Mixed Uranyl SulfateâSelenates: Evolution of Structural Topology and

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Mixed Uranyl Sulfate-Selenates: Evolution of Structural Topology and Complexity vs. Chemical Composition Vladislav V. Gurzhiy, Olga S. Tyumentseva, Sergey V. Krivovichev, Vladimir G. Krivovichev, and Ivan G. Tananaev Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00611 • Publication Date (Web): 23 Jun 2016 Downloaded from http://pubs.acs.org on June 23, 2016

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Crystal Growth & Design

Mixed Uranyl Sulfate-Selenates: Evolution of Structural Topology and Complexity vs. Chemical Composition Vladislav V. Gurzhiy,*,† Olga S. Tyumentseva,† Sergey V. Krivovichev,† Vladimir G. Krivovichev,† and Ivan G. Tananaev‡ †

Institute of Earth Sciences, St. Petersburg State University, 199034, University emb. 7/9, St.

Petersburg, Russian Federation ‡

Far Eastern Federal University, 690950, Suhanova st. 8, Vladivostok, Russian Federation

Uranyl, selenate, sulfate, potassium, crystal structure, topology, structural complexity, structural evolution

Phase formation in the aqueous system of uranyl nitrate, potassium hydroxide and variable amount of sulfuric and selenic acids has been investigated. Four different types of crystalline phases with variable S and Se contents were isolated and characterized using single-crystal X-ray diffraction (XRD) and IR spectroscopy. Topological analysis and information-based complexity calculations demonstrated: (a) the absence of a continuous solid solution in the system, (b) the

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absence of isotypic sulfate and selenate phases, (c) the discovery of two layered topologies unprecedented among inorganic oxysalts.

Introduction Within the last fifteen years, structural chemistry of actinide-containing compounds experienced continuous and extensive development.1-4 However, the field is still far from being mature, and there many ‘white spots’ worth of investigation. Even the structural chemistry of uranium, not to mention transuranium elements, contains many unsolved problems and unanswered questions. Uranium compounds containing oxoanions of hexavalent cations (S, Mo, Cr, Se) are of special importance from the environmental and mineralogical points of view. It is of great interest that natural and synthetic chemistry of these compounds are remarkably different. For instance, uranyl sulfate minerals do not contain any noticeable amounts of Se, and, whereas uranyl sulfates are one of the most widespread natural secondary phases, uranyl selenates are unknown as minerals, though there are seven natural uranyl selenites reported to date.2,5 It is noteworthy that there are several isotypic uranyl sulfates and selenates known,6,7 which points out to the possibility of Se6+ - S6+ substitution in minerals and synthetic compounds. For instance, adolfpateraite, K(UO2)(SO4)(OH)(H2O),8 has both sulfate9 and selenate10 synthetic analogues. The study of the phase formation in mixed uranyl sulfate-selenate systems seems to be a promising avenue from both geochemical (mineralogical) and chemical points of view.


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Herein we report on the results of our investigations of the phase formation in the aqueous uranyl nitrate – sulfuric acid – selenic acid – potassium hydroxide system, which provide some interesting implications for mineralogy and explorative inorganic chemistry of uranium.

Synthesis In order to investigate mixed uranyl sulfate-selenate system, the K+-UO22+-TO42--NO3--H2O system was chosen, which was previously investigated for T = Se.11-13 The mixture of uranyl nitrate hexahydrate (0.2 g, 0.4 mmol, (UO2)(NO3)2·6H2O, Vekton, 99%), potassium hydroxide (0.05 g, 0.85 mmol, KOH, Vekton, 98%), and deionized distilled water (2 ml, 110.2 mmol) had been taken unchanged for each synthetic experiment, while the amounts of selenic acid (40 wt. % in H2O, Aldrich, 99.95%) and sulphuric acid (Aldrich, 98%) had been taken in different molar Se:S ratios according to the Table 1. Aqueous solutions were stirred to homogenization and left to evaporate in watch glasses at room temperature. After 2-5 days, yellowish-green crystals were detected in each synthetic experiment. In total, fourteen crystalline phases were isolated as single crystals numbered as 2-15 in Table 1 (the compound 1 is a pure selenate phase, which was reported previously12).

Table 1. Chemical formulae and the details of synthesis of the compounds 1 – 15.

Compound

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Formula

1

K(H5O2)[(UO2)2(SeO4)3(H2O)]12

2

K(H5O2)[(UO2)2(SeO4)2.9(SO4)0.1(H2O)]

3

K(H5O2)[(UO2)2(SeO4)2.7(SO4)0.3(H2O)]

H2SeO4:H2SO 4 in ml (in mmol) 0.20 : 0 (4.0 : 0) 0.19 : 0.01 (3.8 : 0.2) 0.17 : 0.02 (3.5 : 0.5)

Se / (Se+S), mol % in solution

in structure

100

100

94.2

95.3

88.3

88.7


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4

K(H5O2)[(UO2)2(SeO4)2.4(SO4)0.6(H2O)]

