A Novel Family of Np(VI) Oxysalts: Crystal Structures, Calorimetry

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A Novel family of Np(VI) oxysalts: crystal structures, calorimetry, thermal behaviour and comparison with U(VI) compounds Ilya V. Kornyakov, Vladislav V. Gurzhiy, Jennifer E. S. Szymanowski, Lei Zhang, Samuel N. Perry, Sergey V. Krivovichev, and Peter C Burns Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Crystal Growth & Design

A Novel family of Np(VI) oxysalts: crystal structures, calorimetry, thermal behaviour and comparison with U(VI) compounds Ilya V. Kornyakov a,b, Vladislav V. Gurzhiy *,a, Jennifer E. S. Szymanowski c, Lei Zhang c,

a

Samuel N. Perry c, Sergey V. Krivovichev a,b and Peter C. Burns c,d

Department of Crystallography, St. Petersburg State University, University Emb. 7/9,

199034 St. Petersburg, Russia

b Kola

c

Science Centre, Fersmana st. 14, 184209 Apatity, Murmansk region, Russia

Department of Civil and Environmental Engineering and Earth Sciences, University of

Notre Dame, Notre Dame, Indiana 46556

d Department

of Chemistry and Biochemistry, University of Notre Dame, Notre Dame,

Indiana 46556

ACS Paragon Plus Environment

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Crystal Growth & Design 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

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Neptunium; Sulfate; Selenate; Cesium; Calorimetry; Crystal Structure; Isomorphism; Xray diffraction

Abstract

Phase formation in the mixed sulfate-selenate Np(VI)-bearing aqueous system has been investigated. The obtained crystalline compounds, Cs2[(NpO2)2(TO4)3] (T = S, Se), crystallize in the tetragonal system, P-421m, a = 9.5737(3)–9.817(3), c = 8.0824(4)– 8.111(3) Å, V = 740.79(6)– 781.8(6) Å3, and have been chemically characterized. Single crystal X-ray diffraction experiments at various temperatures were used to define the thermal behaviour of the crystal structures relative to the S/Se ratio. The thermal behaviour of mixed sulfate-selenate and pure selenate compounds is anisotropic with the highest thermal expansion in the direction perpendicular to the sheets of Np-bearing polyhedra, while the structure of the pure sulfate compound experiences almost isotropic thermal expansion with slightly higher expansion within the layer, which is explained by the influence of stronger Cs – O bonding. High-temperature drop solution calorimetry was used to derive the enthalpies of formation of the Cs+-bearing neptunyl


2

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oxysalts. Within this family of isotypic structures, cesium neptunyl sulfate has a more negative enthalpy of formation than the selenate compound.

Introduction

The crystal chemistry of actinide compounds attracts significant attention due to its importance in the nuclear fuel cycle, most stages of which (i.e. mining, fuel production and reprocessing, waste management, etc.) involve structural features of actinides. Structural architectures of uranium compounds are fairly well studied, especially for U4+and U6+-species, due to their stability and geological significance (about 250 U-bearing mineral species are known to date) and relative ease of handling in a laboratory1-4. The crystal chemistry of neptunium, however, is dramatically less explored.

237Np,

a

synthetic element, is mainly produced by the β-decay of uranium and has a long half-life (2.14 × 106 years)5-6. Although both U and Np show a wide range of oxidation states, the chemistry of Np is distinct from U in part owing to the relatively high stability of its pentavalent oxidation state. Although Np is radioactive element, the structures of about 550 compounds are known to date (Cambridge Structural Database, Version 5.39; and


3


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Inorganic Crystal Structure Database, Version 2018.1). Among them Np5+-bearing compounds (for example, see

7-11)

are the most common, whereas the number of Np6+

compounds reported to date is much smaller

12-16.

The electronic structures of U6+ and

Np6+ differ, especially in the presence of a 5f electron in the case of Np6+, and this may result in different stereochemistries of their compounds. It is therefore interesting to compare phase formation processes involving these ions within chemically similar systems. In order to analyse the crystallization features of the Np oxysalts, the recently studied Cs2[(UO2)2(TO4)3] (T = Se, S) system17 provides a useful model. Herein we report the synthesis and structural characterisation of four novel Np(VI)bearing phases that were obtained from mixed sulfate-selenate aqueous solutions. Crystal chemical analysis of Np and isotypic U compounds has been conducted to evaluate the features of the Np6+ coordination environment and its influence on the structural architecture. In addition, we report on the thermal behaviour and the values of the enthalpies of formation of Cs2[(NpO2)2(TO4)3] (T = Se, S) compounds.


