Oxo-selenite

Jan 22, 2018 - A study of neptunium (Np) chemistry in the complex oxo-selenium system has been performed. Hereby, two sets of precipitation experiment...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Unexpected Behavior of Np in Oxo-selenate/Oxo-selenite Systems Eike M. Langer,† Olaf Walter,‡ Jean-Yves Colle,‡ Dirk Bosbach,† and Evgeny V. Alekseev*,†,§ †

Institute of Energy and Climate Research (IEK-6), Forschungszentrum Jülich GmbH, D-52428, Jülich, Germany European Commission, DG Joint Research Centre, Directorate G - Nuclear Safety and Security, Postfach 2340, D-76125, Karlsruhe, Germany § Institute of Crystallography, RWTH Aachen University, D-52066, Aachen, Germany ‡

S Supporting Information *

ABSTRACT: A study of neptunium (Np) chemistry in the complex oxo-selenium system has been performed. Hereby, two sets of precipitation experiments were conducted, investigating the influence of the initial oxidation state of selenium using SeIVO2 and H2SeVIO4 with NpV in alkali nitrate solution, keeping the ratio of Np/Se constant. Surprising results were observed. Five novel neptunium and selenium bearing compounds have been obtained by slow evaporation from aqueous solution. The novel NpIV phase K4‑x[Np(SeO3)4‑x(HSeO3)x]·(H2O)1.5 (1) crystallizes in green-colored, plate-shaped crystals and was obtained by adding SeO2 and ANO3 to a NpV stock solution. Single-crystal X-ray diffraction reveals one-dimensional chain structures composed of square antiprismatic NpO8 polyhedra linked via four trigonal pyramidal SeO3 and HSeO3 units. Raman spectral analysis supports the presence of both selenite and hydroselenite due to the presence of corresponding modes within the spectra. The addition of selenic acid to a NpV stock solution resulted in the precipitation of elongated rose prisms of K2[(NpO2)2(SeO4)3(H2O)2]·(H2O)1.5 (2), Rb2[(NpO2)2(SeO4)3(H2O)2]·(H2O)2 (3) and K9[(NpO2)9(SeO4)13.5(H2O)6]· (H2O)12 (4) as well as light red plates of Cs2[(NpO2)2(SeO4)3] (5). To our knowledge, this is the first report of NpVI selenates. All four structures show two-dimensional layered structures with alkali cations acting as charge balancing counter cations. Hereby the layers of compounds 2 and 3 are found to be orientational geometric isomers. Distinctly different phenomena are made responsible for the phase formation within these systems. The kinetically driven process of NpV disproportionation led to the formation of the NpIV selenites in the SeIV-based system, whereas the oxidation of NpV by reduction of nitrate in acidic conditions is responsible for the formation of the NpVI selenates in the SeVI system. The influence of air oxygen is also discussed for the latter reaction.

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NpIV and NpVI in strong acidic conditions has been shown to be essential in NpV chemistry.5,6 Np occurs as a byproduct within the 235U nuclear fuel cycle, and 237Np is considered as one of the most challenging actinides for final deep geologic waste disposal.10,11 Reasons for this are the combination of the long half-life of 2.14 × 106 years, a relatively weak complexing ability, and a high solubility for compounds containing neptunyl ions resulting in a high mobility within the environment.10−12 Next to Np, 79Se is also considered crucial for the safe assessment of long-term nuclear waste disposal.13 The combination of radiotoxicity, a long half-life (3.27 × 105 years14), and especially the high geochemical mobility make 79Se one of the radionuclides of greatest environmental impact.15 The occurrence of actinide bearing selenites in nature in the form of the minerals marthozite,16 demesmaekerite,17 derriksite,18 haynesite,19 guilleminite,20 piretite,21 and larisaite22 shows that such phases can form and potentially show long-time stability. Selenium is stable in oxidation states −II, 0, +IV, and +VI in the form of Se2−, Se, SeO32−, and SeO42− ions.23 Hereby SeVI is

INTRODUCTION Neptunium (Np) is the fifth element of the actinide series in the Periodic Table, residing next to its neighbors uranium (U) and plutonium (Pu). Np shows a rich chemistry stably adopting a wide variety of oxidation states, ranging from +III to +VII,1 both in aqueous solution and in the solid state. The variety of Np electronic structures is manifold due to the presence and availability of 5f electrons. In the pentavalent and hexavalent state, Np has the tendency to form almost linear dioxo-cations NpO2n+ (n = 1, 2), denominated as neptunyl ions. This results in an anisotropic coordination of Np adopting tetragonal, pentagonal, or hexagonal bipyramids.7,8 The trivalent and tetravalent states however show distinctly different coordination environments, typically ranging from 8-fold distorted dodecahedra to 9-fold tricapped trigonal prisms.2,9 In solution, pentavalent Np is often erroneously considered to be the only viable oxidation state for Np in water with dissolved oxygen. This largely relies on the fact that comproportionation of NpIV and NpVI to NpV is predicted due to standard reduction potentials.2 However, this has been proven wrong by many studies in which high concentrations, temperature influence, low pH, and strong complexants all influence this equilibrium.2−4 Disproportionation of NpV into © XXXX American Chemical Society

