Anion Topology Research Articles

Synthesis and characterization of uranyl chromate sheet compounds containing edge-sharing dimers of uranyl pentagonal bipyramids

Eight uranyl chromates have been crystallized from aqueous solution and characterized: Mg(H2O)6[(UO2)2(CrO4)2(OH)2](H2O)3 (1), (NH4)2[(UO2)2(CrO4)2(OH)2](H2O)3, Rb2[(UO2)2(CrO4)2(OH)2](H2O)3 (3), Cs[(UO2)(CrO4)(OH)]H2O (4), Rb[(UO2)(CrO4)(OH)]H2O (5) Co(H2O)4(Co(H2O)6)2[(UO2)4(CrO4)6(OH)2](H2O)4 (6), Li2[(UO2)2(CrO4)3](H2O)7 (7), and Zn(H2O)6[(UO2)2(CrO4)3](H2O)3 (8). The structural units of 1 through 8 each consist of a sheet of uranyl pentagonal bipyramids and (Cr(VI)O4)2− tetrahedra. In each case two uranyl pentagonal bipyramids share an equatorial edge, giving a dimer that is linked into the sheet through vertex sharing with (Cr(VI)O4)2− tetrahedra. The sheets are based upon three distinct sheet anion topologies, and the sheets based on a given anion topology can differ in the orientations of the non-bridging O atoms of (CrO4)2− tetrahedra. The interlayers of these compounds contain either monovalent or divalent cations, as well as H2O groups that are either bonded to the interlayer cation or are held in place by H bonding only. We explore the relationships between sheet topologies and interlayer configuration in these compounds.

THE CRYSTAL STRUCTURE OF NATURAL ZIPPEITE, K1.85H+0.15[(UO2)4O2(SO4)2(OH)2](H2O)4, FROM JACHYMOV, CZECH REPUBLIC

The crystal structure of natural zippeite,K1.85H+0.15[(UO2)4O2(SO4)2(OH)2](H2O)4, from Jachymov, Czech Republic, has been determined by single-crystal X-ray diffraction. Zippeite is monoclinic, space group C2/m, with a 8.7802(6), b 13.9903(12), c 8.8630(6) A, beta 104.524(7), V 1053.92(12) A3. The structure consists of structural sheets of the zippeite uranyl anion topology and an interlayer, in which split K atoms and disordered O atoms (of the H2O groups) are located. The structure unit [(UO2)4O2(SO4)2(OH)2]2– is novel for both natural and synthetic compounds, but generally consistent with the known zippeite-type structures.

Linear Alkyl Diamine-Uranium-Phosphate Systems: U(VI) to U(IV) Reduction with Ethylenediamine

Mild-hydrothermal reactions in acidic medium using 1,3-diaminopropane, 1,4-diaminobutane, and 1,5-diaminopentane as structure directing agents led to three-dimensional (3D) uranyl phosphates (CH₂)₃(NH₃)₂{[(UO₂)(H₂O)][(UO₂)(PO₄)]₄} (C3U5P4), (CH₂)₄(NH₃)₂{[(UO₂)(H₂O)][(UO₂)(PO₄)]₄} (C4U5P4) and (CH₂)5(NH₃)₂{[(UO₂)(H₂O)][(UO₂)(PO₄)]₄} (C5U5P4). The structures of (C4U5P4) and (C5U5P4) were solved in the space group Cmc2₁ using single-crystal X-ray diffraction data. The compounds are isostructural to the corresponding uranyl vanadates and contain the same 3D inorganic framework built from uranyl-phosphate layers of uranophane-type anion topology pillared by [UO₆(H₂O)] pentagonal bipyramids. In neutral or basic medium the alkyl diamines decompose to give ammonium uranyl phosphate trihydrate. In the same conditions by using ethylenediamine, unexpected reduction of uranium(VI) to uranium(IV) occurs leading to the formation of (CH₂)₂(NH₃)₂[U(PO₄)₂] (C2UP2) single crystals. C2UP2 undergoes a reversible phase transition from triclinic to monoclinic symmetry at about 230 °C. The structure of the two forms results from the stacking of inorganic layers (∞)¹[U(PO₄)₂]²⁻, and organic layers containing ethylene diammonium ions, the two layers being linked by hydrogen bonds. Single crystals of (CH₂)₂(NH₃)₂[PO₃OH] (C2HP) are formed by evaporation of the solution after filtering of C2UP2 single crystals. The structure of C2HP contains infinite (∞)¹[PO₃OH]²⁻ chains connected by (CH₂)₂(NH₃)₂²⁺ ions through hydrogen bonds.