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Experimental

Synthesis Caution: Nepunium-237 is radioactive alpha (α) emitter that is potentially harmful. Extreme care must be taken when using this material, and it should be handled at properly licensed facilities with proper engineering controls. The neptunium starting material came from an available stock of 100mM Np(V) in 1M HCl. The initial starting material was dried down and resuspended in concentrated nitric acid. The concentrated nitric acid/Np solution was dried and resuspended 3x before being resuspended in 2M HNO3. The stock was then oxidized to Np(VI) using excess KBrO3, and the oxidation to Np(VI) was confirmed using UV-vis. The Np(VI) was precipitated from solution using NaOH, and washed several times with deionized water and centrifugation. The Np(VI) was redissolved in 2M HNO3 for use in experiments. H2SO4 (BDH 98%), H2SeO4 (40 wt. % in H2O, Aldrich, 99.95%), and CsNO3 (Alfa Aesar 99%) were used as received. The mixture of Np(VI), CsNO3 (0.028 g, 0.14 mmol), and deionized distilled water (2 mL, 110.2 mmol) were constant for each


5


synthesis, whereas selenic and sulphuric acids were added to achieve different molar Se:S ratios according to Table 1. The resulting dark brown aqueous solutions were stirred and left to evaporate in dram vials covered in parafilm within a radiological hood at room temperature. Crystals of pure selenate phase (5) formed within 1 month, whereas crystals with higher sulfate content took longer to crystalize. Thus the formation of pure sulfate compound (1) required 4 months. Moreover, higher concentrations of sulphuric acid led to lower yields and more hygroscopic crystals.

Table 1. Chemical formulae and the details of synthesis of the compounds 1 – 5.

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

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H2SeO4:H2SO4 in

Formula

mL (in mmol) 0 : 0.20

1

Cs2[(NpO2)2(SO4)3]

2

Cs2[(NpO2)2(SeO4)0.18(SO4)2.82]

3

Cs2[(NpO2)2(SeO4)0.55(SO4)2.45]

4

Cs2[(NpO2)2(SeO4)1.19(SO4)1.81]

5

Cs2[(NpO2)2(SeO4)3]

(0 : 3.91) 0.05 : 0.15 (1.02 : 2.94) 0.10 : 0.10 (2.04 : 1.96) 0.15 : 0.05 (3.06 : 0.98) 0 : 0.20 (4.08 : 0)

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

in structure

0

0

25.8

5.8

51.0

18.2

75.8

39.7

100

100


6

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Single crystal X-ray study Single crystals of 1–5 were selected for data collection under an optical microscope, encased in an epoxy and mounted on glass fibers. Data were collected using a Bruker Quazar three-circle diffractometer equipped with an Apex II CCD area detector operated with microfocused monochromated MoKα radiation (λ[MoKα] =0.71073 Å). Diffraction data were collected over a range of temperatures (100-295 K) with frame widths of 0.5o in ω and φ, and exposures of 30-80 s (depending on the crystal size) spent per frame. Data were integrated and corrected for background, Lorentz, and polarization effects using empirical spherical models by means of the Bruker programs APEX2 and

XPREP18, and CrysAlisPro19. A multi-scan semi-empirical absorption correction for the data processed with the Bruker software was applied using equivalent reflections in

SADABS-12. Empirical absorption corrections for the data processed with the CrysAlisPro software was applied using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. The unit-cell parameters (Tables 2 and S1-S5) were refined by least-square techniques. Initial structure models were obtained by direct methods and refined using SHELX20 incorporated in the OLEX2 program package21.