Received: November 21, 2017

A

DOI: 10.1021/acs.inorgchem.7b02961 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry considered the most mobile speciation and also the least sorbed selenium species.23 In general, the sorption of Se decreases with increasing oxidation number.23 The selenate anion, SeO42−, adopts a tetrahedral coordination, whereas the selenite anion, SeO32−, typically is coordinated as a trigonal pyramid due to the presence of an active electron lone pair atop the apical vertex. This can favor the formation of polar structures,24 and due to this, promising physical properties could be potentially observed, such as piezo- and ferroelectricity, second-harmonic generation, and unique magnetic properties.25,26 However, the previously published Np selenite compounds do not possess non-centrosymmetric space groups.2,24,27,28 Challenging handling issues and low availability of appropriate laboratories for transuranium elements have led to thorium and uranium being used as structural surrogates. Structural similarities of NpIV to ThIV and UIV as well as structural similarity of NpV and NpVI to UVI exist. This can however only be an indicator of phases which might form, however, not much as to under what kinds of conditions. The chemical behavior, especially redox and pH dependency, can vary distinctly due to the different electronic configurations of the present 5f electrons between the surrogate systems and the real transuranic systems. In general, the chemistry of the Np−Se oxo-system is only scarcely investigated, exemplarily indicated by the low number of known phases within the system in comparison to the corresponding systems with Th and U. The goal of this study was to investigate the influence of different initial oxidation states of Se on the precipitation of solid phases from aqueous acidic solutions in combination with Np. Hereby the Np was initially in its most stable oxidation state - NpV.

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Table 1. Numbering of Phases 1−6 According to Their Chemical Formula, Actinide to Se (An/Se) Molar Ratio As Well As the Oxidation State of the Actinide and Selenium no. 1 2 3 4 5 6

compound K4−x[Np(SeO3)4−x(HSeO3)x]· (H2O)1.5 K2[(NpO2)2(SeO4)3(H2O)2]· (H2O)1.5 Rb2[(NpO2)2(SeO4)3(H2O)2]· (H2O)2 K9[(NpO2)9(SeO4)13.5(H2O)6]· (H2O)12 Cs2[(NpO2)2(SeO4)3] Cs2[(UO2)2(SeO4)3]

An/Se ratio

ox. state Np/U

ox. state Se

1:4 (Np)

+IV

+IV

2:3 (Np)

+VI

+VI

2:3 (Np)

+VI

+VI

2:3 (Np)

+VI

+VI

2:3 (Np) 2:3 (U)

+VI +VI (U)

+VI +VI

Figure 1. Exemplary photo for the simultaneous formation of NpIV and NpVI phases. Hereby the former is visible as yellow-green Rb4−x[NpIV(SeO3)4−x(HSeO3)x]·(H2O)1.5 and the latter as red Rb(NpVIO2)(NO3)3.29

For 2, 3, 4, and 5, an initial 0.037 mmol of NpV was used. H2SeO4 and ANO3 (K, Rb, K, Cs) were added resulting in molar ratios of 1:3:6, 1:3:3, 1:3:3, and 1:3:6, respectively. 2, 3, and 4 crystallized in light red-/rose-colored elongated prisms and 5 in a light red plate. The purities were optically estimated to be between 60 and 80%. Green, plate-shaped crystals of 6 were obtained in analogue conditions with 0.037 mmol of UVI from a (UO)2(NO3)2(H2O)6 solution; H2SeO4 and CsNO3 were added in resulting molar ratios of 1:3:6. As a byproduct cesium selenate was also found; the major product however was phase 6. Crystallographic Studies. The as-obtained, neptunium-bearing crystals were selected for data collection. The crystals were mounted on glass fibers and optically aligned on a single-crystal X-ray diffractometer (SC-XRD). X-ray diffraction (XRD) measurements were performed on a Bruker Apex II Quazar diffractometer with monochromated MoKα-irradiation collecting at least two full spheres of data.30 Frames were collected with a mixed ω- and ϕ-scan technique with Δω = Δ ϕ = 0.5°; irradiation time per frame was adjusted appropriately to the size and diffracting abilities of the crystals. Data were integrated with SAINT30 and corrected to Lorentz and polarization effects, and an empirical adsorption correction with SADABS30 was applied. The structures were solved by direct methods and refined to an optimum R1 value with SHELXL31 operated within the WinGX v2014.1 program suite.32 The data and crystallographic information are given in Table 2. The structures were solved by direct methods and refined to R1 = 0.0433 for 1, R1 = 0.02414 for 2, R1 = 0.0297 for 3, R1 = 0.0459 for 4, R1 = 0.0204 for 5, and R1 = 0.0389 for 6, respectively. The structures have been deposited at The Cambridge Crystallographic Data Centre (CCDC) with the reference CCDC numbers 1586290−1586295; they contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the CCDC via www.ccdc.cam.ac.uk/data_request/cif. For structures 3 and 4, level A and B alerts have occurred as well as B alerts for structure 2 and an A alert for structure 6. These arise from disordering of incorporated solvent water molecules and alkali metal

