Crystal Growth and Structure Characterization of Three Layered … · 2019-03-05 · Crystal Growth and Structure Characterization of Three Layered Uranyl Phosphates and Their Relation

Crystal Growth and Structure Characterization of Three LayeredUranyl Phosphates and Their Relation to the PhosphuranyliteFamilyChristian A. Juillerat and Hans-Conrad zur Loye*Department of Chemistry and Biochemistry, and the Center for Hierarchical Waste Form Materials, University of South Carolina,Columbia, South Carolina 29208, United States

*S Supporting Information

ABSTRACT: Three new materials related to the phosphuranylite familywere synthesized by using alkali chloride fluxes at 875 °C: CsNa3-[(UO2)3O2(PO4)2] (1), Cs2Na4[(UO2)5O5(PO4)2] (2), and Rb6[(UO2)5-O5(PO4)2] (3). CsNa3[(UO2)3O2(PO4)2] (1) crystallizes in the triclinicspace group P1 with the lattice parameters, a = 6.9809(3) Å, b = 9.3326(4)Å, c = 12.9626(5) Å, α = 71.5620(10)°, β = 78.9430(10)°, and γ =68.0840(1)°; Cs2Na4[(UO2)5O5(PO4)2] (2) crystallizes in the triclinicspace group P1 with the lattice parameters, a = 6.9890(3) Å, b =12.9652(6) Å, c = 13.2086(6) Å, α = 96.224(2)°, β = 101.433(2)°, and γ =105.459(2)°; and Rb6(PO4)2[(UO2)5O5] crystallizes in the P21/m spacegroup with the lattice parameters, a = 6.9255(3) Å, b = 24.773(1) Å, c =7.07647(3) Å, and β = 90.741(1)°. The sheets of 1 are based on thephosphuranylite topology, while the sheets of 2 and 3 contain sheets based on the U3O8 and uranophane topologies but differ inthe orientation of the phosphate tetrahedra. The regions between the sheets contain the alkali cations and are not all identical instructures 1 and 2. The geometrical isomers found in these sheet structures and their relationship to known sheet topologies arediscussed.

■ INTRODUCTIONLayered uranium(VI) phosphate materials continue to receiveattention in the actinide community due to their largestructural variety and low solubility, which makes them ofinterest to the waste form community. Historically mostinvestigations were focused on obtaining new uraniumcontaining compositions and to crystallize them in novelstructure types with the long-term goal being to betterunderstand uranium crystal chemistry, in general, and thelocal coordination chemistry, in specific. The low solubility ofactinide phosphates is a valuable property for nuclear wasteapplications, where issues including environmental mobility,environmental remediation, waste processing, and the develop-ment of novel nuclear waste storage materials are beinginvestigated.1

The phosphuranylite sheet anion topology is one of themost dominant structure types for uranium phosphate andarsenate minerals, and this sheet topology is also found insynthetic materials. To date, there are 17 synthetic compoundsand 17 structurally characterized minerals belonging to thephosphuranylite class. The phosphuranylite materials are acompositionally varied class of materials composed of sheets,sometimes connected by U or Th polyhedra, separated bymonovalent and/or divalent cations and water in mineralstructures. The phosphuranylite sheet anion topology containstriangles, squares, pentagons, and hexagons where there aretypically chains of UO8 square bipyramids and dimers of UO7

pentagonal bipyramids that are connected together bytetrahedral (P, As, V), trigonal pyramidal (Se, Te), or trigonalplanar (C) building units.2−7 This arrangement always leads tovacant square coordination sites, and the hexagonal uraniumsites can at times also be vacant, as seen in the johannitemineral, although materials that have vacant hexagons will notbe considered further in this work. Also related to the family ofphosphuranylite materials, are a few that are frameworkstructures constructed of phosphuranylite-type chains, andthese include A3[Al2O7(PO2)3][(UO2)3O2] (A = Rb, Cs),6 thearsenate mineral nielsbohrite,3 and [(UO2)3(PO4)O(OH)-(H2O)2](H2O), which will be included in the discussion ofsynthetically derived phosphuranylite materials.8

