Si10 O23 three out of five tetrahedra are four-connected to neighboring tetrahedra. Extending the scope of this brief discussion to compounds with different elements on the Tsites several zeolite type materials can be classified as interrupted frameworks. Examples are chiavennite [29] and wenkite [30]. The differences between the unit cell data reported for Na2 Si3 O7 by Williamson and Glasser [2] mentioned in the introduction and the values observed for sodium trisilicate in this study point to the existence of different modifications depending on the synthesis route. Preparative and structural investigations on this unknown sodium trisilicate phase are currently in progress.

[13] [14]

[15]

[16] [17] [18] [19] [20] [21]

[22] [23] [24] [25] [26] [27] [28] [29] [30]

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