Na7UIVO2(UVO)2(UV/VIO2)2Si4O16]: A Mixed‐Valence Uranium Silicate

For U′r eyes only: A mixed-valence uranium silicate containing three different valence states of uranium has been synthesized under high-temperature, high-pressure hydrothermal conditions. The structure contains uranium silicate sheets with uranophane anion topology that are connected by UO6 octahedra to form a 2D layered structure with Na+ ions within and between the layers.

A new series of pillared uranyl-vanadates based on uranophane-type sheets in the uranium-vanadium-linear alkyl diamine systems

A new family of three-dimensional (3D) uranyl vanadates (C 3N 2H 12){[(UO 2)(H 2O)][(UO 2)(VO 4)] 4}·1H 2O (C3UV), (C 4N 2H 14){[(UO 2)(H 2O)][(UO 2)(VO 4)] 4}·2H 2O (C4UV), (C 5N 2H 16) {[(UO 2)(H 2O)][(UO 2)(VO 4)] 4} (C5UV), (C 6N 2H 20) {[(UO 2)(H 2O)][(UO 2)(VO 4)] 4} (C6UV) and (C 7N 2H 22) {[(UO 2)(H 2O)][(UO 2)(VO 4)] 4} (C7UV) was prepared from mild-hydrothermal reactions using 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane and 1,7-diaminoheptane as structure directing agents. The five compounds are orthorhombic, space group Cmc2 1, with a≈15.6, b≈14.1, c≈13.6 Å. The structures were solved using single-crystal X-ray diffraction data. The compounds contain the same three-dimensional inorganic framework built from uranyl-vanadate layers of uranophane-type anion topology pillared by [UO 6(H 2O)] pentagonal bipyramids. The doubly protonated diamines reside in the cavities created by the inorganic framework and are linked to the inorganic framework through hydrogen bonds involving one nitrogen atom. The structure is compared with that of uranyl-phosphates and uranyl-arsenates containing alkaline metals. The use of alkaline metals for the synthesis of uranyl-vanadates leads to carnotite-type compounds.

Hydrothermal Synthesis, Structure and Thermal Stability of Diamine Templated Layered Uranyl‐Vanadates.

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Hydrothermal synthesis, structure and thermal stability of diamine templated layered uranyl-vanadates