The phosphuranylite topology consists of chains of uranylpentagonal and hexagonal bipyramids whose edges aredecorated with tetrahedral building units that connect thechains into layers through edge and corner sharing (Figure 1a).Related to this phosphuranylite topology is the sheet aniontopology observed in “extended phosphuranylite” systems, thestructural relationship of which is illustrated in Figure 1b, c,which is observed in the synthetic materials Cs1.7K4.3[(UO2)5-O5(PO4)2], Rb1.6K4.4[(UO2)5O5(PO4)2],

5 K6[(UO2)5O5-(PO4)2],

9 M6[(UO2)5O5(VO4)2] (M = K, Na),10 K6[UO2)5-

Received: November 1, 2018Revised: December 12, 2018Published: December 18, 2018

Article

pubs.acs.org/crystalCite This: Cryst. Growth Des. 2019, 19, 1183−1189

© 2018 American Chemical Society 1183 DOI: 10.1021/acs.cgd.8b01643Cryst. Growth Des. 2019, 19, 1183−1189

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O5(AsO4)2],4 α- and β-Rb6[(UO2)5O5(VO4)2],

11 and K4-[(UO2)5(TeO3)2O5].

12 This topology contains mirror imagechains of pentagons that are connected together through anadditional uranium site not found in the regular phosphur-anylite topology to create wider chains that are connected intolayers through edge and corner sharing via the tetrahedralbuilding units. This topology can be obtained if one envisionsextending the phosphuranylite topology by cutting the chainsof hexagons and pentagons in half and inserting additionaluranium sites (Figure 1b). This topology will be referred to asthe “extended phosphuranylite” topology throughout thispaper.Herein, we report the flux crystal growth and structural

characterization of three new materials CsNa3[(UO2)3O2-(PO4)2] (1), Cs2Na4[(UO2)5O5(PO4)2] (2), and Rb6[(UO2)5-O5(PO4)2] (3) that crystallize in the phosphuranylite andextended phosphuranylite topologies. We will discuss thestructures of these materials and their geometrical isomers ofthe phosphuranylite and extended phosphuranylite topologiesand present a new general classification scheme for theextended phosphuranylite topologies based on the one for thephosphuranylite topologies.

■ EXPERIMENTAL SECTIONUF4 (International Bio-Analytical Industries, powder, ACS grade),UO2(NO3)2·6H2O, AlPO4 (Alfa Aesar, powder, 99.99%), CsCl (AlfaAesar, powder, 99%), RbCl (Alfa Aesar, powder, 99.8%), and NaCl(Fisher Chemical, powder, 99.0%) were used as received. Caution!Although the uranium precursor used contained depleted uranium,standard safety measures for handling radioactive substances must befollowed.Crystal Growth. Crystals of all three phases were obtained using

molten alkali chloride fluxes.13 Small orange needles of Cs2Na4-[(UO2)5O5(PO4)2] (2) were produced in a large excess of AgCl

byproduct by loading 0.5 mmol of UF4, 2 mmol of AlPO4, 4 mmol ofNaCl, and 20 mmol of CsCl into silver tubes measuring 5.7 cm tall by1.8 cm wide. The reaction was heated to 875 °C in 1.5 h, held for 12h, and slowly cooled to 400 °C at 6 °C/h. Both CsNa3[(UO2)3O2-(PO4)2] (1) and Rb6[(UO2)5O5(PO4)2] (3) were synthesized inalumina crucibles that were covered by larger inverted crucibles aspreviously described5 and heated under the same conditions as 2. Toobtain the yellow single crystals of CsNa3[(UO2)3O2(PO4)2] (1)(Figure 2), 0.5 mmol of UF4, 0.33 mmol of AlPO4, 5 mmol of CsCl,

and 5 mmol of NaCl were used. Rb6[(UO2)5O5(PO4)2] (3) wassynthesized using 0.5 mmol of UF4, 0.33 mmol of AlPO4, and 20mmol of RbCl, and this reaction produced orange plates of 3 andyellow rods of Rb7[Al2O7(PO2)3][(UO2)6O4(PO4)2.