5

K3(H5O2)[(UO2)2(SeO4)3.0(SO4)1.0(H2O)2](H2O)4

6

K3(H5O2)[(UO2)2(SeO4)3.1(SO4)0.9(H2O)2](H2O)4

7

K3(H5O2)[(UO2)2(SeO4)3.2(SO4)0.8(H2O)2](H2O)4

8

K3(H5O2)[(UO2)2(SeO4)3.1(SO4)0.9(H2O)2](H2O)4

9

K3(H5O2)(H3O)2[(UO2)5(SeO4)5.3(SO4)2.7(H2O)](H2O)z

10 11 12 13

K5(H5O2)3(H3O)[(UO2)6(SO4)4.8(SeO4)5.2(H2O)4](HSO4)0.8(HSeO4)0.2 (H2O)2.8 K5(H5O2)3(H3O)[(UO2)6(SO4)5.5(SeO4)4.5(H2O)4](HSO4)0.8(HSeO4)0.2 (H2O)3.4 K5(H5O2)3(H3O)[(UO2)6(SO4)6.1(SeO4)3.9(H2O)4](HSO4)0.9(HSeO4)0.1 (H2O)3.5 K5(H5O2)3(H3O)[(UO2)6(SO4)7.2(SeO4)2.8(H2O)4](HSO4)0.9(HSeO4)0.1 (H2O)3.5

14

K5(H5O2)3(H3O)[(UO2)6(SO4)8.4(SeO4)1.6(H2O)4](HSO4)(H2O)3.25

15

K5(H5O2)3(H3O)[(UO2)6(SO4)10(H2O)4](HSO4)(H2O)3.25

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0.15 : 0.05 (3.1 : 0.9) 0.15 : 0.06 (3.1 : 1.1) 0.14 : 0.06 (2.8 : 1.1) 0.15 : 0.07 (3.1 : 1.3) 0.15 : 0.08 (3.1 : 1.5) 0.15 : 0.08 (3.1 : 1.5) 0.13 : 0.07 (2.6 : 1.3) 0.10 : 0.07 (2.0 : 1.3) 0.10 : 0.10 (2.0 : 1.9) 0.08 : 0.12 (1.6 : 2.3) 0.05 : 0.15 (1.0 : 2.8) 0 : 0.20 (0 : 3.7)

76.5

81.7

73.0

76.8

71.6

78.5

69.9

80.5

67.0

76.8

67.0

63.9

66.8

53.9

60.7

46.2

52.0

35.6

41.9

26.1

26.5

16.2

0

0

Single Crystal X-ray Diffraction Single crystal X-ray diffraction studies of 2 – 15 were carried out using a Bruker Smart singlecrystal X-ray diffractometer equipped with an APEX II CCD area detector operated with monochromated MoKα radiation at 50 kV and 40 mA (for 10 – 14) and Bruker Kappa Duo diffractometer equipped with an APEX II CCD area detector operated with microfocused monochromated MoKα radiation at 50 kV and 0.6 mA (for 2 – 9, 15). Diffraction data were collected at room-temperature conditions with frame widths of 0.5° in ω and φ, and exposition of 5 to 240 s (depending on the size of the crystal) spent per each frame. Data were integrated and corrected for background, Lorentz, and polarization effects using an empirical spherical model by means of the Bruker programs APEX2 and XPREP. Absorption correction was applied using the SADABS program.14 The unit-cell parameters (Table 2) were refined by the least-squares techniques. The structures were solved by direct methods and refined


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using the SHELX programs15 incorporated in the OLEX2 program package.16 The final models included coordinates and anisotropic displacement parameters for all non-hydrogen atoms except for the several partially occupied positions of interlayer water molecules in the structure of 9 due to the low quality of its crystals. Positions of H atoms of H2O and hydronium molecules were not localized. The crystal structures of 9 and 14 have been refined as inversion twins and the crystal structures of 11–13 have been refined as two-component twins with (100) as a twinning plane using the [-100/010/00-1] matrix. Supplementary crystallographic data have been deposited at Inorganic

Crystal

Structure

Database

(Table

2)

and

can

be

obtained

from

Fachinformationszentrum Karlsruhe via www.fiz-karlsruhe.de/request_for_deposited_data.html.

Table 2. Crystallographic and refinement parameters for 1 – 15.

Compound

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Se content in structure

S.G.

a, Å / α, °

1

100

P21/c

11.456(2)

2

95.3

P21/c

11.4448(12)

3

88.7

P21/c

11.4176(12)

4

81.7

P21/c

11.4014(3)

5

78.5

C2/c

17.7579(18)

6

76.8

C2/c

17.774(4)

7

80.5

C2/c

17.7681(16)

8

76.8

C2/c

17.7245(4)

9

63.9

P21

8.2050(7)

10

53.9

Amm2

11

46.2

Amm2

b, Å / β, °

V, Å3

R1 (|Fo| ≥ 4σF)

ICSD

14.809(2)

1698.4(4)

0.0547

423517 12

14.8096(16)

1700.1(3)

0.0245

430719

14.7868(15)

1691.3(3)

0.0224

430718

c, Å / γ, °

10.231(1) / 101.901(4) 10.2486(11) / 101.834(2) 10.2339(10) / 101.796(2) 10.2270(3) / 101.710(1) 8.1510(7) / 97.035(3) 8.1557(18) / 97.089(4) 8.1487(7) / 96.984(2) 8.1359(2) / 97.001(1) 57.399(5) / 93.265(2)

14.7706(4)

1686.44(8)

0.0293

430717

17.7750(17)

2553.5(4)

0.0318

430725

17.785(4)

2558.5(10)

0.0204

430722

17.7917(15)

2556.9(4)

0.0236

430726

17.7502(4)

2540.58(10)

0.0257

430728

10.3222(8)

4853.4(7)

0.0649

430727

12.9496(10)

18.5797(14)

13.5134(11)

3251.3(4)

0.0269

430724

12.9158(5)