Six new layered uranyl vanadates (NH 4) 2[(UO 2) 2V 2O 8] ( 1), (H 2EN)[(UO 2) 2V 2O 8] ( 2), (H 2DAP)[(UO 2) 2V 2O 8] ( 3), (H 2PIP)[(UO 2) 2(VO 4) 2].0,8H 2O ( 4), (H 2DMPIP)[(UO 2) 2V 2O 8] ( 5), (H 2DABCO)[(UO 2) 2(VO 4) 2] ( 6) were prepared from mild-hydrothermal reactions using 1,2-ethylenediamine (EN); 1,3-diaminopropane (DAP); piperazine (PIP); 1-methylpiperazine (MPIP); 1,4-diazabicyclo[2,2,2]octane (DABCO). The structures of 1, 4, 5 and 6 were solved using single-crystal X-ray diffraction data while the structural models of 2 and 3 were established from powder X-ray diffraction data. In compounds 1, 2, 3 and 5, the uranyl-vanadate layers are built from dimers of edge-shared UO 7 pentagonal bipyramids and dimers of edge-shared VO 5 square pyramids further connected through edge-sharing. In 1 and 3, the layers are identical to that occurring in the carnotite group of uranyl-vanadates. In 2 and 5, the V 2O 8 dimers differ in orientation leading to a new type of layer. The layers of compound 4 and 6 are built from chains of edge-shared UO 7 pentagonal bipyramids connected by VO 4 tetrahedra and are of uranophane-type anion topology. For the six compounds, the ammonium or organoammonium cation resides in the space between the inorganic layers. Crystallographic data: 1 monoclinic, space group P2 1/ c with a=6.894(2), b=8.384(3), c=10.473(4) Å and β=106.066(5)°, 2 monoclinic, space group P2 1/ a with a=13.9816(6), b=8.6165(3), c=10.4237(3) Å and γ=93.125(3)°, 3 orthorhombic, space group Pmcn with a=14.7363(8), b=8.6379(4) and c=10.4385(4) Å, 4 monoclinic, space group C2/ m with a=15.619(2), b=7.1802(8), c=6.9157(8) Å and β=101.500(2)°, 5 monoclinic, space group P2 1/ b with a=9.315(2), b=8.617(2), c=10.5246(2) Å and γ=114.776(2)°, 6 monoclinic, space group C2/ m with a=17.440(2), b=7.1904(9), c=6.8990(8) Å and β=98.196(2)°.

The A1‐xUNbO6‐x/2 Compounds (x = 0, A: Li, Na, K, Cs and x = 0.5, A: Rb, Cs): From Layered to Tunneled Structure.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Amide‐Based Ligands for Anion Coordination

Anion recognition is an active area of research in supramolecular chemistry. The rapidly increasing amount of structural data now allows anion coordination chemistry to be formalized in terms of coordination numbers and geometries based on hydrogen-bonding interactions between the host (ligand) and the guest (anion). This Minireview targets just one class of anion receptors, namely, amide-based ligands. The structural data for a series of five anion shapes are compiled according to coordination number, and distinct commonalities are observed within a given anion topology. The results also indicate a number of similarities between the coordination of anions and transition-metal ions.

Ba(NpO2)(PO4)(H2O), its relationship to the uranophane group, and implications for Np incorporation in uranyl minerals

Single crystals of Ba(NpO 2 )(PO 4 )(H 2 O) were obtained using hydrothermal synthesis techniques. The structure was determined using single-crystal X-ray diffraction data collected using MoKα radiation and an APEX II CCD detector and was refined on the basis of F 2 for all unique data to R 1 = 2.41%. Ba(NpO 2 )(PO 4 )(H 2 O) crystallized in monoclinic space group P2 1 /n with a = 6.905(3), b = 7.108(3), c = 13.321(6) Å, β = 105.02°, and V = 631.4 Å 3 . The structure contains chains of edge-sharing neptunyl pentagonal bipyramids that link through phosphate tetrahedra to form infinite sheets. This sheet-type is identical to the anion topology of the uranophane group, in particular to that of oursinite, Co[(UO 2 )(SiO 3 OH)] 2 (H 2 O) 6 . Similarities between Ba(NpO 2 )(PO 4 )(H 2 O) and the uranophane group of minerals suggests a charge-balancing mechanism for incorporation of Np 5+ into uranyl minerals.

The A 1−xUNbO 6−x/2 compounds ( x=0, A=Li, Na, K, Cs and x=0.5, A=Rb, Cs): from layered to tunneled structure