6

Structure. Single crystal X-ray diffraction data were collected on aBruker D8 Quest single crystal X-ray diffractometer equipped with aMo Kα microfocus source (λ = 0.71073 Å). The raw data werereduced and corrected using SAINT+ and SADABS within the APEX3 software.14 The SHELXT intrinsic phasing solution program wasused to obtain an initial structure that was subsequently refined usingSHELXL.15,16 PLATON programs ADDSYM and TwinRotMap wereused to check for missing symmetry elements and minor twincomponents.17 Energy dispersive spectroscopy (EDS) performed on aTESCAN Vega-3 SBU equipped with an EDS detector was used toobtain qualitative elemental analysis in order to verify the elementalcontents of the structures. All metal atoms were allowed toindividually freely refine, and no significant deviation from unitywas observed. Full crystallographic data are reported in Table 1, andtables of selected bond distances and bond valence sums are includedin Tables S1−S3.

CsNa3[(UO2)3O2(PO4)2] (1) crystallizes in the triclinic spacegroup P1 with the lattice parameters, a = 6.9809(3) Å, b = 9.3326(4)Å, c = 12.9626(5) Å, α = 71.5620(10)°, β = 78.9430(10)°, and γ =68.0840(1)°. Within the asymmetric unit there are three U sites, twoP sites, four Na sites, and 16 O sites where all lie on general positions(Wykoff site 2i), except Na3 and Na4 that lie on Wyckoff sites 1a and1b, respectively, and have 1 symmetry.

Cs2Na4[(UO2)5O5(PO4)2] (2) crystallizes in the triclinic spacegroup P1 with the lattice parameters, a = 6.9890(3) Å, b = 12.9652(6)Å, c = 13.2086(6) Å, α = 96.224(2)°, β = 101.433(2)°, and γ =105.459(2)°. The asymmetric unit contains five symmetrically uniqueU sites, two P sites, two Cs sites, five Na sites, and 23 O sites. Similarto 1, all atoms lie on general positions, except Na3 and Na5 that lie onWyckoff sites 1c and 1g, respectively, with 1 symmetry. Afterrefinement, a large, but of acceptable magnitude considering the heavyscatterers in the structure, electron density peak of 5.886 remainsatempts to collect on additional crystals and at lower tempertatures inorder to improve the structure solution were unsuccessful. The lowtemperature, 100 K, data collection resulted in the best refinementand is reported in Table 1.

Rb6(PO4)2[(UO2)5O5] crystallizes in the P21/m space group withthe lattice parameters, a = 6.9255(3) Å, b = 24.773(1) Å, c =7.07647(3) Å, and β = 90.741(1)°. The solution was refined as a two-component twin using twin law −1 0 −0.025 0 −1 0 0 0 1 with avolume fraction of 1%. The addition of the twin law with smallvolume fraction significantly improved the R1 value from 0.0272 to0.0232 and the maximum and minimum residual density peaks from

Figure 1. (a) The phosphuranylite sheet anion topology. (b) Theexpansion of the phosphuranylite topology by cutting the chains ofuranyl pentagonal and hexagonal bipyramids in half. (c) The insertionof additional uranium sites to obtain the extended phosphuranylitetopology.

Figure 2. Optical images of crystals of (a) CsNa3[(UO2)3O2(PO4)2](1) and (b) Rb6[(UO2)5O5(PO4)2] (3).

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6.3/−3.5 to 5.3/−2.4. The asymmetric unit contains 3 U sites, 1 Psite, 4 Rb sites, and 13 O sites. U3, Rb1, O5, O6, and O8 lie onWyckoff site 2e with m symmetry and Rb4A lies on site 2a with 1symmetry, while all other sites lie on general positions. There isdisorder on the Rb4 site, which is split into Rb4A and Rb4B, where asump command was used to constrain the sum of the occupancies ofthe two Rb4A, due to the inversion symmetry, and one Rb4B site toone and this constraint resulted in occupancies of 0.288(17) and0.356(9) for Rb4A and Rb4B, respectively.