EXPERIMENTAL SECTION

Syntheses. Commercially available SeO2, H2SeO4 (40 wt % solution with ρ = 1.407 g·cm−3), KNO3, RbNO3, and CsNO3 were used as received. A NpV stock solution (c = 170 mg·mL−1) on nitrate basis was utilized. Evaporation experiments were conducted in 30 mL containers under air in slightly underpressurized glove boxes in a laboratory specially dedicated to actinide chemistry. Caution! 237Np (t1/2 2.14 × 106 years) represents a serious health risk, because of its α and γ emission, and especially because of its decay to the short-lived isotope 233Pa (t1/2 = 27.0 days), which is a potent β and γ emitter. Precipitation Experiments. Alkali nitrates, NpV stock solution, and either SeO2 or H2SeO4 were filled into an evaporation container and filled up to a total solution volume of 1.212 mL by water. After the majority of water evaporated, the containers were sealed with a lid to reduce the final crystallization speed. Within this work, six phases are reported, and Table 1 gives an overview of the formulas, Np/Se ratio, and oxidation states. To increase readability, from hereon the phases will only be named by their assigned number 1−6 shown in Table 1. Yellow green prism crystals of 1 were obtained using 58.8 μL of NpV stock solution (0.037 mmol), 12.38 mg of SeO2 (0.112 mmol), and 22.51 mg of KNO3 (0.223 mmol), resulting in a molar ratio of 1:3:6 for Np/Se/K. The structure of an isostructural Rb analogue (Rb4‑x[Np(SeO3)4−x(HSeO3)x]·(H2O)1.5) (lattice parameters a = 12.831(2) Å, c = 10.251(2) Å) could not be refined to a degree better than R1 ≈ 10% due to twinning of all measured crystals. The experimental conditions were accordingly the same with RbNO3 being used instead of KNO3. Figure 1 shows an exemplary image of Rb4‑x[Np(SeO3)4−x(HSeO3)x]·(H2O)1.5 (yellow-green) coprecipitating with a rubidium neptunyl nitrate (Rb(NpO2)(NO3)3 (red). In general the phase purity was difficult to distinguish for all phases shown here. A rough estimate would be roughly 50% for 1, relying on optical impressions through the microscope. B

DOI: 10.1021/acs.inorgchem.7b02961 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Crystallographic Data of All Phases Reported in This Work (1−6)

a

compound

1

2

3

4

5

6

mass (g mol−1) space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z λ (Å) ρcalcd (g cm−3) R1(F) for Fo2 > 2σ(Fo2)a wR2 (Fo2)b

1850.61 P4/mnc 12.6562(7) 12.6562(7) 10.2096(8) 90 90 90 1635.4(2) 2 0.71073 3.758 0.0433 0.0901

4468.32 P21/m 11.9244(9) 13.3800(10) 13.6256(10) 90 109.4920(10) 90 2049.4(3) 1 0.71073 3.595 0.02414 0.0647

1201.92 P1̅ 8.3870(2) 11.7728(3) 13.2102(4) 102.5414(6) 106.9367(7) 103.0102(6) 1158.47(5) 2 0.71073 3.445 0.0297 0.0908

19965.18 P63/mmc 19.2741(9) 19.2741(9) 28.889(2) 90 90 120 9294.2(12) 1 0.71073 3.567 0.0459 0.1100

1232.70 P4̅21m 9.867(5) 9.867(5) 8.141(4) 90 90 90 792.6(9) 2 0.71073 5.165 0.0204 0.0564

1234.76 P4̅21m 9.8949(6) 9.8949(6) 8.2016(7) 90 90 90 803.01(11) 2 0.71073 5.107 0.0389 0.0778

R(F) = ∑||F0| − |Fc||/∑|F0|. bRw(F02) = {∑w(F02 − Fc2)2/∑wF04}1/2.

atoms as well as from extinction in context of heavy elements and are discussed in the cif files within the supplementary data. Raman Studies. Small crystalline fragments in the range between 0.1 and 0.5 mm were encapsulated in an α-shielded Plexiglas special container equipped with a quartz window through which micro-Raman spectra can be recorded. The container was connected to the microscope slide holder of the Raman spectrometer like previously described.33,34 The Raman microscope was equipped for those measurements with a ×50 magnification long working distance (10.6 mm) objective with a 0.5 numerical aperture. The spectra were registered on a Jobin-Yvon T 64000 Raman spectrometer equipped with a 1800 grooves per mm grating, a low noise liquid nitrogencooled symphony CCD detector. The excitation source used in this work is the 647 nm (1.91 eV) polarized line of a Coherent continuous wave (CW) laser. The typical laser power used at the sample surface ranged within a few mW. Using a ×50 objective allows a spectral resolution around 1 cm−1, with a spatial resolution of 2 μm × 2 μm. The spectrograph is calibrated with the T2g excitation of a silicon single crystal, set at 520.5 cm−1.35 The instrument is calibrated on a daily basis prior to measurements. Bond-Valence Analysis. Bond-valence sums (BVS) for all atom positions in the six reported actinide selenium oxo-compounds were calculated. The bond-valence parameters for SeIV−O, SeVI−O and AI− O (A = K, Rb, Cs) are used according to Brese and O’Keefee.36 NpIV −O values were taken from Diefenbach et al. as a = 1.972 Å and b = 0.538 Å 2. For NpVI−O, Wang et al. obtained the parameters a = 2.025 Å and b = 0.444 Å by analyzing 23 known NpVI−O compounds.7 The values for UVI−O were taken from Burns et al. as a = 2.051 Å and b = 0.519 Å.37 The calculated BVS sums can be found in the Supporting Information.

Figure 2. Structure of 1. (a) Top view parallel to [001] with square antiprismatic NpO8 polyhedra in dark green, trigonal pyramidal SeO3 in orange, K in purple, and water in red. For better clarity the partially occupied K2 positions are omitted. (b) Polyhedral representation of the 1D chains [Np(SeO3)4−x(HSeO3)x](4−x)‑. (c) Topologic representation: NpO8 polyhedra presented as black nodes, SeO3 in white.