Attempts to prepare alkaline metal uranyl niobates of composition A 1− x UNbO 6− x /2 by high-temperature solid-state reactions of A 2CO 3, U 3O 8 and Nb 2O 5 led to pure compounds for x=0 and A=Li ( 1), Na ( 2), K ( 3), Cs ( 4) and for x=0.5 and A=Rb ( 5), Cs ( 6). Single crystals were grown for 1, 3, 4, 5, 6 and for the mixed Na 0.92Cs 0.08UNbO 6 ( 7) compound. Crystallographic data: 1, monoclinic, P2 1/ c, a=10.3091(11), b=6.4414(10), c=7.5602(5) Å, β=100.65(1), Z=4, R 1=0.054 (w R 2=0.107); 3, 5 and 7 orthorhombic, Pnma, Z=8, with a=10.307(2), 10.272(4) and 10.432(3) Å, b=7.588(1), 7.628(3) and 7.681(2) Å, c=13.403(2), 13.451(5) and 13.853(4) Å, R 1=0.023, 0.046 and 0.036 (w R 2=0.058, 0.0106 and 0.088) for 3, 5 and 7, respectively; 6, orthorhombic, Cmcm, Z=8, and a=13.952(3), b=10.607(2) Å, c=7.748(2) Å, R 1=0.044 (w R 2=0.117). The crystal structure of 1 is characterized by ∞ 2 [ UNbO 6 ] - layers of uranophane sheet anion topology parallel to the (100) plane. These layers are formed by the association by edge-sharing of ∞ 1 [ UO 5 ] 4 - chains of edge-shared UO 7 pentagonal bipyramids and ∞ 1 [ NbO 4 ] 3 - chains of corner-shared NbO 5 square pyramids alternating along the [010] direction. The Li + ions are located between two consecutive layers and hold them together; the Li + ions and two layers constitute a neutral “sandwich” {(UNbO 6) −–(Li) 2 2+–(UNbO 6) −}. In this unusual structure, the neutral sandwiches are stacked one above another with no formal chemical bonds between the neutral sandwiches. The homeotypic compounds 3, 5, 6, 7 have open-framework structures built from the association by edge-sharing in two directions of parallel ∞ 1 [ UO 5 ] 4 - chains of edge-shared UO 7 pentagonal bipyramids and ∞ 1 [ Nb 2 O 8 ] 6 - ribbons of two edge-shared NbO 6 octahedra further linked by corners. In 3, 5 and 7, the mono-dimensional large tunnels created in the [001] direction by this arrangement can be considered as the association by rectangular faces of two columns of triangular face-shared trigonal prisms of uranyl oxygens. In 3 and 7, all the trigonal prisms are occupied by the alkaline metal, in 5, they are half-occupied. In 6, the polyhedral arrangement is more symmetric and the tunnels created in the [010] direction are built of face-sharing cubes of uranyl oxygens totally occupied by the Cs atoms. This last compound well illustrates the structure-directing effect of the conterion.

A novel arrangement of silicate tetrahedra in the uranyl silicate sheet of oursinite, (Co0.8Mg0.2)[(UO2)(SiO3OH)]2(H2O)6

Oursinite is a rare Co-bearing uranyl silicate of the uranophane group. The structure of oursinite, (Co 0.8 Mg 0.2 )[(UO 2 )(SiO 3 OH)] 2 (H 2 O) 6 , is orthorhombic, space group Cmca, a = 7.0494(5), b = 17.550(1), c = 12.734(1) Å, V = 1575.4(2) Å 3 , Z = 4. It was solved by direct methods and refined on the basis of F 2 for all unique reflections using least-squares techniques to an agreement index (R1) of 2.66%. The structure contains an approximately linear (UO 2 ) 2+ uranyl ion that is present as a uranyl pentagonal bipyramid, one symmetrically distinct SiO 3 OH acid silicate group, and one M 2+ (OH,H 2 O) 6 octahedron (M is dominated by Co). The uranyl pentagonal bipyramids and silicate tetrahedra are linked by the sharing of edges and vertices, giving a sheet based upon the uranophane anion topology. Adjacent sheets are linked by M 2+ (OH,H 2 O) 6 octahedra located in the interlayer, and by hydrogen bonds. Each M 2+ (OH,H 2 O) 6 octahedron contains two OH groups that are apical ligands of silicate tetrahedra in adjacent uranyl silicate sheets. Although several uranophane-group minerals contain sheets that are based upon the uranophane anion topology, the oursinite sheet involves novel orientations of silicate tetrahedra.