■ RESULTS/DISCUSSIONSynthesis. Among the 32 structures containing the

phosphuranylite chains and sheets, 16 were syntheticallyobtained, while the rest are naturally occurring minerals.Structure 2 in this text, the six A4[(UO2)3O2(PO4)2]compositions,5,18 and aluminophosphates Rb7[Al2O7(PO2)3]-[(UO2)6O4(PO4)2 and A3[Al2O7(PO2)3][(UO2)3O2] (A = Rb,Cs)6 were obtained by molten flux methods using alkalichloride fluxes, while A4[(UO2)3O2(AsO4)2] (A = K, Rb),4

Li2(H2O)6[(UO2)3(SeO3)2O2],7 Sr[(UO2)3(SeO3)2O2]-

(H2O)4,8 and [(UO2)3(PO4)O(OH)(H2O)2](H2O)

8 wereobtained by various hydrothermal methods. Similarly, withthe extended phosphuranylite-type structures, all weresynthesized using alkali halide fluxes, except for K6[UO2)5O5-(AsO4)2]

4 which was synthesized by high pressure, hightemperature hydrothermal methods. These reports suggest thatalkali halide flux growth methods are a good synthetic route forthe synthesis of new structures of the phosphuranylite andextended phosphuranylite types.

Typically, uranium phosphates crystals have been obtainedby hydrothermal, solid state, or molten flux methods.Regardless of the synthesis route, UO2(NO3)2 is among themost widely used uranium source, although the commonuranium oxides U3O8, UO2, UO3 are also used. The phosphatesources are more widely varied where wet chemical routes tendto use solutions of H3PO4 or H3PO3,

19−24 and less commonlysolutions of Na4P2O7 and K4P2O7,

25 where solid state andmolten flux synthetic routes most often use P2O5, (NH4)2-HPO4, or NH4H2PO4.

26−32 While P2O5 is very common insolid state and molten flux synthesis, sometimes as a flux, it isbetter suited for synthesis in closed systems due to the fact thatP2O5 is very reactive to atmospheric water and should behandled in the glovebox for accurate masses. (NH4)2HPO4 andNH4H2PO4, where (NH4)2HPO4 loses NH3 at 70 °C tobecome NH4H2PO4, are often used instead of P2O5 due to theease of handling the ammonium based reagents in air. AlPO4 isa fairly unique phosphate source for molten flux methods andcan lead to the synthesis of both phosphates andaluminophosphates and leads to the synthesis of differentphosphate products as compared to the use of (NH4)2HPO4and NH4H2PO4 phosphate sources. Thus far, the AlPO4starting material has led to the discovery of 15 new uraniumphosphates/aluminophosphates including those described inthis article. The large variety of phosphate sources available tothe solid state chemist including, BPO4,

33 A4P2O7 (A = Na,K),34 AlPO4, Na3PO4, APO3 (A = Na, K),35 AH2PO4 (A = Na,K), and A2HPO4 (A = Na, K), are underexplored in uraniumphosphate chemistry with few examples of syntheses using

Table 1. Full Crystallographic Data for CsNa3[(UO2)3O2(PO4)2], Cs2Na4[(UO2)5O5(PO4)2], and Rb6[(UO2)5O5(PO4)2]

CsNa3[(UO2)3O2(PO4)2] Cs2Na4[(UO2)5O5(PO4)2] Rb6[(UO2)5O5(PO4)2]