18.5289(7)

13.4353(5)

3215.3(2)

0.0440

430723


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12

38.8

Amm2

12.879(2)

18.497(3)

13.402(2)

3192.5(9)

0.0422

430720

13

29.2

Amm2

12.8777(6)

18.4628(6)

13.3817(5)

3181.6(2)

0.0331

430721

14

16.2

Amm2

12.8530(17)

18.369(2)

13.402(2)

3164.2(8)

0.0451

430716

15

0

Amm2

12.8259(6)

18.2778(9)

13.3388(7)

3127.0(3)

0.0184

430715

Results Correlation between the solution and solid state chemical composition The phases 2–4 crystalize in the monoclinic system, P21/c, and are isotypic to the previously reported K(H5O2)[(UO2)2(SeO4)3(H2O)].12 The unit-cell parameters decrease gradually with the increase of the S content (Table 2). In order to analyze the rates of the S-for-Se substitution in the crystalline phases, the correlation plot of the Se:S ratio in the original solution versus the Se:S ratio in the solids has been studied (Table 1, Figure 1). According to the plot, the compound 112 corresponds to the upper right point (100:100), while the pure sulfate compound 15 appears in the lower left corner (0:0). The points corresponding to the compounds 1–4 (Figure 1) indicate linear correlation with R2 = 0.98 that can be described by the following Se-S distribution function: εcryst = 0.79εsol + 20.76

(1),

where εcryst and εsol are the Se:S ratios in crystalline phase and solution, respectively. The respective S/Se distribution coefficient, Kd, was calculated according to the formula Kd = [crystxSe][solxSe]-1[crystxS][solxS]-1 where

cryst

xT and

sol

(2),

xT are concentrations of T (= Se, S) in the crystalline phase and solution,

respectively.


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Figure 1. Correlation graph of the Se to S ratio in the original solution and in the resulted crystal structures for the compounds 1 – 15. Legend: following from right to left; blue rhombs = compounds 1 – 4, green triangles = compounds 5 – 8, purple circle = compound 9, dark red squares = compounds 10 – 15; linear (black, solid) and polynomial (dark red, dashed) trends are shown with the correlation coefficients. For the phases 1-4, Kd ~ 1, which means that S and Se are distributed almost equally between solution and crystalline phase. It is also remarkable that the slope of the curve in Figure 1 is 0.8, which means that Se:S ratio in crystalline phases is smaller than that in the mother liquid, which may be explained by the lower solubility of sulfate component compared to the selenate one. Continuation of the observed linear function to εsol = 0 (i.e. for a pure sulfate phase) would correspond to the impossible value εcryst ~ 20. Therefore, in order to be real, the Se-S distribution function has to change its character in the middle of the diagram, which is what actually happens.


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In the ‘sufate’-rich region of the diagram (εsol = 0–66), crystallization of the phases 10–15 have been observed. The phases are isotypic, crystalize in the orthorhombic system, space group Amm2, and have the ideal formula K5(H5O2)3(H3O)[(UO2)6(TO4)10(H2O)4](HTO4)(H2O)y (where T = S, Se and y = 3.25 for the pure sulfate 15). This structure type is novel and had never been observed before (Table 2). As for the compounds 1–4 (εsol = 78–100), there is a positive linear correlation in unit-cell parameters increase with the increase of the Se content in the structure. The Se-S distribution function (R2 = 0.97) is linear and can be described by the following equation: εcryst = 0.79εsol – 3.0 1

(3).

Note that the slope of the function is again 0.8, but the y-intercept is different and equals ~ – 3.0 for the pure sulfate system (εsol = 0). Slightly more realistic (R2 = 0.99) approximation can be obtained with the second-degree polynomial function: εcryst = 0.006εsol2 + 0.37εsol + 0.4

(4).

In this case, the y-intercept is equal ~ 0 as it should be for the pure sulfate system (εsol = 0). The Kd value calculated according to the equation (2) for this part of the diagram is equal to ca. 0.71, which is drastically different from the value of (1) observed for the phases 1–4. Thus, the system displays two regions with the linear relation between εcryst and εsol (0–66 and 78–100) and the intermediate region, where the relation should obviously be modified. It was observed that crystallization in this intermediate region proceeds with difficulty and requires more time that in the ‘linear’ parts. The compounds 5–8 crystalize in the monoclinic system, space group C2/c, with the ideal formula K(H5O2)[(UO2)2(TO4)3(H2O)] (T = S, Se). These compounds are isotypic to the recently published pure selenate compounds.11 According to the data given in Table 2, there is a positive correlation between the decrease of the Se amount in the structure and the


8

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decrease in the unit-cell parameters of crystalline phases. It is of interest that Se-S distribution function shows rather variable scatter of points: compounds 5–7 show the negative tendency (less Se in the solution – more Se in the crystal), whereas the compound 8 demonstrates the positive trend. However, the number of points is small and does not allow for unambiguous conclusions. The most interesting phase was the phase 9 found as tiny plates in the same synthetic experiment that produced crystals of 8. The compound 9 is monoclinic, P21, and has the ideal formula K3(H5O2)(H3O)2[(UO2)5(SeO4)5.3(SO4)2.7(H2O)](H2O)z with z ~ 7.5. In conclusion, the system under study has two regions of linear relations between the chemical composition of solutions and crystalline phases separated by the intermediate region, where the function alters its shape and where structure types experience dramatic changes.