Alfred Werner Revisited: The Coordination Chemistry of Anions

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Alfred Werner Revisited: The Coordination Chemistry of Anions

A series of macrocyclic receptors were designed to probe the influence of four factors, hydrogen bonding, charge, dimensionality, and topology, on anion binding. Monocyclic and bicyclic polyammonium and polyamide receptors were synthesized from either 2,2'-diaminodiethylamine derivatives (dien) or 2,2',2''-aminoethylamine (tren) building blocks, plus aromatic or heterocyclic spacers. Supramolecular complexes of these hosts with three simple anion topologies were probed: spherical (halides), trigonal planar (nitrate), and tetrahedral (sulfate). Results indicate a number of corollaries with transition-metal coordination chemistry in terms of binding concepts such as the chelate effect and dual valencies, as well as geometries for anion complexes that are strikingly similar to those observed in transition-metal coordination chemistry.

Anionic dicyanamide frameworks as possible components of multifunctional materials

The dicyanamide anion, [N(CN) 2 ] - (hereafter abbreviated dca), has been shown to be a versatile building block for the assembly of coordination polymers which are components of molecular superconductors, magnets and conducting/magnetic hybrid materials. Significantly different anion topologies have been observed. We report the first study of the use of alkylammonium cations as templates for [M(dca) 3 ] - (M = Mn and Ni) anionic networks, including a novel three-dimensional anionic structure in the (Bu 3 NH) 4 [Mn 3 (dca) 10 ] salt. The molecular superconductors with the highest superconducting transition temperature, κ-(ET) 2 [Cu(dca)X] [X = Cl, Br; ET, commonly abbreviated as BEDT-TTF, is bis(ethylenedithio)tetrathiafulvalene] contain chains of diamagnetic Cu(I) ions linked by dca anions. The successful introduction of magnetically active transition metals into the charge-compensating dca layer will also be discussed. Specifically, we have synthesized and characterized the (ET) 2 [Mn(dca) 3 ] salt that contain a novel triangular, spin frustrated, anion lattice separated by conducting layers of ET radical cations.

Two new lithium uranyl tungstates Li 2(UO 2)(WO 4) 2 and Li 2(UO 2) 4(WO 4) 4O with framework based on the uranophane sheet anion topology

Two new lithium uranyl tangstates Li 2(UO 2)(WO 4) 2 and Li 2(UO 2) 4(WO 4) 4O have been prepared by high-temperature solid state reactions of Li 2CO 3, U 3O 8 and WO 3. For each compound, the crystal structure was determined by single crystal X-ray diffraction data, using a Brucker diffractometer, equipped with a SMART CCD detector and Mo Kα radiation. The crystal structures were solved at room temperature by direct methods followed by Fourier difference techniques, and refined by a least square procedure on the basis of F 2 for all independent reflections, to R 1=0.035 for 65 refined parameters and 807 reflections with I⩾2 σ( I) for Li 2(UO 2)(WO 4) 2 and to R 1=0.051 for 153 refined parameters and 1766 reflections with I⩾2 σ( I) for Li 2(UO 2) 4(WO 4) 4O. The crystal structure of Li 2(UO 2)(WO 4) 2 is formed by perovskite sheets of WO 6 octahedra, one octahedron thickness, connected together by (UO 5) ∞ infinite chains, and creating tunnels parallel to the c-axis. The lithium atoms are localized in the tunnels. The structure can be deduced from that of U MO 5 ( M=Mo, V, Nb) compounds by the replacement of half U atoms by Li. The crystal structure of Li 2(UO 2) 4(WO 4) 4O consists of UO 7 pentagonal bipyramids, UO 6 tetragonal bipyramids and WO 6 distorted octahedra linked together to form a three-dimensional framework creating paralleled channels filled with lithium cations. The structure can also be described by the stacking of layers with the uranophane sheet anion topology similar to those obtained in U MO 5 ( M=Mo, V, Nb, Sb) compounds with an ordered population of pentagons by U and Li and of squares by U and W. The measured conductivities are comparable to those of the better Li + ion conductor solid electrolytes such as Lisicon or Li- β-alumina. Crystallographic data: Li 2(UO 2)(WO 4) 2, orthorhombic symmetry, space group Pbcn and unit cell parameters a=7.9372(15) Å, b=12.786(2) Å, c=7.4249(14) Å, ρ cal=6.87(2) g/cm 3, ρ mes=6.89(1) g/cm 3 and Z=4. Li 2(UO 2) 4(WO 4) 4O, monoclinic symmetry, space group C2/ c and unit cell parameters a=14.019(4) Å, b=6.3116(17) Å, c=22.296(6) Å, β=98.86(3)°, ρ cal=7.16(2) g/cm 3, ρ mes=7.25(3) g/cm 3 and Z=4.