compound 1 2 3space group P1 P1 P21/ma (Å) 6.9809(3) 6.9732(3) 6.9255(3)b (Å) 9.3326(4) 12.9576(5) 24.7730(10)c (Å) 12.9626(5) 13.1389(10) 7.0547(3)α (deg) 71.5620(10) 96.4130(10) 90β (deg) 78.9430(10) 101.377(2) 90.7410(10)γ (deg) 68.0840(1) 105.3960(10) 90V (Å3) 740.65(5) 1105.05(8) 1210.24(9)crystal color yellow orange-yellow orangecrystal size (mm3) 0.02 × 0.02 × 0.05 0.02 × 0.02 × 0.05 0.02 × 0.05 × 0.06temperature (K) 300 100 300density (g cm−3) 5.533 5.944 5.851θ range (deg) 2.442−36.330 2.498−36.332 2.466−36.364μ (mm−1) 35.528 40.087 45.536collected reflns 75239 113264 123881unique reflns 7198 10726 5988Rint 0.0450 0.0445 0.0393h −11 < h < 11 −11 < h < 11 −11 < h < 11k −15 < k < 15 −21 < k < 21 −41 < k < 41l −21 < l < 21 −21 < l < 21 −11 < l < 11Δρmax (e Å−3) 1.909 5.886 5.278Δρmin (e Å−3) −2.498 −2.281 −2.432GoF 1.108 1.144 1.199extinction coefficient 0.00036(3) 0.000112(19) 0.00090(4)R1(F) for Fo2 > 2σ(Fo2)

a 0.0181 0.0272 0.0232Rw(Fo2)

b 0.0353 0.0627 0.0566aR1 = ∑||Fo| − |Fc||/∑|Fo|.

bwR2 = [∑w(Fo2 − Fc

2)2/∑w(Fo2)2]1/2; P = (Fo

2 + 2Fc2)/3; w = 1/[σ2(Fo

2) + (0.0017P)2 + 1.7972P] forCsNa3[(UO2)3O2(PO4)2], w = 1/[σ2(Fo

2) + (0.0201P)2 + 15.1774P] for Cs2Na4[(UO2)5O5(PO4)2], and w = 1/[σ2(Fo2) + (0.0115P)2 +

16.9843P] for Rb6[(UO2)5O5(PO4)2].

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these reagents. The use of different phosphate sources shouldbe studied in order to obtain new and unique structures as wellas to increase our understanding of why different phosphatesources lead to different products.In the synthesis of Cs2Na4[(UO2)5O5(PO4)2] (2), the large

amount of AgCl produced by the chloride flux interaction withthe silver reaction vessel during the synthesis made isolatingthe very small and brittle orange needles difficult. As in therecently reported synthesis of the A4[(UO2)3O2(PO4)2] family,reactions with the same reactant loading and heating werecarried out in alumina reaction vessels. All attempts in aluminacrucibles, including conducting multiple crystal growthreactions utilizing different amounts of flux, yielded only theyellow crystals of CsNa3[(UO2)3O2(PO4)2] (1). In thesynthesis of the A4[(UO2)3O2(PO4)2] materials, it wasshown that increasing the flux to 40 mmol improved theyield of the desired phase, A4[(UO2)3O2(PO4)2], over sideproducts of A6[(UO2)5O5(PO4)2]. In order to target Cs2Na4-[(UO2)5O5(PO4)2] (2), the reverse was attempted. However,decreasing the flux to 5 mmol was unsuccessful and theCsNa3[(UO2)3O2(PO4)2] (1) phase still preferentially formed.Rb6[(UO2)5O5(PO4)2] (3) also proved difficult to isolate

over the recently published products Rb6[(UO2)7O4(PO4)4],36

Rb3[Al2O7(PO2)3][(UO2)3O2], and Rb7[Al2O7(PO2)3]-[(UO2)6O4(PO4)2.

6 All form under similar conditions of 0.5mmol of UF4, 0.2−0.5 mmol of AlPO4, 10/20 mmol of RbCl,and 775−875 °C reaction temperature. The lower temperatureof 775 °C favored the formation of Rb3[Al2O7(PO2)3]-[(UO2)3O2], while 10 mmol of RbCl flux at 875 °C primarilyproduced Rb6[(UO2)7O4(PO4)4], and 20 mmol producedRb7[Al2O7(PO2)3][(UO2)6O4(PO4)2 with small amounts ofthe title compound Rb6[(UO2)5O5(PO4)2] (3). Attempts to

optimize the synthesis for Rb6[(UO2)5O5(PO4)2] (3) bychanging the amount of flux (2.5−40 mmol), varying thetemperature, and using a non-Al containing phosphorussource, (NH4)2HPO4, were unsuccessful. Reactions using(NH4)2HPO4 as the phosphate source favored the synthesisof the Rb6[(UO2)7O4(PO4)4] phase regardless of the uraniumto phosphorus ratio used, and 3 could not be obtained withthis reagent.