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The final models included coordinates and anisotropic displacements parameters for all atoms. The crystal structure of 1 was refined as a two-component inversion twin using the [-100/0-10/00-1] matrix. Selected interatomic distances and angles for compounds 1 – 5 measured over the temperature range of 100-295 K are listed in Tables 3 and S6S10. For the comparative study of crystal chemical characteristics of the neptunyl and uranyl17 compounds, crystallographic data and interatomic parameters discussed herein are for the data collected at T = 150 K, unless otherwise reported. Supplementary crystallographic data have been deposited to the Inorganic Crystal Structure Database and

can

be

obtained

from

Fachinformationszentrum

Karlsruhe

via

https://www.ccdc.cam.ac.uk/structures/ on quoting the depository numbers ICSD 434352 – 434375 and 434378 – 434384. The main coefficients of the thermal-expansion tensor were determined using a linear approximation of temperature dependencies for the unit cell parameters using the TEV program22. TEV was also used to determine the orientation of the principal axes of the thermal expansion tensor with respect to the crystallographic axes, and to visualize the thermal expansion.


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Table 1. Crystallographic and refinement parameters for 1 – 5 at T = 150 K.

Se content

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


structur

1

0

P-421m

9.5737(3)

9.5737(3)

8.0824(4)

2

7.91

P-421m

9.6220(6)

9.6220(6)

8.0897(7)

3

18.2

P-421m

9.6272(18)

9.6272(18)

8.0970(15)

4

39.7

P-421m

9.6604(13)

9.6604(13)

5

100

P-421m

9.817(3)

9.817(3)

in

S.G.

a, Å / α, °

b, Å / β, °

c, Å / γ, °

V, Å3

R1 (|Fo| ≥ 4σF)

ICSD

e 740.79(6)

0.0159

434353

0.0224

434359

750.5(3)

0.0175

434365

8.1046(11)

756.3(2)

0.0136

434372

8.111(3)

781.8(6)

0.0337

434379

748.97(11 )

Chemical analysis Chemical analyses for compounds 1 and 5 were done using a Nu Instruments AttoM high resolution inductively coupled plasma mass spectrometer (ICP-MS) and Perkin Elmer Avio 200 inductively coupled plasma optical emission spectrometer (ICP-OES). Briefly, compounds were dissolved in 2% nitric acid and diluted to appropriate concentrations of each analyte. Matrix matched standards and internal standards were used to monitor analyses. Molar ratios of Np, Cs, S, and Se were calculated from the


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results with an analytical uncertainty of ~3.5%. Compound 1: S/Np = 1.77 and Cs/Np = 1.08. Compound 5: Se/Np = 1.78 and Cs/Np = 1.11.

High-temperature oxide-melt Calorimetry Enthalpies of drop solution of the cesium neptunyl sulfate (compound 1) and selenate (compound 5) into molten sodium molybdate (3Na2O·4MoO3) at 973 K were measured using a Setaram AlexSYS 1000 high temperature Tian-Calvet calorimeter. The calorimeter was calibrated against the heat content of 5 mg α-Al2O3 pellets23-25. Operating procedures for high temperature oxide melt drop solution calorimetric experiments for neptunium-bearing materials were first successfully developed for neptunium oxides26. It was shown that neptunium compounds readily dissolve in molten sodium molybdate solvent at 973 K, and that Np(V) is the stable state after dissolution under oxidizing conditions in the calorimeter. In each experiment for a Np compound, about 5 mg of material was pressed into a 1.5 mm diameter pellet inside a negative pressure glove box, weighed on a Mettler-Toledo XSE105 microbalance (± 0.01 mg), transported to the calorimetry lab in triple containment, and dropped into 10 g of molten


10

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sodium molybdate solvent contained in silica glass crucibles in the calorimeter. The calorimetric assembly was flushed continuously with oxygen at 43 mL/min and oxygen was bubbled in the molten solvent at 5 mL/min through silica glass tubes to facilitate dissolution of neptunium oxides and provide an oxidizing atmosphere.

Results and Discussion

Correlation between the solution and solid state chemical composition Compounds 1-5 crystalize in the tetragonal space group P-42m and are isotypic to the previously reported uranyl compounds Cs2[(AnO2)2(TO4)3] (An = U, Np; T = S, Cr, Se, Mo)16-17,27-29. The unit-cell parameters of 1-5 increase gradually with Se content, due to the volume of the tetrahedral oxyanions (from ~1.64 Å3 for sulfate, to ~2.16 Å3 for selenate). For uranyl compounds a lower sensitivity of the c unit-cell parameter to Se content relative to a and V values was observed17. In the case of Np compounds, the c parameter responds similarly (Fig. 1) to the Se-for-S substitution.