Spriggite, Pb3[(UO2)6O8(OH)2] (H2O)3, a new mineral with β-U3O8–type sheets: Description and crystal structure

Spriggite, Pb 3 [(UO 2 ) 6 O 8 (OH) 2 ](H 2 O) 3 , is a new hydrated Pb uranyl oxyhydroxide found near Arkaroola, Northern Flinders Ranges, South Australia. The new mineral's name honors geologist and conservationist Reginald Claude Sprigg (1919-1994), founder of the Arkaroola Tourist Station. Together with beta-uranophane, soddyite, kasolite, Ce-rich francoisite-(Nd), metatorbernite, billietite, Ba-bearing boltwoodite, schoepite, metaschoepite, and weeksite, spriggite results from the supergene alteration of U-Nb-REE-bearing hydrothermal hematite breccia. Spriggite forms prismatic crystals up to about 150 μm in length and up to 40 μm across. It is transparent, bright orange in color with vitreous luster, biaxial, n m i n = 1.807; n m a x = 1.891 (Na D , 22.5 °C), non-fluorescent, brittle with an uneven fracture. It has a pale orange streak, Mohs' hardness ∼4, good cleavage along (100) and D c a l c = 7.64(6) g/ cm 3 . The empirical formula is (Pb 2 . 7 7 Ca 0 . 0 6 Ba 0 . 0 4 ) Σ 2 . 8 7 U 6 O 1 9 . 9 (OH) 2 .3H 2 O, and the simplified formula is Pb 3 [(UO 2 ) 6 O 8 (OH) 2 ](H 2 O) 3 . Spriggite is monoclinic, C2/c, a = 28.355(9), b = 11.990(4), c = 13.998(4) A, β = 104.248(5); V = 4613(3) A 3 , Z = 8. The strongest eight lines in the powder X-ray diffraction pattern are [d in A (I)(hkl)]: 6.92(60)(400), 6.02(30)(112;020), 3.46(80)(800), 3.10(100)(204;604;332;532), 2.74(30)(440), 2.01(30)(336), 1.918(60)(10.04;14.04;11.32;13.31), 1.738(30)(536;11.36). The structure has been solved from a crystal twinned on (001) and refined to Rl = 9.7%. The structure is based upon the [(UO 2 ) 6 O 8 (OH) 2 ] 6 + sheets of uranyl polyhedra of the β-U 3 O 8 anion topology with Pb 2 + cations and H 2 O groups in the interlayer. Billietite and spriggite contain only hexavalent U in the uranyl sheets, whereas the similar sheets in β-U 3 O 8 contain U 5 + and U 6 + , and those in ianthinite U 4 + and U 6 + . Spriggite has the highest Pb:U ratio among the known hydrated Pb uranyl oxyhydroxide minerals.