Structure descriptions The crystal structures of 1–4 contain two crystallographically nonequivalent U atoms each with two short U6+=O2– bonds (1.755(4)–1.767(4) Å) forming approximately linear UO22+ uranyl ions (Ur). The U(1)O22+ cation is coordinated by five oxygen atoms (U1‒Oeq = 2.381(4) – 2.417(4) Å) that belong to the sulfate-selenate tetrahedra and that are arranged in the equatorial plane of the Ur(1)O5 pentagonal bipyramid. The U(2)O22+ cation is coordinated by four oxygen atoms (2.340(4)–2.408(4) Å) belonging to the (TO4)2– (T = S, Se) tetrahedra and one oxygen atom of water molecule with the U(2)–H2O(12) bonds are elongated (2.477(5)–2.482(4) Å) compared to other U‒Oeq bonds, which is typical for hydrated uranyl complexes. Three symmetrically nonequivalent T6+ positions are tetrahedrally coordinated by four O2– atoms each. The (TO4)2– groups are tridentate, sharing three vertices with adjacent uranium polyhedra,


9


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respectively. Non-shared vertices of the selenate tetrahedra are oriented either up or down relative to the plane of the layer. Crystal structures of 1–4 are based upon the [(UO2)2(TO4)3(H2O)]2– uranyl selenate (in 1) or sulfate-selenate (in 2–4) layers formed by linkage of U and T coordination polyhedra via common O atoms (Figure 2a, b). The layers are parallel to (001) and are modulated. The modulation vector is parallel to [010] and equals to the b unit-cell parameter; the modulation amplitude is about 6 Å. The charge of the layer is compensated by potassium and Zundel (H5O2+) cations arranged in between the 2-D uranyl complexes. The crystal structures of 5–8 contain one crystallographically nonequivalent U atom each with two short bonds in the range of 1.756(4)–1.786(3) Å. The UO22+ cation is coordinated by four oxygen atoms (2.320(4)–2.400(5) Å) belonging to the (TO4)2– (T = S, Se) tetrahedra and one oxygen atom of H2O molecule with the elongated U–H2O(6) bond (2.471(4) – 2.480(4) Å). Two crystallographically nonequivalent T6+ positions are tetrahedrally coordinated by four O2– atoms each. The (TO4)2– groups are bidentate, bridging uranyl coordination polyhedra and thus forming layered complexes. Crystal structures of 5–8 are based upon the [(UO2)(TO4)2(H2O)]2– uranyl sulfate-selenate layers formed by the linkage of U and T coordination polyhedra via common O atoms (Figure 2c, d). The layers are parallel to (10-1). The negative charge of the layer is compensated by potassium and Zundel cations arranged in between the 2-D uranyl complexes.


10

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Figure 2. Crystal structure and crystals: compounds 1 – 4 (a and b); compounds 5 – 8 (c and d). Legend: U polyhedra = yellow, TO4 (T = S, Se) tetrahedra = orange, K atoms = cyan, O atoms = red. The crystal structure of 9 is remarkable by its unusually large b unit-cell parameter of 57.399(5) Å. It contains ten crystallographically nonequivalent U atoms with U6+=O2– bonds ranging from 1.71(2) to 1.82(2) Å. The Ur(1) and Ur(4) cations are coordinated by four oxygen atoms belonging to the (TO4)2– (T = S, Se) tetrahedra and one H2O molecule with the U(1)–


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H2O(3) = 2.53(2) Å and

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U(4)–H2O(26) = 2.45(2) Å. The Ur(2-3) and Ur(5-10) cations

coordinated by five oxygen atoms that belong to the tetrahedral oxyanions. The Ur–Oeq bond lengths are in the range 2.33(2) – 2.44(2) Å. There are eighteen crystallographically nonequivalent T6+ positions in the structure of 9, tetrahedrally coordinated by four oxygen atoms each. Condensation of uranyl and (TO4)2–polyhedra via sharing common vertices leads to the layered structure. The crystal structure of 9 is based upon the [(UO2)5(TO4)8(H2O)]6– modulated uranyl sulfateselenate layers parallel to (100) (Figure 3). The modulation vector is parallel to [010] and equals to the half of the b unit-cell parameter; the modulation amplitude is about 7 Å. The charge of the layer is compensated by potassium, hydronium and Zundel cations arranged in between the uranyl sulfate-selenate layers. The structure type of the compounds 10–15 contains two crystallographically nonequivalent U atoms in pentagonal bipyramidal coordination by oxygen atoms: two short bonds in the range of 1.687(16)–1.739(17) Å forming Ur entities and five longer bonds (2.345(11)–2.525(13) Å) in the equatorial planes of Ur. Ur(2) shares all five Oeq with the neighboring TO4 polyhedra, while Ur(1) shares only four Oeq and the fifth oxygen atom belongs to the H2O(7) water molecule. There are four T6+ positions in the structures of 10–15 tetrahedrally coordinated by four O2– atoms each: three of them involved in the formation of the layers and the T(4) position is arranged in between the 2-D complexes, in every second interlayer space, with the T(4)-O(17)H bond elongated to ~ 1.7 Å.