Structure. CsNa3[(UO2)3O2(PO4)2] (1), Cs2Na4[(UO2)5-O5(PO4)2] (2), and Rb6[(UO2)5O5(PO4)2] (3), contain thewell-known phosphuranylite or extended phosphuranylitetopologies discussed in the Introduction and shown in Figure1a, c, respectively. CsNa3[(UO2)3O2(PO4)2] (1) adopts thephosphuranylite topology; however, it is the first reportedexample of the uuuuuuS geometric isomer of this sheet aniontopology. The geometric isomers of the phosphuranylite familyhave previously been identified by the pattern of the directionin which the phosphate tetrahedra (or As, V, or Se buildingunits) point, whether that is below or above the plane of thelayer. First, the pattern in which all phosphate tetrahedrabetween two uranyl chains are identified with “u” and “d” tosignify up and down, respectively. Second, the pattern of pairsof phosphate tetrahedra that edge share with the hexagonaluranyl bipyramids are identified where the letters “S” and “O”are used to describe whether the two tetrahedra point in thesame or opposite directions, respectively. All documentedisomers are represented in Figure 3. Isomers c and e−g areonly found in natural minerals, while b and d have only beenobserved in synthetic materials of A4[(UO2)3O2(PO4)2] andSr[(UO2)3(SeO3)2O2](H2O)4. CsNa3[(UO2)3O2(PO4)2] (1)contains a new isomer as described in Figure 3a, where allphosphate tetrahedra within the same layer point in the same

Figure 3. Known geometric isomers of the phosphuranylite topology where uranium polyhedra are yellow, tetrahedra or trigonal pyramids in theup orientation are pink, and those in the down orientation are purple. Examples of compounds exhibiting these isomers are (a)CsNa3[(UO2)3O2(PO4)2] (1), (b) A1.4K2.6[(UO2)3O2(PO4)2],

5 (c) vanmeersscheite,37 (d) Sr[(UO2)3(SeO3)2O2](H2O)4,8 (e) phosphuranylite,38

(f) phurcalite,39 and (g) bergenite.40

Figure 4. Structure of CsNa3[(UO2)3O2(PO4)2] (1) where the sheet topology and the overlaid cations are shown in (a) and (b), and the sheetstacking is shown in (c). The structure of Cs2Na4[(UO2)5O5(PO4)2] (2) is depicted in an analogous way in (d)−(f). Uranyl polyhedra are yellow,oxygen atoms are red, and phosphate tetrahedra are magenta.

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direction. Unlike other reported phosphuranylite-type layeredsystems, the layers within 1 are not all the same, the phosphatetetrahedra in subsequent layers point in opposite directions,and the phosphate tetrahedra do not align vertically (Figure 4).As a consequence, this creates two different interlayer distanceswhere there is a smaller distance between layers in which thephosphate tetrahedra point toward each other, and largerdistances where they point away from each other. This is likelya consequence of having two significantly different sizedcations, Cs and Na, and as expected solely Na cations liebetween layers where PO4 units point in toward each other,whereas both Cs and Na cations lie between layers in whichPO4 units point away from each other. In these latter layers,the larger Cs cations are located in the gap created by theoutward pointing PO4 units and the Na cations lie betweenuranyl polyhedra. This follows a similar trend as observed inthe A1.4K2.6[(UO2)3O2(PO4)2] (A = Rb, Cs) structures,5

where the sites between phosphate tetrahedra were occupiedby a mixture of K and Cs or Rb, and the site between uranylpolyhedra were solely K.Cs2Na4[(UO2)5O5(PO4)2] (2) and Rb6[(UO2)5O5(PO4)2]