The Np center is situated on a special position regarding x and y, however on a general position regarding z just slightly off from (0, 0, 1/4). This results in two distinct distances of oxygen positions to Np forming a square antiprismatic coordination (2.324(8) Å and 2.345(8) Å). This bonding-behavior is typical for NpIV and supported by BVS calculations yielding 4.06 v.u for Np. Both Se atoms are coordinated by three oxygen atoms forming trigonal pyramids with Se atoms positioned atop. The shape, as well as BVS, makes it clear that SeIV is present. The trigonal pyramids of the SeO3 polyhedra are strongly distorted, which can be easily explained by the structural connection with the NpO8 square antiprisms. Two antiprisms are connected by four corner-sharing selenite polyhedra, making two oxygen atoms necessary for the connection, leaving one oxygen of the trigonal pyramid terminal. This bond distance with 1.753(11) Å is distinctly longer than the two others (1.675(8) Å to 1.684(9) Å), as there is a higher degree of freedom. BVS calculation indicates a partial presence of a hydroxyl group at the terminal oxygen position. Charge balance is achieved by introducing the x into the sum formula with x potentially ranging from 0 to 4. The connection between Np atoms and Se oxo-groups extends infinitely along the c-axis leading to one-dimensional (1D) chains within the structure with the [Np(SeO 3 ) 4−x (HSeO3)x](4−x)− composition. The chain structure can be described by its topology and then systematically compared to other inorganic oxo-com-

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RESULTS The experimental regimes with different initial selenium valence show distinctly different results. NpV in combination with SeIV in the form of SeO2 resulted in a coprecipitation of NpIV selenites and NpVI nitrates. In the system of NpV with SeVI in the form of selenic acid, four novel phases were found, all of which are NpVI selenates. To our knowledge no NpVI selenates have been reported to date. Surprisingly, no NpV phases were found in both experimental regimes. Structure Descriptions. Neptunium Selenites. 1 crystallizes in the tetragonal space group P4/mnc with Np occupying one crystallographically independent position and Se two. The alkali atom occupies one well-defined and one disordered position. The structure is depicted in Figure 2, and the according crystallographic data are presented in Table 2. C

DOI: 10.1021/acs.inorgchem.7b02961 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry pounds. This form of description was introduced for actinide oxo-compounds by Krivovichev and Burns38 and later expanded by Krivovichev.39 Hereby actinide centers are represented by black nodes and oxoanions by white nodes. Connections are drawn as singular lines for corner sharing, double lines for edge sharing, and triple lines for face sharing polyhedra. The advantage of such a description allows a comparison regardless of elemental content, cell parameters, and symmetry. Figure 2c shows the according topology for this compound. This 1D topology has only been reported for Ce, Np, and Pu iodites.3,40 In these structures, the connection behavior is the same; however, the heavy cation is always coordinated in a trigonal dodecahedral fashion. Charge balance is achieved by K+ cations filling the interspace between the singular chains. Hereby the K1 position is well-defined; however, K2 is distinctly disordered with having three potential positions each occupied by 1/3. Neptunium Selenates. The structure of 2 adopts the monoclinic space group P2 1 /m. 2 is isostructural to (H3O)2[(UO2)2(SeO4)3(H2O)2](H2O)3.5,41 a direct uranyl selenate analogue, except for the localization of the solvent water positions. The structure is built upon two-dimensional (2D) neptunyl-selenate sheets with interlayer water and potassium (K) to achieve a neutral charge balance. Table 2 shows the basic crystallographic information for phase 2. Np occupies two crystallographically nonequivalent sites, both coordinated by seven oxygen atoms resulting in pentagonal-bipyramidal NpO7 polyhedra. The (Np1O2)2+ cation bridges by corner sharing to five SeO4 tetrahedra (Np1−O 2.336−2.403 Å) and the (Np2O2)2+ cation to three SeO4 tetrahedra (Np2−O 2.347−2.392 Å) and to two water molecules in cis-position relative to each other (Np2−OH2 2.405−2.411 Å). The presence of water within Np coordination is strongly indicated by bond-valence calculations and by the slightly prolonged bond distance. The moiety of neptunyl O NpO is nearly linear for both Np1 and Np2 with 178.8(2)° and 179.8(3)°, respectively. The NpO bond lengths range from 1.742 to 1.756 Å. BVS calculations yields 6.00 v.u. and 5.99 v.u. for Np1 and Np2, respectively. Three crystallographically nonequivalent positions are present for SeVI. Se is coordinated by four oxygen atoms resulting in a typical tetrahedral shape with average angles of 109.4−109.5°, which matches almost the ideal tetrahedron (109.47°). The Se1O4 and Se2O4 tetrahedra are tridentatebridging, connecting three independent NpO7 polyhedra. Se3O4 tetrahedra only connect to two NpO7 polyhedra, are hence described as bidentate-bridging. All terminal Se−O bond lengths are shorter with 1.612−1.637 Å, whereas the binding oxygen atoms are within 1.635−1.659 Å. BVS calculations yield 5.98 v.u. to 6.02 v.u. and are therefore in accordance for hexavalent selenium. The neptunyl and selenate polyhedra form [(NpO2)2(SeO4)3(H2O)2]2− sheets which lie parallel to the (010) plane. The interlayer space is filled with strongly disordered potassium and oxygen positions. Individual layers are linked together by potassium as well as water molecules interacting via hydrogen bonds. Unfortunately the hydrogen positions cannot be assigned by X-ray techniques. Layered structures can be well characterized and compared by investigating their topology. Using the method described above, the corresponding topology for 2 is depicted in Figure 3c. This topology has so far only been reported for two phases, and these are uranyl selenates. The first is the previously

Figure 3. 2D sheet structure of 2 and 3. (a) Sheet layer of 2 parallel to the (010) plane. NpO7 polyhedra are colored in rose and the SeO4 tetrahedra in light green. (b) Sheet layer of 3 parallel to the (101̅) plane. (c) Common topologic description of both sheets, 2 and 3. NpO7 polyhedra represented in black, SeO4 tetrahedra reduced to white nodes. (d) and (e) show the orientational geometric isomerism of 2 and 3. Selenate polyhedra facing “up” are depicted in magenta; polyhedra facing “down” are depicted in cyan. Chains of (uddu)∞ shown in (d) and (du)∞ in (e).