11


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To analyze the extent of Se-for-S substitution in 1 – 5, the Se:S ratio in the initial solution is compared to the Se:S in the crystal structures in Figure 2. Compound 1 corresponds to the lower left point (0:0), whereas the pure selenate compound 5 appears at the upper right corner (100:100) of Figure 2. The data for compounds 1-5 are correlated with R2 =0.98 described by the second-degree polynomial function:

εcryst = 0.014εsol2 – 0.43εsol + 3.5

(1),

where εcryst and εsol are the Se:S ratios in the crystalline phases and solution, respectively. The respective Se:S distribution coefficient, Kd, was calculated according to the formula

Kd = [crystxSe][solxSe]-1[crystxS][solxS]-1

(2),


12

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where

crystx

T

and

solx

T

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

solution, respectively. Thus, for 1-5, Kd = 0.68, indicating that Se and S are distributed unevenly between solution and crystalline phases.


13


Page 14 of 44

Figure 1. Unit-cell parameters as a function of Se:S ratio in the crystal structures of 1 – 5; trend lines are shown with the correlation coefficients; ESDs of the unit cell parameters are within the limits of the symbols.

Figure 2. Correlation graph of the Se:S ratio in the original solution and in the resulting crystal structures for compounds 1 – 5; a second degree polynomial trend is shown with the correlation coefficient.

Structure description


14

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The crystal structures of 1 – 5 contain one crystallographically nonequivalent Np(VI) cation with two short Np6+=O2- bonds (1.72(2)-1.77(2) Å) forming approximately linear NpO22+ neptunyl ions (Npt). Each NpO22+ cation is coordinated by five oxygen atoms (Np1-Oeq = 2.33(2) – 2.42(2) Å) that belong to sulfate or selenate groups arranged in the equatorial plane of the NptO5 pentagonal bipyramid. Two symmetrically nonequivalent

T6+(T= S, Se) positions are tetrahedrally coordinated by four O2- atoms each. The (T1O4)2- group shares three vertices with three adjacent neptunium polyhedra. Unshared vertices of the tetrahedra are oriented either up or down relative to the plane of the [(NpO2)2(TO4)3]2- layer (Fig. 3). The (T2O4)2- group is 4-connected and bridging, sharing all four oxygen atoms with the adjacent Np polyhedra. The crystal structures of 1-5 are based upon the [(NpO2)2(TO4)3]2- neptunyl sulfate (in 1), selenate (in 5) or sulfate-selenate (in 2-4) layers formed by the linkage of the Np and

T coordination polyhedra via common O atoms. The layers are parallel to (001) and are corrugated due to the presence of 4-connected tetrahedra in an “egg tray” manner with ~6 Å diameter of the “tray’s cell” (Fig. 3c). The negative charge of the [(NpO2)2(TO4)3]2sheet is compensated by Cs+ cations that are arranged in the interlayer space. Two


15


Page 16 of 44

crystallographically nonequivalent Cs atoms link adjacent Np-bearing layers through ionic bonds. The Cs1 atoms fill the “egg tray cells” forming bonds with nine O atoms (Cs1–O =3.059(8) – 3.49(2) Å) whereas Cs2 atoms are arranged in between the “tray partitions”, and are coordinated by twelve oxygen atoms (Cs2–O =3.16(2) – 3.663(3) Å). The average bond length does not change significantly with increasing Se ( = 3.20 Å for the pure sulfate compound 1 and 3.23 Å for the pure selenate compound 5) in the structure, whereas Cs2 site is more sensitive: = 3.46 Å for 1 and 3.37 Å for 5.


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Figure 3. Crystal structure (a, c) of compounds 1 – 5; the arrangement of figures of thermal expansion coefficients in the structures of 1 (b) and 4 (d). Legend: Np polyhedra = green, TO4 (T = S, Se) tetrahedra = yellow, Cs atoms = cyan, O atoms = red; the dashed black line designates a cavity similar to an “egg tray’s cell” (see text for details); figures of TEC are arranged relative to the projection shown in (a), green = expansion.