CRYSTAL CHEMISTRY OF URANYL MOLYBDATES. IX. A NOVEL URANYL MOLYBDATE SHEET IN THE STRUCTURE OF Tl2[(UO2)2O(MoO5)

Crystals of a new thallium uranyl molybdate, Tl 2 [(UO 2 ) 2 O(MoO 5 )], have been synthesized by high-temperature solid-state reactions. The structure [monoclinic, P 2 1 / n , a 8.2527(3), b 28.5081(12), c 9.1555(4) A, β 104.122(1)°, V 2088.91(15) A 3 , Z = 8] was solved by direct methods and refined to R 1 = 0.039 ( wR 2 = 0.081) on the basis of 8609 unique reflections. The structure is novel in that Mo 6+ cations occur in both tetragonal pyramidal and trigonal bipyramidal coordination environments, and it contains a uranyl molybdate sheet of composition [(UO 2 ) 2 O(MoO 5 )] with more uranium than molybdenum. The uranyl molybdate sheets are composed of Ur O 5 ( Ur : uranyl ion, UO 2 2+ ) pentagonal bipyramids and MoO 5 polyhedra that are linked by the sharing of edges. Within the sheet, eight Ur O 5 bipyramids share edges, resulting in a Z-shaped complex that is oriented approximately parallel to [010]. The MoO 5 polyhedra each contain one O atom that is not shared within the sheet; these are oriented both up and down relative to the plane of the sheet. The anion topology of the sheet consists of pentagons, squares and triangles. In the sheet, all pentagons are populated by U, four-fifths of the squares contain Mo, and the triangles are empty. The uranyl molybdate sheets are parallel to (101) and are linked via Tl + cations located in the interlayer.

A new uranyl carbonate sheet in the crystal structure of fontanite, Ca[(UO2)3(CO3)2O2](H2O)6

The structure of fontanite, Ca[(UO 2 ) 3 (CO 3 ) 2 O 2 ](H 2 O) 6 , is monoclinic, space group P2 1 /n, a = 6.968(3), b = 17.276(7), c = 15.377(6) Å, β = 90.064(6)°, V = 1851(1) Å 3 , Z = 4. The structure was solved by direct methods and refined on the basis of F 2 for all unique reflections using leastsquares techniques to an agreement index (R1) of 9.9%. The structure contains two symmetrically distinct uranyl pentagonal bipyramids, one uranyl hexagonal bipyramid, and two CO 3 triangles. The uranyl polyhedra form chains by sharing equatorial edges, and CO 3 groups occur on either side of the chains, where they share equatorial edges of the uranyl hexagonal bipyramids. The CO 3 groups share their third ligand with a uranyl pentagonal bipyramid of an adjacent chain, resulting in uranyl carbonate sheets of composition [(UO 2 ) 3 (CO 3 ) 2 O 2 ] 2- . The single symmetrically unique Ca 2+ cation is located between the sheets, and is coordinated by two O atoms of uranyl ions of adjacent sheets, and six H 2 O groups. The uranyl carbonate sheet in fontanite is novel, but is based upon the phosphuranylite anion topology that is the basis of uranyl phosphate, uranyl selenite, and uranyl sulfate sheets in a variety of minerals.

The Crystal Structure of Triuranyl Diphosphate Tetrahydrate

The hydrated neutral uranyl phosphate, (UO2)3(PO4)2(H2O)4, was synthesized by hydrothermal methods. Intensity data were collected using MoKα radiation and a CCD-based area detector. The crystal structure was solved by direct methods and refined by full-matrix least-squares techniques to agreement indices wR2=0.116 for all data, and R1=0.040, calculated for the 2764 unique observed reflections (∣Fo∣≥4σF). The compound is orthorhombic, space group Pnma, Z=4, a=7.063(1) Å, b=17.022(3) Å, c=13.172(3) Å, V=1583.5(5) Å3. The structure consists of sheets of phosphate tetrahedra and uranyl pentagonal bipyramids, with composition [(UO2)(PO4)]− and the uranophane sheet anion topology. The sheets are connected by a uranyl pentagonal bipyramid in the interlayer that shares corners with a phosphate tetrahedron on each of two adjacent sheets, resulting in an open framework with isolated H2O groups in the larger cavities of the structure.