mentioned isostructural (H3O)2[(UO2)2(SeO4)3(H2O)2]· (H2O)3.5,41 and the second is Rb2[(UO2)2(SeO4)3(H2O)2]· (H2O)4,42 which is isostructural to 3. A topology comparison of the two structures will be given later. 3 crystallizes in the triclinic space group P1̅, and as noted above, it is isostructural to the direct uranyl selenate analogue Rb2[(UO2)2(SeO4)3(H2O)2]·(H2O)4,42 except for the disordered solvent water oxygen positions. The crystallographic data are presented in Table 2. The structure of 3 resembles the structure of 2 in many respects: Np occupies two independent positions and Se three, Np is coordinated as a pentagonal bipyramid, Se as SeO4 tetrahedra, and the connectivity of these is the same as described for 3, including the two water molecules in cisposition to each other in Np2. The same layers of [(NpO2) 2(SeO4)3(H2O)2]2− are formed, with the only D

DOI: 10.1021/acs.inorgchem.7b02961 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Structural representation of 4; (a) double layer structure (perpendicular to [001]) including the interlayer cations within the quasi-double sheets; same color scheme used as in Figure 3, additionally K is depicted in purple and water in red. The trace of the (110) plane is marked in black. (b) Side view of the quasi-double sheet. (c) Schematic cut along (110) showing the resulting cavities and excavations and showing the different unit elements #1, #2 and #3.

exception, that the orientation of terminal selenate polyhedra differsthis will be compared below. The interlayer space differs between compounds 2 and 3. Whereas potassium ions were assigned for charge balance in 2, rubidium is present as counter cations in 3. Two water molecules are present per formula unit within the interlayer space of 3 and only 1.5 in 2. Orientational Geometric Isomerism between 2 and 3. As already stated above, compounds 2 and 3 are isotopological (Figure 3c). However, investigating the 2D sheets reveals that there is a distinct difference within the orientation of the terminal oxygen atoms within the selenate tetrahedra. The terminal oxygen atom of the SeO4 tetrahedra is either oriented up top of the plane or down below. In Figure 3d and Figure 3e, the topology representation is combined with the orientational behavior of the selenate tetrahedra; hereby upward oriented polyhedra are denominated as up or simply “u” and downward oriented polyhedra as down or “d”. Most of the polyhedra are in accordance with each other except for the infinite chains in one sheet direction: compound 2 shows chains of (uddu)∞ oriented tetrahedra and compound 3 (du)∞. Such behavior makes these two structures orientational geometric isomers. This behavior has already been described previously by Krivovichev8 for the uranyl analogues of 2 and 3. 4 is the most complex structure described here and adopts the hexagonal space group P63/mmc. Its structure is depicted in Figures 4 and 5, and its topology is shown in Figure 6. Three independent Np positions are occupied as well as four Se positions. Crystallographic data are denoted in Table 2.

Figure 6. Topological descriptions of 4: (a) observed topology for a single layer of [(NpO2)9(SeO4)13.5(H2O)6]9−. (b) Idealized topology. (c) Idealized topology expected if no deformation would be present. Hereby monolayers would be expected.

All Np positions are hepta-coordinated by oxygen atoms: two close distance bonds with 1.731−1.773 Å assigned to the almost linear neptunyl group (177.7−179.6°) and five within the perpendicular equatorial plane resulting in a pentagonal bipyramid. Distances and shape resemble once again NpVI. The Np1 and Np2 pentagonal bipyramids are each connected to five selenate tetrahedra via equatorial oxygen positions in a vertex sharing fashion. For Np3 however, only three of the five equatorial oxygen positions are also corner shared with Se tetrahedra, the two remaining are part of two water molecules. The presence of water is indicated by BVS calculations; however, the hydrogen positions cannot be detected with the chosen method. All four Se positions are coordinated by four oxygen atoms forming the typical tetrahedral shape. However, the bond distances are strongly distorted within these tetrahedra: Se1, Se2, and Se3 each show one distinctly shorter bond (1.613 Å, 1.584 Å, 1.603 Å), which is explained by its connections to the NpO7 bipyramids. Each selenate tetrahedron corner shares three oxygen positions with equatorial oxygen positions of three different NpO7 polyhedra. The fourth oxygen position remains terminal resulting in the mentioned distortion. The corresponding long bonds range between 1.634 and 1.655 Å. Se4 only vertex shares with one NpO7 bipyramid, and hence

Figure 5. Layer structure of 4, all units shown perpendicular to [001]; (a) complete view of a single layer with Se1 tetrahedra combining all three elements (#1, #2, and #3). NpO7 tetragonal bipyramids depicted in rose, selenate tetrahedra in green, and partially occupied selenate in orange. (b) first unit element (#1) containing Np1 and Se3, (c) second unit element (#2) containing Np2 and Se2, (d) third unit element (#3) containing Np3 and Se4. E