(3), shown in Figures 4 and 5, both adopt new isomers of the

“extended phosphuranylite” sheet topology. These layereduranyl phosphates contain chains of edge-sharing uranylpentagonal bipyramids that are connected to a mirror imageuranyl pentagonal bipyramid chain through edge sharing withan additional uranyl polyhedron (referred to as the interior U),as in the U3O8 structure. These [(UO2)5O5] units areconnected via edge-sharing and corner-sharing phosphatetetrahedra, similar to the uranophane topology. The extendedphosphuranylite sheet topology can contain either α-U3O8 orβ-U3O8 units (Figure 6d) depending on the coordination ofthe interior uranyl polyhedra. The difference between the α-U3O8 or β-U3O8 topologies stems from the 7- or 6-coordinateinterior uranium polyhedra. In order to determine which bestdescribes the structures Cs2Na4[(UO2)5O5(PO4)2] (2) andRb6[(UO2)5O5(PO4)2] (3), the U−O bond distances andbond valence sums were investigated for the uranium inquestion, which is U1 and U3 for structures 2 and 3,respectively. Bond valence sums (BVS) were calculated using

Rij and b parameters by Burns specific to the coordinationgeometry of the uranium center.41 For 2, if one considers U1as 6-coordinate with equatorial U−O bonds between U1 andO3, O4, O5, and O6, the bond distances fall between 2.215and 2.296 Å and are usual for uranium coordination and yield aBVS of 5.911, in good agreement with the expected value of 6.If one includes the U1−O14 bond with length 2.815 Å, whichis on the long end of the range of reported values for uranylpentagonal bipyramid U−O distances (1.7−2.8 Å),42 the BVScomes out to 5.835 as a result of the different Rij and b valuesreported for 6- and 7-coordinate uranium. Similarly for 3,considering U3 as a square bipyramid yields a BVS of 5.941,but when the 2.998 Å U3−O8 bonding interaction is included,the sum is 5.800. The bond valence sums suggest thecoordination environment is most accurately described as 6-coordinate, and we have represented the structures as such, butit is important to note that these long U−O distances can stillbe considered as interaction especially considering that thesum of the crystallographic van der Waals radii is 3.57 Å.43

The topologies of Cs2Na4[(UO2)5O5(PO4)2] (2) andRb6[(UO2)5O5(PO4)2] (3) only differ in the orientation ofthe phosphate tetrahedra. In Cs2Na4[(UO2)5O5(PO4)2] (2),all the phosphate tetrahedra within the same sheet point in thesame direction, but adjacent layers have phosphate tetrahedrapointing in the opposite direction. When the layers are stacked,the phosphate tetrahedra are staggered. This geometric isomerof the extended phosphuranylite sheet topology can bedescribed in a similar method to the isomers of thephosphuranylite topology. By first looking at a section ofphosphate tetrahedra between [(UO2)5O5] units and labeling“u” and “d” as appropriate, and then looking at the pairs ofphosphate tetrahedra that edge share with U2O14 dimers, theisomer obtained is “uuuuuuS” as seen in the related structure,CsNa3[(UO2)3O2(PO4)2] (1). The positions of the alkalimetals are similar to those observed in CsNa3[(UO2)3O2-(PO4)2] (1), where the smaller sodium cations lie betweenlayers in which phosphate tetrahedra in adjacent sheets pointtoward each other, while the mixed Cs/Na layer occurs whenadjacent sheets have phosphate tetrahedra pointing away fromeach other. While the geometric isomers could be labeled todescribe the presence of the interior square uranyl bipyramidor pentagonal bipyramid, we have decided to only use theisomer label to describe the orientation of the phosphatetetrahedra. The isomer observed in 3 can be labeled as“udududS” which is analogous to the phosphuranylite basedmineral vanmeersscheite.37

There are nine additional compounds that belong to thisfamily: Cs1.7K4.3[(UO2)5O5(PO4)2], Rb1.6K4.4[(UO2)5O5-(PO4)2],