17


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Geometries of Np6+ and U6+ coordination polyhedra In order to analyze the influence of Np6+ on the crystal structure architecture, it is useful to examine the first coordination spheres of Np6+ and U6+ cations. The crystal structures of 1-5 and isotypic uranyl analogs17 contain one crystallographically inequivalent Np or U atom with two short An6+ = O2- bonds forming actinyl ions. At T = 150 K the average actinyl bond lengths are 1.746 Å and 1.760 Å for Npt and Ur ions, respectively. Each of the linear actinyl ions is coordinated by five O atoms arranged in the equatorial plane of the corresponding pentagonal bipyramid and belong to sulfate or selenate groups with average bond lengths equal to 2.385 Å (at T = 150 K) for both Npt and Ur cations. Thus, it can be assumed that the decrease in the cation size with incorporation of an additional electron into the configuration of Np (in comparison with U) leads to reduction of the multiple actinyl bond lengths and slight elongation (0.010.02 Å) of the equatorial bond lengths, which results in the flattening of neptunyl pentagonal bipyramids slightly as compared to the uranyl analogs, which is in agreement with the previous studies.30-33


18

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Table 3. Selected interatomic distances (Å) and angles (o) for compounds 1 – 5 at T = 150 K.

Bond length, Å

1

Np1–O1

1.755 (7)

Np1–O2 Np1–O3 Np1–O4 Np1–O6

T1–O3 T1–O5 T1–O6 T2–O4

2

3

4

5

1.757

1.750

1.751

(11)

(8)

(7)

1.744

1.745

1.736

(12)

(9)

(7)

1.749

1.750

1.747

1.743

1.74

2.405 (5)

2.401 (8)

2.404

2.398

2.50 (3)*

(6)

(5)

2.380

2.379

(6)

(5)

2.366

2.359

2.354

(11)

(9)

(7)

2.386

2.386

2.385

2.382

2.386

1.491 (5)

1.515 (8)

1.535

1.580

1.66 (3)*

(6)

(5)

1.457

1.445

1.502

(13)

(9)

(8)

1.503

1.520

1.567

(11)

(8)

(7)

1.475

1.498

1.509

1.557

1.611

1.469 (5)

1.475 (8)

1.478

1.486

1.61 (3)*

(6)

(5)

1.743 (7)

2.372 (5) 2.375 (7)

1.441 (7) 1.477 (8)

2.382 (8)

1.73 (2) 1.75 (2)

2.39 (3)* 2.35 (2)

1.63 (2) 1.639 (19)


19


Cs1–O1 Cs1–O2 Cs1–O3 Cs1–O5 Cs2–O3 Cs2–O4 Cs2–O5

3.071 (7)

3.090

3.071

3.064

(12)

(8)

(7)

3.166

3.167

3.173

(11)

(9)

(7)

3.315 (8)

3.316

3.344

(6)

(5)

3.263

3.253

3.201

(12)

(9)

(8)

3.230

3.245

3.242

3.243

3.218

3.356 (5)

3.358 (9)

3.352

3.326

3.34 (3)*

(7)

(6)

3.383

3.374

(6)

(5)

3.646

3.656

(3)

(3)

3.460

3.452

3.159 (7) 3.290 (5) 3.259 (7)

3.388 (6) 3.623 (3)

3.382 (8) 3.638 (5)

3.456

3.459

Angles, o

1

2

T1–O3–Np1

132.8 (3)

133.1 (4)

T1–O6–Np1 T2–O4–Np1

Page 20 of 44

142.6 (4) 142.1 (3)

141.9 (7) 142.4 (5)

3

4

132.8

131.8

(3)

(3)

141.6

139.6

(5)

(4)

142.6

143.5

(4)

(3)

3.09 (2) 3.16 (2) 3.48 (2)* 3.20 (2)

3.32 (3)* 3.20 (2) 3.302 5

136.8 (10) 129.1 (2)* 138.7 (2)*

* - averaged value for two crystallographically disordered O sites (O3A-O3B and O4A-O4B, respectively).