DOI: 10.1021/acs.inorgchem.7b02961 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the bonds are prolonged. The Se4 position is only occupied by 1/2 as otherwise two equivalent selenium positions would occur within 2.264 Å. The connection of the polyhedra results in a 2D quasi-double sheet structure of two sheets lying closely opposite to one another (Figure 4). A single sheet is made up of three distinctive units. The first unit is a triangle of three Np1O7 bipyramids and three Se3O4 tetrahedra (#1, Figure 5b), the second a triangle of three Np2O7 bipyramids and three Se2O4 tetrahedra (#2, Figure 5c), and the third a ring of three Np3O7 and 1.5 Se4O4 (#3, Figure 5d). The latter unit is distinctively out of plane, whereas the first and second unit are in plane. Finally, the three elements are connected by the Se1 tetrahedra. This results in layers of [(NpO2)9(SeO4)13.5(H2O)6]9−. Combining both layers reveals that the #1 unit has another #1 unit as its counterpart, and for #2 and #3, the positions exchange (Figure 4). A schematic cut through the sheet along the (110) plane is depicted in Figure 4c. This leads to a more or less closed cavity between the two #1 units, which is filled by two potassium cations (K1 position) and three oxygen (water) positions. These water molecules will most likely stabilize this quasi-double layer structure by forming hydrogen bonds to water molecules from the tilted Np3O7 bipyramids. The match of the #2 units on top of the #3 units leads to a half open excavation with the #3 unit acting as the open side. Within this space, three partially occupied potassium sites (K3) are present (in total an occupation of 1). The interspace between two separate double layers is occupied by several disordered potassium positions as well as disordered water molecules. Viewing the topology of 4 (Figure 6a,b) helps to understand the morphology of the sheets. If the terminally bonded Se4 were bonded to Np3O7, the entire structure could adopt the regular topology depicted in Figure 6c and would be flat. Hereby the Se4 position would be occupied by 100%, and hence the structural unit would be [(NpO2)3(SeO4)5]4−. Such layers and topologies can for example be found in M2[(UO2)3(SeO4)5](H2O)16 (M = Co, Zn)43 and Na6[(NpVO2)2(NpVIO2)(MoO4)5](H2O)13.44 In this case the structure would likely not be a quasi-double sheet structure but a structure made up of monolayers. Possibly, steric hindrance as well as to stabilization via hydrogen bonds from interlayer water molecules with inner layer water molecules could be the reason for the stabilization of these quasi-double layers. Compound 5 is made up of one symmetrically independent Np, two Se, and two Cs sites. It crystallizes in the tetragonal space group P4̅21m, and its structure is depicted in Figure 7. Np is coordinated 7-fold by oxygen, two of which are part of the neptunyl dioxo bond and the remaining five within the perpendicular equatorial plane. The bond distances within the plane range from 2.361(11) Å to 2.404(9) Å and within the neptunyl group from 1.731(11) Å to 1.747(10) Å. These distances are typical for NpVI coordination, and the BVS of 6.04 v.u. is also well in agreement with the proposed oxidation state. The neptunium pentagonal bipyramids are connected by corner sharing SeO4 tetrahedra. The Se1O4 tetrahedron connects three bipyramids and the Se2O4 tetrahedron four bipyramids. This results in an undulated 2D sheet structure with a composition of [(NpO2)2(SeO4)3]2−. Se2 lies exactly within the 4-fold inversion axis, and as Se1 is on a general position, the ratio between Se1/Se2 is 3:2. For Se1, the bond distances are distinctly distorted by structural force: 1.569(14) Å for the terminal oxygen bond and 1.632(9)−1.646(10) Å for

Figure 7. Structural representation of 5. (a) Side view on the stacked layers, NpO7 pentagonal bipyramids depicted in rose, tetragonal SeO3 in light green, Cs in pink. (b) Top view of the [(NpO2)2(SeO4)3]2− layer with corner-sharing neptunyl and selenate polyhedra. (c) Topologic representation: NpO7 polyhedra presented as black nodes, SeO4 in white, same topology and structure as for 6.

the vertex connecting atoms. The Se2−O bond is isotropic due to its special position with 1.604(9) Å. BVS calculations for both Se positions yield in a slight overestimation of the charge, with 6.32 and 6.58 v.u., respectively. The structure is isostructural to the UVI analogue 6, as well as to Cs2(AnO2)2(SO4)3 (An = U, Am)45,46 and Cs2(UO2)2(CrO4)347 and also isotopologic to β-Cs2[(UO2)2MoO4)3].48 The Cs2[(UO2)2(SeO4)3], has not been reported to date, and the crystallographic data of this are shown in Table 2. This was obtained by an analogue slow evaporation experiment, but instead of the NpV stock solution, (UO2)(NO3)2(H2O)6 was used as the actinide source. The ratios and concentrations were identical to the experiment described above. Raman Spectroscopy. The linear neptunyl cations possess D∞h symmetry. This symmetry results in three Raman-active modes, which include the symmetric stretching mode ν1, the bending mode ν2, and the asymmetric stretching mode ν3. In nitric acid solution, ν1 has been observed at 863 cm−1, in perchloric acid solution at 859 cm−1, and within the solid state, a band at 872 cm−1 has been assigned for NpO2(IO3)2(H2O).49 The ideal trigonal pyramidal selenite possesses C3v symmetry. Hereby four Raman-active vibrational frequencies can be observed: two nondegenerate modes ν1 and ν2 and two doubly degenerate modes ν3 and ν4. The according values are given by Nakamoto to be 807, 432, 737, and 374 cm−1, respectively.50 In the case of a hydroselenite ([HSeO3]−), the symmetry degrades to Cs in which both ν3 and ν4 are nondegenerate, resulting in six Raman-active modes (ν1, ν2, ν3′, ν3″, ν4′, and ν4″). Selenate polyhedra with ideal tetrahedral symmetry (Td) result in four normal modes, the symmetric and the asymmetric stretch (ν1 and ν3) as well as the bending modes ν2 and ν4. The frequencies for free SeO42− ions in aqueous solution are 833 cm−1, 335 cm−1, 875 cm−1, and 432 cm−1 for ν1, ν2, ν3, and ν4, respectively. Due to crystal-field effects as well as mutual interactions, the vibrational modes are expected to shift in solid-state compounds.51 Figure 8 depicts the Raman spectrum for a mixture of 1 and K[(NpO2)(NO3)3] in the range of 125−1100 cm−1. The F