12

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Figure 3. Crystal structure (a) and crystals (b) of the compound 9; polyhedral representation of layered complex in the structure of 9 (c) and its graphical representation. Legend: U polyhedra = yellow, TO4 (T = S, Se) tetrahedra = orange, K atoms = cyan, O atoms = red; black nodes = U atoms, white nodes = T atoms. The crystal structures of 10–15 are based upon the [(UO2)3(TO4)5(H2O)2]4– layers formed by linkage of U and T coordination polyhedra via common O atoms (Figure 4). The layers are flat and parallel to (100). The negative charge of the layers is compensated by three


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crystallographically nonequivalent potassium cations, one hydronium ion and two Zundel cations, arranged in the interlayer space along with three additional water molecules. With the increase of the Se amount in the structures (from 15 to 10), positions of protonated and neutral H2O molecules split with the change in their occupancies that could be associated to the tilting of (TO4)2– tetrahedra.

Figure 4. Crystal structure (a) and crystals (b) of the compounds 10 – 15; polyhedral representation of layer in the structures of 10 – 15 (c) and topology of its interpolyhedral linkage. For legend see Figure 3.


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Structural topology The topology of linkage of the U and T coordination polyhedra can be described using blackand-white graphs, where U6+ and T6+ coordination polyhedra are symbolized by the black and white nodes, respectively. Two nodes are linked by an edge if the corresponding polyhedra have a shared O atom. Analysis of the topology of the uranyl-containing layers using graphs17,18 indicates that the topology of the 2-D layers in the structures of 1–4 belongs to the cc2-2:3-4 type (linkage of 4- and 6-membered rings creating edge-shared chains of the later arranged along [010], Figure 5a, c), one of the most widespread in uranyl compounds. The topology of the layers in the structures of 5–8 belongs to the cc2-1:2-3 type (sequential linkage of 4- and 12-membered rings along [101], Figure 5b, d). The topology of the layers in the structure of 9 belongs to the cc2-5:8-5 type (linkage of 4-membered rings creating dimers of vertex-sharing 6-membered rings, Figure 3c, d). It is worthy to note that topologies with the M:T ratio = 5:8 are very rare – only six compounds with four different topological types are known.6,18 The layer topology in 9 is unprecedented for the structural chemistry of inorganic oxysalts and belongs to the novel (5th) type, though it has some similarities with the known ones. The arrangement of the dimers of 6membered rings in cc2-5:8-2 (Figure 6b)6,19 has a sequential change of direction …-up-down-updown-… whereas, in cc2-5:8-3, the dimers has a coaxial …-up-up-up-up-… arrangement (Figure 6c).2 The cc2-5:8-5 topology observed in 9 represents more complicated sequence with the double zones …-up-up-down-down-up-up-down-down-… (Figure 6a). The topology of the 2-D complexes in the structures of 10–15 belongs to the cc2-3:5-5 type (linkage of 4-membered rings tetramers creating large 10-membered crowns, Figure 4c, d), it is unprecedented for the structural chemistry of inorganic oxysalts as well and is quite different from the other topologies with M:T = 3:5.


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Figure 5. Polyhedral representation of layers in the structures of 1 – 4 (a), 5 – 8 (b) and topology of its interpolyhedral linkage (c and d, respectively). For legend see Figure 3.


16

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Figure 6. Idealized black-and-white 2D graphs with the dimers of 6-memberd rings arrangement shown by orientation vectors and shaded fragments for the topological types cc2-5:8-5 in 9 (a), cc2-5:8-2 (b)6,19 and cc2-5:8-3 (c)2.

Occupancy of the T6+ sites


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The refinement of the occupancies of the T6+ sites in the crystal structures of 2–14 shows the absence of a uniform statistical distribution (Tables 3, 4). Furthermore, for all structure types there is a certain trend of selenium incorporation in each sulfate position (or vice versa).

Table 3. Occupancy of the T6+ positions by Se atoms (p.f.u.) in the structures of 1 – 8 and 10 – 15. Dark grey color = relatively low content of Se; light grey color = relatively high content of Se. Se / (Se+S), mol % Compound

T1

T2

T3

T4 in in solution structure

1

1.00

1.00

1.00

100

100

2

0.97

0.98

0.91

94.2

95.3

3

0.91

0.93

0.82

88.3

88.7

4

0.86

0.88

0.71

76.5

81.7

5

0.86

0.67

73.0

78.5

6

0.87

0.70

71.6

76.8

7

0.88

0.73

69.9

80.5

8

0.84

0.69

67.0

76.8

10

0.85

0.49

0.39

0.16

66.8

53.9

11

0.83

0.41

0.30

0.15

60.7

46.2

12

0.72

0.35

0.25

0.08

52.0

35.6

13

0.62

0.23

0.16

0.04

41.9

26.1

14

0.41

0.12

0.08

0.005

26.5

16.2

15

0.00

0.00

0.00

0.00

0

0


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Table 4. Occupancy of the T6+ positions by Se atoms (p.f.u.) in the structure of 9. Dark grey color = relatively low content of Se; light grey color = relatively high content of Se. Pos.

Occ.

Pos.

Occ.