Occupancy of the T6+ sites


20

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It has recently been shown17,34 that the occupancies of the T6+ sites in mixed uranyl sulfate-selenates relate to their connectivity with a clear preference of S6+ for more tightly bonded positions. Refinement of the occupancies of the T6+ sites in the crystal structures of 1–5 confirms this preference. The occupancy of the 3-connected T1 site by Se atoms obeys the second-degree polynomial correlation with R2 = 0.99, unlike the recently reported family of uranyl compounds17 where direct linear dependence was observed. Substitution of the larger U6+ by Np6+ cations leads to a decrease of the An = OAn bond lengths accompanied by a decrease of the average bond length from 3.313 Å to 3.218 Å, and the average bond length from 3.487 Å to 3.302 Å for U17 and Np selenates, respectively. It is noteworthy that the change of the bond lengths for the sulfate compounds is less significant with the bond length decreasing from 3.255 Å to 3.230 Å, and the bond length decreasing from 3.474 Å to 3.456 Å for the U17 and Np sulfates, respectively. Hence, the influence of the Cs+ cations upon the conformation of the [(AnO2)2(TO4)3]2- layers increases with Se content. The excess of internal tension induced by the incorporation of the smaller


21


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actinide cation likely regulates a low rate of S substitution by Se in the structures due to the increasing volume of the tetrahedra from 1.65 Å3 to 2.16 Å3 for (SO4)2- and (SeO4)2-, respectively. It is noteworthy that the O3 and O5 atoms of the (T1O4)2- tetrahedron in the structure of pure neptunyl sulfate (1) are disordered, which can be explained by the increased influence of Cs+ ions. The O6 atom belonging to the same tetrahedral group is ordered due to the absence of bonds to the Cs atoms. The same situation is observed for the T2 site as for the T1 site, but the influence of the excess of internal tension is much higher due to the T2 tetrahedron being 4connected to four adjacent neptunyl polyhedra. When the Se amount at the T2 site reaches 100%, all the O atoms of the (Se2O4)2- become disordered. Throughout the 2–4 range of compounds, the T2 site is much less occupied by Se than its amount in solution. The occupancy of the T2 site can approximately be described by the seconddegree polynomial function:

OT = 0.0002εsol2 – 0.0119εsol + 1.474

(3),


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Another confirmation of the strongly uneven Se-for-S substitution rate can be found by analyzing the geometry of tetrahedral oxyanions. In the ideal case, the distances should display a linear dependence with the Se:S ratio. However, the distances deviate from the linear correlation with the Se:S ratio in the solid state. The plot in Fig. 4 shows an almost linear correlation for 1-5, for which the Se contribution from the T2 site is not so significant. When the Se amount at the T2 site rapidly increases, the “T1 line” becomes flattened. And vice versa, the very flat start of “T2 line” approaches to the values in the Se-dominant region.


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Figure 4. Occupancy of the T6+ positions by Se atoms (p.f.u.) in the structures of 1 – 5; (a) variation of the average T6+–O distances with the increase of Se in the structures; (b) Linear and polynomial trends are shown with the correlation coefficients. See text for details.


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Figure 5. Variations of the Np – Obr – T angles with the increase of Se in the crystal structures of 1 – 5 in comparison with the U – Obr – T angles17. Legend: Squares = Np compounds, shaded triangles = U compounds.

Special attention is warranted for the behavior of the Np-O-T angles in 1 to 5. Recently35 the flexibility of actinyl structural units that are connected with adjacent coordination polyhedra through common vertices was examined and this mechanism was compared with a flexible ball-in-socket joint. The main idea is that the lower An – Obr – T angles corresponds to the direction of the stronger layer corrugation, whereas


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the higher An – Obr – T angle values correspond to the flatter zones. Thus, it can be assumed that the increase of the unit-cell parameters (expansion of the structure) caused by the increase of Se should be accompanied by flattening of the layers and hence the increase of the Np – Obr – T angles. However, all three nonequivalent angles at the bridged O atoms decrease, which means an increase of the layer deformation. According to Figure 5, the T1 site is largely responsible for the layer deformation, whereas the angles at the 4-connected (more rigidly bound) T2 site are changing less. For the T sites fully occupied by Se atoms with disordered O ligands, an average value of the Np – Obr – T angles has been used. The same tendency was observed for the isotypic uranyl compounds17, although with smaller gradient, which shows the higher deformation of the Np-bearing layered complexes. Distribution of the Np – Obr – T angle values as a function of Se concentration in the structures of 1–5 once again demonstrates that disparity of the ionic radii of S and Se directly and largely affects the changes in the lattice geometry more than the influence caused by polyhedral tilting.