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Figure 8. Raman spectrum from 125 to 1100 cm−1 of 1 and K[(NpO2)(NO3)3] with determined peak positions. The mode assignments are given in Table 3.

corresponding band assignments are given in Table 3. The low wavenumber region (125−250 cm−1) is assigned to lattice

modes and addressing specific assignments is complex. The strongest band at 882 cm−1 is attributed to the symmetric stretch of a neptunyl group. The slightly larger Raman shift compared to the previously mentioned shift of 872 cm−1 in NpO2(IO3)2(H2O) is well in agreement, as the Np−O bondlength is also slightly shorter with 1.747 Å compared to the 1.760 Å in the latter compound.49 The sharp band at 1046 cm−1 and the rather weak band at 722 cm−1 are attributed to ν1 and ν4 of the trigonal planar nitrate. These values are well in agreement with other nitrate, as for example KNO3.52 The third Raman-active nitrate mode is beyond the range of the Raman spectrum depicted here (ν3 in the range of 1320−1390 cm−1).52 The structure refinement of 1 strongly indicated a presence of both selenite and hydroselenite groups, in order to achieve charge balance. Raman spectroscopy is a method which yields information concerning short-range order, and hence this method allows distinguishing between selenite as well as hydroselenite modes. The symmetric stretch and symmetric bend modes are nondegenerate, and these are assigned first. For the selenite, ν1 and ν2 can be assigned at 871 and 459 cm−1, which is well in agreement for other selenites.53 For the hydroselenite, 657 and 399 cm−1 are assigned correspondingly, well in agreement with assignments made by Cody et al.54 For the asymmetric modes, a distinct assignment for ν3(SeO3)2−

Table 3. Raman Modes Determined for a Mixture of 1 and K[(NpO2)(NO3)3] and Proposed Band Assignmentsa wave number [cm−1] 130 145 166 205 216 232 325 343 354 383 399 436 459 494

w m w w w w w w w m m w m w

assignment lattice mode lattice mode lattice mode lattice mode lattice mode lattice mode ν4′/ν4″ (HSeO3)− ν4′/ν4″ (HSeO3)− ν4′/ν4″ (HSeO3)− ν4 (SeO3)2− ν2 (HSeO3)− ν2 (SeO3)2−

wave number [cm−1] 606 644 657 722 741 761 779 801 827 842 871 882 914 1046

w sh s sh m m w w w w sh s w s

assignment

ν1 (HSeO3)− ν4 (NO3)− ν3 (SeO3)2− ν3″ (HSeO3)−

ν3′ (HSeO3)− ν1 (SeO3)2− ν1 (NpO2)2+ ν1 (NO3)−

Hereby the accuracy of the assignments is assumed to be ±1 cm−1. s = strong, m = medium, w = weak and sh = shoulder. a

Figure 9. Raman spectrum for 3. The corresponding mode assignments are given in Table 4. G

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Inorganic Chemistry and ν3″(HSeO3)− is difficult to make, as both of these have bands in a similar range. The band at 741 cm−1 is assigned to ν3(SeO3)2−, at 761 cm−1 to ν3″(HSeO3)− and at 842 cm−1 to ν3′(HSeO3)−. The latter is hereby well in agreement with the assignment of 813−859 cm−1 in KHSeO3.54 The asymmetric bend for the selenite can be found at 383 cm−1. For the hydroselenite, ν4′ and ν4″ are found in the region of 324 to 354 cm−1, in which the lower Raman shift should be addressed to the ν4′ and the higher to the ν4″. The Raman spectrum in the range of 125−1000 cm−1 for phase 3 is depicted in Figure 9, and band assignments are given in Table 4. The low wave numbers (125−300 cm−1) are

the form of nitrates. Figure 1 shows a photo of the obtained phase mixture. In the case of initial SeVI being used, the first ever reported NpVI selenate compounds have been stabilized. To best of our knowledge, only one NpVI oxo-selenium compound has been reported as a (NpO2)(SeO3) selenite.2 It was synthesized by hydrothermal methods and formed simultaneously with the NpIV bearing Np(SeO3)2, this concurrent precipitation being explained by the disproportionation of NpV. The formation of several different selenates, as well as the inability to find any NpIV or NpV species within solid phases, indicates at a complete oxidation of the initial NpV within the solution (reaction II1). Possible reaction pathways could be Oxidation:

Table 4. Raman Modes and Proposed Band Assignments for 3a

a

wave number [cm−1]

assignment

wave number [cm−1]

137 152 167 177 194 211 227 237 253 301 324 335 352 367 378 398

lattice mode lattice mode lattice mode lattice mode lattice mode lattice mode lattice mode lattice mode lattice mode ν2 (SeO4)2− ν2 (SeO4)2− ν2 (SeO4)2− ν2 (SeO4)2− ν2 (SeO4)2− ν2 (SeO4)2− ν4 (SeO4)2−

432 463 785 796 812 837 860 876 885 901 913 921 931 948 962 974

w w w w w w w w w w w w w w m w

w w sh s s s m sh sh w w m m w w w

2[NpO2 ]+ ⇌ [NpO2 ]2 + + 2e− E 0 = −1.159 V (ref 1)

assignment ν4 (SeO4)2− ν4 (SeO4)2− ν1 ν1 ν1 ν1

Reduction: (NO3)− + 3H+ + 2e− ⇌ HNO2 + H 2O

(SeO4)2− (SeO4)2− (SeO4)2− (NpO2)2+

E 0 = +0.934 V (ref 56)

(III)

(SeO4 )2 − + 2H+ + 2e− ⇌ (SeO3)2 − + H 2O E 0 = +0.880 V (ref 57)

(IV)