T1

0.89

T10

0.57

T2

0.83

T11

0.62

T3

0.51

T12

0.74

T4

0.79

T13

0.73

T5

0.36

T14

0.53

T6

0.43

T15

0.76

T7

0.47

T16

0.29

T8

0.87

Se in solution

67.0

T9

0.81

Se in structure

63.9

Thus, for 1–4, the T1 and T2 sites are equally occupied by Se and S, while the S-for-Se substitution in the T3 site is twice as extensive. In 1–4 all tetrahedra are tridentate, having three vertices shared with adjacent U polyhedra and only the fourth one ‘pending’. The T1 and T2 sites are arranged around large 6-membered rings, whereas the T3 position is a connection point for three small 4-membered cycles. Therefore, the T3 position has much less degree of freedom, especially considering that its fourth non-bridged vertex is involved in a strong ionic bond to K+ ion. In contrast, apical vertices of the T1O4 and T2O4 tetrahedra are involved only in relatively weak hydrogen bonding to interlayer Zundel cations (Table 3). In the crystal structures of 5–8, both symmetrically independent TO4 tetrahedra are bidentate. The T1 tetrahedra are involved in the formation of large 12-membered cycles whereas the T2


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tetrahedra form dense 4-membered rings. Therefore, the preference of S for the more dense and tightly bonded site has a mechanistic structural explanation (Table 3). Another situation is observed in the structures of 10–14. They contain four different T sites (Table 3): two tridentate one bidentate and one located in the interlayer. The T1 position is predominantly Se-preferred compared to others (with much higher occupancy than the concentration of Se in solution). Analyzing the higher coordination spheres of the T positions, it could be noted that the triangular fragment U(1)–U(2)–U(1) (with U(1)–U(2) = 6.2 Å, U(1)–U(1) = 6.6 Å) around the T1 position is the most regular and rigid. Taking into account the flat geometry of the layers, it looks like the insertion of Se into the T1 site does not require considerable changes in structural geometry. In contrast, the increase of Se in the T3 site, for instance, results in the change of the bi-dentate tetrahedra tilt. The interlayer T4 site corresponding to the loosely bonded (HTO4)– anion is predominantly occupied by S: it is evident that the incorporation of larger Se into this site would result in the expansion of the interlayer distance and the weakening of the interlayer connectivity. There are sixteen tetrahedral positions in the structure of 9, and all of them are tridentate. As it can be seen from Table 4 and Figure 1, the amount of Se in the structure of 9 better than in the other structures correlates with the Se concentration in the solution; however, the occupancies of the T sites are much more diverse and vary from 0.29 to 0.89 Se per formula unit (p.f.u.). The positions occupied predominantly by Se are mostly arranged at the hollow 6-membered rings and the S-rich positions are arranged at the point of the association of three 4-membered rings. Thus the general principles that govern the Se-S substitution in the system can be formulated as following: (a) smaller S6+ cations tend to incorporate themselves into more tightly bonded


20

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arrangements than the larger Se6+ cations; (b) the interlayer species are preferentially occupied by S6+ cations.

Structural complexity analysis In order to compare structural variations in the system under study from the viewpoint of structural complexity, the latter was estimated as a Shannon information content per atom (IG) and per unit cell (IG,total). According to this approach developed in,20-22 complexity of a crystal structure can be quantitatively characterized by the amount of Shannon information it contains measured in bits (binary digits) per atom (bits/atom) and per unit cell (bits/cell), respectively. The concept of Shannon information, also known as Shannon entropy, employed here originates from the information theory and its application in graph theory, chemistry, biology, etc. The amount of Shannon information reflects diversity and relative proportion of different objects, e.g., the number and relative proportion of different sites in an elementary unit cell of a crystal structure. For a crystal structure, the calculation involves the use of the following equations20-22: IG = –∑ i log2 pi

(bits/atom)

IG,total = – v IG = – v∑ i log2 pi

(5), (bits/cell)

(6),

where k is the number of different crystallographic orbits (independent crystallographic Wyckoff sites) in the structure and pi is the random choice probability for an atom from the i-th crystallographic orbit, that is: pi = mi / v

(7),

where mi is a multiplicity of a crystallographic orbit (i.e. the number of atoms of a specific Wyckoff site in the reduced unit cell), and v is the total number of atoms in the reduced unit cell. The information-based structural complexity parameters for the crystal structures of 1–15 have


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been calculated using software package TOPOS23 and are given in Table 5. Analysis of these data shows that the structural evolution of the system under changing S:Se ratio has a pronounced non-linear behavior. Whereas crystalline phases on the sides on the diagram are of the same level of complexity (4.2 – 5.2 bits/atom and 300 – 560 bits/cell), the crystallization of the phase 9 corresponds to the explosion of the structural information amount measured as both per atom and per unit cell (Figure 7).

Table 5. Information-based structural complexity parameters for the crystalline phases 1‒15.

Compounds IG (bits/atom)

IG,total (bits/cell)

1‒4

4.644

464.386

5‒8

4.236

313.500

9

7.119

1979.066

10, 11

5.162

536.846

12, 13

5.200

551.160

14, 15

5.002

480.156


22

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Figure 7. Evolution of information-based complexity parameters for the crystalline phases in the K+-UO22+-TO42--NO3--H2O system (T = Se, S; phases 1-15).

IR spectroscopy The IR spectra of 2–7 and 13–15 were recorded using KBr pellets on the Bruker Vertex 70 spectrometer in the region 4000‒300 cm-1 (Figure 8). The band assignment shows that the vibrational spectroscopic data are in a good agreement with the results of single-crystal X-ray diffraction study.24-32 Analyzing the spectroscopic data, several intervals could be distinguished.