Thermal behavior


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Figure S1 shows the behaviour of the unit-cell parameters of 1–5 as a function of temperature. Regardless of the Se concentration and the number of points, there is a direct linear correlation of the parameters increasing with temperature. The poorer linear dependence for the pure selenate phase (5) is most likely caused by disorder of the O sites in the structure and by the poor quality of the crystals relative to the 1–5 series. It is noted that it is not entirely rigorous to operate with the exact eigenvalues of the thermal expansion tensors (Table 4), because the data being calculated from the single crystals were not averaged. However, almost parallel trends leave no doubt about the similar thermal behaviour of all the phases within the system.

Table 4. The main axes of the thermal expansion tensors in the structures of 1 – 5.

Temp., K

α11

α22

α33

Cs2[(NpO2)2(SO4)3] (1) 100

23.2

23.2

22.9

200

23.1

23.1

22.8

300

23.1

23.1

22.8

Cs2[(NpO2)2(SeO4)0.24(SO4)2.76] (2) 100

14.0

14.0

27.2


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200

14.0

14.0

27.1

300

14.0

14.0

27.0

Cs2[(NpO2)2(SeO4)0.55(SO4)2.45] (3) 100

19.1

19.1

27.6

200

19.1

19.1

27.5

300

19.1

19.1

27.4

Cs2[(NpO2)2(SeO4)1.19(SO4)1.81] (4) 100

17.3

17.3

28.5

200

17.3

17.3

28.4

300

17.3

17.3

28.3

Cs2[(NpO2)2(SeO4)3] (5) 100

29.4

29.4

36.0

200

29.3

29.3

35.8

300

29.2

29.2

35.7

α – coefficient of thermal expansion [α11, α22, α33—eigenvalues (main values)].

As shown previously36-38, the maximal thermal expansion should be along the direction of the weakest bonding, unless a hinge of shift mechanisms are involved. Considering the layered motif in the structures of 1–5, the highest thermal expansion in the direction perpendicular to the sheets of Np-bearing polyhedra is expected. This is well matched by the behaviour of compounds 2–5, although the values for the pure


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selenate phase (5) are higher than for the mixed S-Se compounds, which may be explained by the slightly poorer approximation. However, the structure of the pure sulfate compound (1) experiences almost isotropic thermal expansion. Moreover, thermal expansion within the layer is slightly higher than that in the direction perpendicular to the layers, which is explained by the influence of stronger (shorter) Cs – O bonding and, at the same time, larger expansion of the layer according to the “ballin-socket joint” mechanism. The latter is confirmed by the greater increase of Np – Obr –

T angles with temperature (see Tables S6-S10) in comparison to compounds 2–5. It should be noted that to determine the potential importance of the noncentrosymmetric and piezoelectric -42m class on the alteration of crystal properties, including anisotropy of thermal behavior, further detailed investigation is needed, which is currently ongoing.

Calorimetry The enthalpies of formation from oxides and elements were calculated by thermochemical cycles listed in Tables S11-S1225,39-42, and the results are shown in Table S13. The dissolution of the cesium neptunyl sulfate and selenate compounds in


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the oxide melt solvent finished within 30 minutes. The enthalpies of formation from oxides are both exothermic; the standard formation enthalpy of Cs2(NpO2)2(SO4)3 is determined to be -4896.7 ± 18.3 kJ/mol, and the one of Cs2(NpO2)2(SeO4)3 is -4151.2 ± 14.8 kJ/mol. The enthalpy of exchange between the cesium neptunyl sulfate and selenate at 298 K for standard states of all species, is calculated as, Cs2(NpO2)2(SO4)3 + 3SeO42- = Cs2(NpO2)2(SeO4)3 + 3SO42∆Hexchange = -171.87 ± 25.76 kJ/mol

(4),

(5).