5 K6[(UO2)5O5(PO4)2],9 M6[(UO2)5O5(VO4)2] (M

= K, Na),10 K6[UO2)5O5(AsO4)2],4 β-Rb6[(UO2)5O5(VO4)2],

α-Rb6[(UO2)5O5(VO4)2], and K4[(UO2)5(TeO3)2O5].12 The

Figure 5. Structure of Rb6[(UO2)5O5(PO4)2] (3) with views in the(a) cb plane and the (b) ab plane. Uranyl polyhedra are yellow,oxygen atoms are red, and phosphate tetrahedra are magenta.

Figure 6. Known geometric isomers of the extended phosphuranylite topology where uranium polyhedra are yellow, tetrahedra or trigonalpyramids in the up orientation are pink, and those in the down orientation are purple. Examples of compounds exhibiting these isomers are (a)Cs2Na4[(UO2)5O5(PO4)2] (2), (b) Cs1.7K4.3[(UO2)5O5(PO4)2],

5 (c) Rb6[(UO2)5O5(PO4)2] (3), (d) α-Rb6[(UO2)5O5(VO4)2]44 and

K4[(UO2)5(TeO3)2O5].12

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latter two, α-Rb6[(UO2)5O5(VO4)2]44 and K4[(UO2)5-

(TeO3)2O5],12 both adopt the udududO isomer, while the

rest of the named compositions contain the uuuuuuO isomer.The trigonal pyramidal (TeO3)2− units in K4[(UO2)5(TeO3)2-O5] can be described in similar manners as (TO4)3− (T = P,As, V) tetrahedra due to the lone pair on Te that causes apyramidal rather than a planar geometry. α-Rb6[(UO2)5O5-(VO4)2] and K4[(UO2)5(TeO3)2O5] are also set apart by thedifference in the interior U coordination. In the vanadate, theinterior uranium is best considered as 7-coordinate, while inthe tellurite it is a 6-coordinate square bipyramid. The orderingof the middle U pentagonal bipyramids in the udududOisomer is analogous to that in udududS as described forRb6[(UO2)5O5(PO4)2] (3). It is natural to wonder if theremaining 3 phosphuranylite isomers (Figure 3e−g) can besynthesized in extended phosphuranylite structures, given thatthere are already four common isomers between the twofamilies.

■ CONCLUSIONThree new crystal structures belonging to the phosphuranyliteand extended phosphuranylite families have been synthesizedas single crystals and structurally characterized. Each adopts anew geometrical isomer of the fairly well-known topologies.The synthetic methods used to obtain the phosphuranylite andextended phosphuranylite materials were discussed, and thealkali molten flux growth method has produced the majority ofthe reported materials including phosphates, vanadates, andtellurites, while hydrothermal methods have been used forselenites and arsenates. Future studies should continue toexplore this diverse family of materials and aim to incorporatepolyhedral building blocks containing Cr, As, Al, and Se via themolten flux method.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.cgd.8b01643.

Bond valence sums and selected bonded distances ofcompounds 1, 2, and 3 (PDF)

Accession CodesCCDC 1874954−1874956 contain the supplementary crys-tallographic data for this paper. These data can be obtainedfree of charge via www.ccdc.cam.ac.uk/data_request/cif, or byemailing [email protected], or by contacting TheCambridge Crystallographic Data Centre, 12 Union Road,Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected]. Phone: (803) 777-6916.Fax: (803) 777-8508.ORCIDChristian A. Juillerat: 0000-0002-8270-1964Hans-Conrad zur Loye: 0000-0001-7351-9098FundingResearch was conducted by the Center for Hierarchical WasteForm Materials (CHWM), an Energy Frontier ResearchCenter (EFRC). Research was supported by the U.S.Department of Energy, Office of Basic Energy Sciences,Division of Materials Sciences and Engineering, under Award

DE-SC0016574. C.A.J. is additionally supported by an NSFIGERT Graduate Fellowship under grant number 1250052.NotesThe authors declare no competing financial interest.

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