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First, and that is clearly seen through all spectra – diffuse bands with the maximum at 1630 cm-1 correspond to the O‒H bending vibrations of water molecules. Second zone with almost no shifts for the characteristic bands in the region of 950 – 810 cm-1 could be attributed to the symmetric stretching modes of the uranyl cations. Much more interesting assignment related to the stretching and bending modes in [TO4]2‒ tetrahedra. Thus, in predominantly selenate phase (compound 2) sharp bands at 1051, 1170 and 1249 cm-1 are due to the vibrations of T–O stretching modes. With the decrease of selenium amount in crystal structures these bands shifts to the lower frequencies, for instance, to 1037, 1128 and 1207 cm-1 for the compound 6 and to 1009, 1069 and 1165 cm-1 for the pure sulfate compound 15. In the low frequencies region vibrations related to the bending deformation modes of T – O bonds shift slightly and more likely to the high values with the decrease of Se. Thus the sharp band at 583 cm-1 (compound 2) shifts to the 593 cm-1 (compound 6) and then to the 596 cm-1 (compound 15). Two bands at 712 and 737 cm-1 well identified for the compound 2, hereinafter with the increase of sulfur amount defined as shoulders at 730 and 746 cm-1 (compound 6), 740 and 750 cm-1 (compound 15). And contrariwise, sharp band at 457 cm-1 for the compound 15 barely visible for the predominantly selenate compound 2 at 425 cm-1. It should be noted that characteristic vibrations of the tetrahedral complexes and uranyl ions are partially overlap, so they cannot be clearly separated without structure-based calculations. Strong peaks at 2300 – 2500 cm-1 correspond to the hydronium OH-stretching modes. The broad bands with the maxima at 3300 – 3500 cm-1 correspond to the O‒H stretching vibrations of water molecules.


24

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Figure 8. IR spectra of 2 – 7 and 13 – 15 compounds. Chemical composition In order to control the chemical composition of crystals obtained during the crystal structure investigation, the EDX spectra have been measured for several selected phases. Small pieces of


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single crystals preliminary checked on the diffractometer have been crushed, pelletized and checked using SEM Hitachi TM 3000 equipped with EDX spectrometer (for spectra see SI). Analytical calculations. Compound 3: atomic ratio from structural data K 1.00, U 2.00, S 0.30, Se 2.70; found by EDX: K 0.88, U 2.12, S 0.32, Se 2.67. Compound 4: Analytical calculations: atomic ratio from structural data K 1.00, U 2.00, S 0.60, Se 2.40; found by EDX: K 0.96, U 2.15, S 0.55, Se 2.34. Compound 11: Analytical calculations: atomic ratio from structural data K 5.00, U 6.00, S 6.30, Se 4.70; found by EDX: K 4.69, U 6.35, S 6.39, Se 4.57. Compound 13: Analytical calculations: atomic ratio from structural data K 5.00, U 6.00, S 8.10, Se 2.90; found by EDX: K 5.18, U 6.16, S 8.35, Se 2.31. Compound 14: Analytical calculations: atomic ratio from structural data K 5.00, U 6.00, S 9.40, Se 1.60; found by EDX: K 5.03, U 6.04, S 9.43, Se 1.50. Compound 15: Analytical calculations: atomic ratio from structural data K 5.00, U 6.00, S 11.00; found by EDX: K 4.97, U 6.12, S 10.91.

Discussion The results of the current study reveal a rather complicated evolution of the structure and composition of crystalline phases in the system under consideration depending upon the changing Se:S ratio. A general description of the basic evolutionary trends may be outlined as follows. In the pure selenate system and for the first phases 1–4, the U:T ratio (T = S, Se) in the solution and the solid state are equal (2:3). The topology of the uranyl selenate-sulfate layer (Figure 5a) is rather dense and contains tridentate TO4 tetrahedra only. This topology is very common for uranyl selenates, but has very rarely been observed in uranyl sulfates. Due to the difference between the ionic sizes of the Se6+ and S6+ ions, the SO4 tetrahedra are essentially smaller than SeO4 tetrahedra, and their incorporation into the layer topology with tridentate


26

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tetrahedra results in the high charge and atomic density of the layer, which most likely make it unstable. This results in the stabilization of the layer topology with bidentate tetrahedra only (phases 5–8; Figure 5b). However, this means that the U:T compositions of the solution and the solid phase differ considerably (2:3 and 1:2, respectively, i.e. the solution becomes U-enriched). In order to decrease the difference, composition and topology of the crystalline solids changes again to that of the phases 10–15. For these compounds, the U:T ratio equals 6:11, whereas the layer topology contains both bidentate and tridentate tetrahedra with the Se6+ preference for the latter. However, the remarkable feature of the structures of 10–15 is that the layer itself has the U:T ratio of 3:5, whereas there is an additional T site not bonded to U, located in the interlayer space and occupied almost solely by S6+. The occurrence of the phase 9 deserves special attention. It was found in a very small amount (as few crystals only), which, along with its unusually high structural complexity points out at its possible transitional or even metastable character. The topology of the uranyl-based layers in the crystal structure of 9 is very complex and contains tridentate tetrahedra only, which represent rather stressed configurations for the incorporation of S6+ cations instead of Se6+. As a result, the layers are strongly modulated with the large identity period of 57.399 Å. It has been recently observed that very complex structures with giant periods may form as transitional architectures between phases with relatively small structural information amounts.33 Therefore we suggest that the phase 9 crystallizes as a transitional state between different ‘stable’ states of the system exemplified by the 1–4, 5–8 and 10–15 series of phases. In general, having a glance at the row of compounds from 1 to 15 and looking at the crystals of the sulfate phases (Figure 4b) the famous phrase “per aspera ad astra” comes to mind describing “sulfur” incorporation into “selenate” structures. But of course not dare to call such remarkable


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Page 28 of 40

complexes as “hardships”, so probably “per structura ad astra” in this case would sounds closer to reality.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications web site at DOI xxxx/acs.cgd.xxxxx. ICSD 430715-430728 contain the supplementary crystallographic data for this paper that can be obtained

free

of

charge

from

Fachinformationszentrum

Karlsruhe

via

www.fiz-

karlsruhe.de/request_for_deposited_data.html. AUTHOR INFORMATION Corresponding Author * Phone: +7 812 350 66 88. E-mail: [email protected].