The synthetic experiments have shown that there is no miscibility gap in the neptunyl solid solution as in recently studied U-bearing system17. However the rate of Se incorporation in Cs neptunyl oxysalts is much lower than it was observed for the uranyl compounds. Thus it would be of great interest to conduct a comprehensive thermodynamic study of these systems combining thermochemical data with entropic data and/or Gibbs free energetic data to probe the phase relations in the system of Cs2(NpO2)2(SO4)3–Cs2(NpO2)2(SeO4)3–Cs2(UO2)2(SeO4)3–Cs2(UO2)2(SO4)3.


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Conclusions

In this paper we have reported four new Cs+-bearing neptunyl oxysalts, representing the solid solution series with variable content of sulfur and selenium. In contrast to the recently studied phase formation within the uranyl system17, no other crystals other than 1-5 were detected. Occupancies of the T6+ sites show clear preference of S6+ for more tightly bonded, four-connected T2 positions, as was recently observed17, but with much lower rates of Se incorporation into the structures. Moreover, the disorder of O atoms in the coordination sphere of the T2 site in the pure selenate phase is observed, which could be explained by the excessive internal tension due to the connection of large Se tetrahedra to four adjacent neptunyl polyhedra. Se incorporation into the crystal structures of Cs2[(NpO2)2(TO4)3] significantly changes the thermal behavior of the compounds: from almost isotropic with slightly higher expansion within the layer in the pure sulfate structure, to essentially anisotropic with the highest thermal expansion in the direction perpendicular to the Np-bearing sheets for the Se structure. In light of this, the results of enthalpies of formation calculations are of special interest. Since


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compounds 1 and 5 are isotypical, the differences in their heats of formation relate to Se-for-S substitution in the oxyanion sheets, which in turn cause changes in interactions between the U-bearing 2D units and Cs+ ions. A similar tendency has been recently reported for the family of uranyl peroxide cage clusters with alkali cations42. It is worth noting that these calculations also demonstrate the relative ease of the selenate phase formation in comparison to the sulfate one, which is in agreement with experimental observations. ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications web site at DOI xxxx/acs.cgd.xxxxx.

ICSD 434352 – 434375 and 434378 – 434384 contain the supplementary crystallographic data for this paper that can be obtained free of charge from Fachinformationszentrum Karlsruhe via https://www.ccdc.cam.ac.uk/structures/.

AUTHOR INFORMATION


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Corresponding Author * Phone: +7 812 350 66 88. E-mail: [email protected], [email protected].

ACKNOWLEDGMENT

This work was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy as part of the Materials Science of Actinides Energy Frontier Research Center (DESC0001089), St. Petersburg State University (to VVG, 3.42.729.2017) and by the President of Russian Federation grants for young scientists (to VVG and IVK, no. MK-4810.2018.5). The ICP-OES analyses were conducted at the Center for Environmental Science and Technology (CEST), at the University of Notre Dame. PXRD, TGA, and high-temperature calorimetry data were collected in the Materials Characterization Facility supported by the Center for Sustainable Energy at the University of Notre Dame (ND Energy). ICP-MS analyses were done at the Midwest Isotopic and Trace Element Research Analytical Center (MITERAC) facility at the University of Notre Dame.

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FOR TABLE OF CONTENTS USE ONLY A Novel family of Np(VI) oxysalts: crystal structures, calorimetry, thermal behaviour and comparison with U(VI) compounds Ilya V. Kornyakov, Vladislav V. Gurzhiy, Jennifer E. S. Szymanowski, Lei Zhang, Samuel N. Perry, Sergey V. Krivovichev and Peter C. Burns

A novel Np(VI)-bearing phases, Cs2[(NpO2)2(TO4)3] (T = S, Se), with variable content of S and Se were synthesized and characterized using ICP-OES, ICP-MS and XRD. The thermal behaviour of mixed sulfate-selenate and pure selenate compounds is anisotropic with the highest thermal expansion in the direction perpendicular to the


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sheets of Np-bearing polyhedra, while the structure of the pure sulfate compound experiences almost isotropic thermal expansion with slightly higher expansion within the layer. Within this family of isotypic structures, cesium neptunyl sulfate has more negative enthalpy of formation than selenate compound.


44

A Novel Family of Np(VI) Oxysalts: Crystal Structures, Calorimetry

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