The most likely source for oxidation is the presence of the high nitrate concentrations (reaction III56) in combination with the higher acidity (lower initial pH) due to the use of selenic acid in comparison to the use of the less acidic selenium dioxide. Oxidation via reduction of SeVI to SeIV (reaction IV57) seems unlikely, as no selenites were found within the precipitates. Additionally the oxidation via oxygen could also be a viable option. Hereby, NpV could be oxidized to NpVI by oxygen supplied by air, in which the strong coordinating ligand of SeO42− could aid in the process. As previously described, the 1D structural units of the reported neptunium selenite 1 shows distinct similarities to neptunium and plutonium iodites. This can indicate a coprecipitation of these phases. This can especially be of importance for the safe assessment of nuclear waste, as iodine in the form of the long-lived fission product 129I (t1/2 1.6 × 107 years) is, similar to 79Se, considered a radionuclide of greatest environmental impact.15 The structural behavior of the reported structures is in accordance with expected behavior for potential NpVI selenates. The neptunyl ion is well-known to cause 2D layered structures, similar to UVI. The absence of cation−cation interactions within these structures is also expectable. The O−Np bond in NpVI is stronger as in the weaker NpV, where this permits linkages of the neptunyl polyhedra through the neptunyl ion oxygen atom.58 Contrary to the structural resemblance, the reaction pathways are very different between the Np and U systems. The analysis of short-range order in phases 1 and K[(NpO2)(NO3)3] as well as in 3 by the assignment of modes in Raman spectra is successful. For 1, this allows the otherwise difficult to distinguish presence of hydroselenite groups in the structure. In SC-XRD with normal X-ray sources, the assignment of hydrogen positions is impossible. However, the presence of the hydroselenite bands in the Raman spectrum supports the indirect assignment through bond-valence and charge neutrality calculations.

ν3 (SeO4)2− ν3 (SeO4)2− ν3 (SeO4)2−

Accuracy and abbreviations are identical to those in Table 3.

attributed to lattice modes. The neptunyl-band is less prominent but addressable at 860 cm−1. In general, only four further regimes should be addressable: ν1 to ν4 for the selenate anion group. The spectrum shows large splitting of possible positions for selenate bands. Considering that three crystallographically unique Se positions are present in the structure of 3, it makes this behavior understandable. The region from 796 to 837 cm−1 is assigned to the symmetric stretch, 301−378 cm−1 to the symmetric bend, 921−948 cm−1 to the asymmetric stretch, and 398 to 463 cm−1 to the asymmetric bend. Similar assignments for the selenates were made with NpV selenates51 and with Rb4Be(SeO4)2(HSeO4)2·4H2O.55

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DISCUSSION The results obtained from these slow evaporation experiments show a very strong dependence on the initial oxidation state of Se. The addition of SeIV in the form of SeO2 resulted in NpIV selenites as well as NpVI nitrates. This kinetically driven phenomenon can be explained by the disproportionation of NpV in acidic conditions, first reported by Sjoblom and Hindman in the 1950s:5 2[NpO2 ]+ + 4H+ ⇌ Np4 + + [NpO2 ]2 + + 2H 2O

(II)

(I)

The Np selenites precipitate first due to their expected lower solubility compared to corresponding nitrates. In the later process, with decreasing solution volumes and hence higher supersaturations, also the present NpVI precipitates in IV

H

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Notes

CONCLUSION We performed a systematic investigation of phase formation in the complex Np-based oxo-system and observed five novel alkali neptunium oxo-selenium crystalline phases with different oxidation states of Np and Se from slow evaporation. Experiments conducted with Se in initial oxidation state SeIV led to the formation of NpIV selenites and simultaneously NpVI nitrates due to disproportionation of NpV in acidic media. We have synthesized the first NpVI selenates to date by using initial SeVI in the form of selenic acid. Hereby the NpV oxidization to NpVI was addressed due to the presence of a high nitrate concentration in strongly acidic conditions, and a possible oxidation via reduction of selenium was rejected as no selenite presence was detected. Oxidation by air oxygen was also postulated as possibly being responsible for the oxidation. BVS calculations, coordination behavior, as well as Raman spectral analysis support the oxidation state assignments, as well as the presence of the proposed nitrate, selenite, hydroselenite, and selenate groups. The structure of the reported NpVI selenates resembles the expected structural behavior similar to the UVI analogue. This can be seen due to the knowledge of two previously reported uranyl isostructures and one isostructure reported here. The reaction pathway however was very much unexpected and does not resemble the chemical behavior of UV/UVI due to the significant differences in their electronic structure and redox chemistry. To further understand the influence of the nitrate, experiments on the basis of alkali chlorides instead of nitrates have been prepared and are currently under investigation. This experiment will make the process more clear, and it will show whether the nitrate is crucial for this reaction or the ambient air oxygen. The investigation of NpVI as initial reactant in contact with Se in oxidation states SeIV and SeVI would also be interesting to investigate and would aid in a further understanding of the new family of NpVI selenium phases. Future experiments also aim at a study of the thermodynamic stability of observed Np selenite phases, as such phases could potentially be relevant for the safety case of an assessment of nuclear waste disposal in deep geologic repository sites.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the support provided by the European Commission in funding the experiments under the scheme of GENTLE SRE Funding at JRC Karlsruhe, Directorate G (former Institute for Transuranium Elements). The work has been supported by the Helmholtz Association within the VHNG-815 grant.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02961. Bond valence sum calculations and tables (PDF) Accession Codes

CCDC 1586290−1586295 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Evgeny V. Alekseev: 0000-0002-4919-5211 I

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b02961 Inorg. Chem. XXXX, XXX, XXX−XXX