ACKNOWLEDGMENT This work was supported by St. Petersburg State University (3.38.136.2014), The Committee on Science and Higher Education of St. Petersburg Government and President of Russian Federation grant for young scientists (no. MK-1737.2014.5). The XRD and EDX measurements have been performed at the X-ray Diffraction Centre and Centre for Microscopy and Microanalysis of St. Petersburg State University. REFERENCES


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(1) Burns, P. C.; Finch R., Eds.; Reviews in Mineralogy and Geochemistry: Uranium: Mineralogy, Geochemistry, and the Environment; Mineralogical Society of America: Washington, DC, 1999; Vol. 38, pp 1-679. (2) Krivovichev, S. V.; Burns, P. C.; Tananaev, I. G., Eds.; Structural Chemistry of Inorganic Actinide Compounds; Elsevier: Amsterdam, 2007; pp 1–494. (3) Kalmykov, S. N.; Denecke, M., Eds.; Actinide Nanoparticle Research; Springer-Verlag: Heidelberg, 2011; pp 1-440. (4) Burns, P. C.; Sigmon, G. E., Eds.; In Uranium: Cradle to Grave. Mineralogical Association of Canada Short Course Ser., 2013; Vol. 43, pp 15–120. (5) Burns, P. C. Can. Mineral. 2005, 43, 1839–1894. (6) Kovrugin, V. M.; Gurzhiy, V. V.; Krivovichev, S. V. Structural Chemistry. 2012, 23, 2003-2017. (7) Gurzhiy, V. V.; Tyshchenko, D. V.; Krivovichev, S. V.; Tananaev, I. G. Zeitschrift für Kristallographie. 2014, 229, 368-377. (8) Plasil, J.; Hlousek, J.; Veselovsky, F.; Fejfarova, K.; Dusek, M.; Skoda, R.; Novak, M.; Cejka, J.; Sejkora, J.; Ondrus, P. American Mineralogist. 2012, 97, 447–454. (9) Forbes, T. Z.; Goss, V.; Jain, M.; Burns, P. C. Inorg. Chem. 2007, 46, 7163-7168. (10)

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FOR TABLE OF CONTENTS USE ONLY Mixed Uranyl Sulfate-Selenates: Evolution of Structural Topology and Complexity vs. Chemical Composition Vladislav V. Gurzhiy, Olga S. Tyumentseva, Sergey V. Krivovichev, Vladimir G. Krivovichev, and Ivan G. Tananaev

Four different types of crystalline phases with variable S and Se contents were isolated from the aqueous UO2(NO3)2 – H2SO4 – H2SeO4 – KOH system. Characterization using single-crystal XRD and IR spectroscopy reveal the discovery of two 2D topologies unprecedented among inorganic oxysalts, the absence of continuous solid solution in the system, and the absence of isotypic sulfate and selenate phases.


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Figure 1. Correlation graph of the Se to S ratio in the original solution and in the resulted crystal structures for the compounds 1 – 15. Legend: following from right to left; blue rhombs = compounds 1 – 4, green triangles = compounds 5 – 8, purple circle = compound 9, dark red squares = compounds 10 – 15; linear (black, solid) and polynomial (dark red, dashed) trends are shown with the correlation coefficients. 229x129mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. Crystal structure and crystals: compounds 1 – 4 (a and b); compounds 5 – 8 (c and d). Legend: U polyhedra = yellow, TO4 (T = S, Se) tetrahedra = orange, K atoms = cyan, O atoms = red. 166x157mm (300 x 300 DPI)


Page 34 of 40

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Figure 3. Crystal structure (a) and crystals (b) of the compound 9; polyhedral representation of layered complex in the structure of 9 (c) and its graphical representation. Legend: U polyhedra = yellow, TO4 (T = S, Se) tetrahedra = orange, K atoms = cyan, O atoms = red; black nodes = U atoms, white nodes = T atoms. 175x168mm (300 x 300 DPI)



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Figure 4. Crystal structure (a) and crystals (b) of the compounds 10 – 15; polyhedral representation of layer in the structures of 10 – 15 (c) and topology of its interpolyhedral linkage. For legend see Figure 3. 161x122mm (300 x 300 DPI)


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Figure 5. Polyhedral representation of layers in the structures of 1 – 4 (a), 5 – 8 (b) and topology of its interpolyhedral linkage (c and d, respectively). For legend see Figure 3. 178x139mm (300 x 300 DPI)



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Figure 6. Idealized black-and-white 2D graphs with the dimers of 6-memberd rings arrangement shown by orientation vectors and shaded fragments for the topological types cc2-5:8-5 in 9 (a), cc2-5:8-2 (b)6,19 and cc2-5:8-3 (c)2. 158x160mm (300 x 300 DPI)


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Figure 7. Evolution of information-based complexity parameters for the crystalline phases in the K+-UO22+TO42--NO3--H2O system (T = Se, S; phases 1-15). 152x206mm (300 x 300 DPI)



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Figure 8. IR spectra of 2 – 7 and 13 – 15 compounds. 104x179mm (300 x 300 DPI)


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