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The preparation and structural characterization of [Li.sub.4][[(U[O.sub.2]).sub.10][O.sub.10]([Mo.sub.2][O.sub.8])] obtained from eight-membered cycles with uranyl ions linked through two- and three-center cation-cation interactions is described. The network of the structure consists of seven-polyhedra-wide uranium oxide tapes interlinked by dimers of edge-sharing [4 1]-distorted Mo[O.sub.5] polyhedra.

The synthesis and structure of the mixed-metal lead rare-earth oxyhalides of [Pb.sub.6]La[O.sub.7]X (X = Cl, Br) is described. These compounds are the first for which the presence of mixed O[Pb.sub.n][Ln.sub.4-n] tetrahedral is recognized.

Research has been conducted on uranyl molybdate with a uranyl molybdate sheet. The synthesis of this compound has been carried out and the results indicate that this sheet is composed of triuranyl clusters connected by MoO (sub)4 tetrahedra.

Dark-green platy crystals of the compound P[b.sub.31][O.sub.22][Br.sub.10][C.sub.l8] is obtained by rapid quenching of a lead oxide halide melt. The structure of P[b.sub.31][O.sub.22][Br.sub.10][C.sub.l8] can be viewed as incorporation of [Pb[X.sub.6]][.sup.4-] halide units (X=Br, Cl) into the defect PbO matrix.

Dark-red crystals of the new compound Li4[(UO2)10O10(Mo2O8)] (1) have been obtained by high-temperature solid-state reactions. The structure of 1 (monoclinic, P21/c, a = 7.9426(4) Å, b = 19.9895-(9) Å, c= 10.0796(5) Å, β = 90.575(2)°, V= 1600.24(13) Å3, Z = 4) consists of a framework of U and Mo polyhedra with Li+ cations in the channels. The framework contains seven-polyhedra-wide uranium oxide tapes interlinked by dimers of edge-sharing [4 + 1]-distorted MoO5 polyhedra. The U—O tapes are parallel to the a axis, and their planes are oriented parallel to (021) and (021) so that they are cross-linked within the framework. The core of the tapes consists of unprecedented one-dimensional arrays of cation-cation-bonded uranyl ions. The arrays are constructed from eight-membered cycles with uranyl ions linked through two- and three-center cation—cation interactions. [ABSTRACT FROM AUTHOR]

Yellowish elongated crystals of the two new compounds Pb6LaO7-Br (1) and Pb6LaO7Cl (2) have been obtained by the method of solid-state reactions. Both structures can be described in the terms of oxo-centered tetrahedra. The structures of 1 and 2 consist of [O7Pb6La]+ chains that are built from oxocentered OA4 (A = Pb, La) tetrahedra. The halogen ions connect the chains through weak Pb-X bonds. An arrangement of eight OA4 tetrahedra that all share the same central La atom forms a [O8Pb10La3]13+ cluster. The clusters are linked into chains, and additional OPb4 tetrahedra are attached to the chains. Incorporation of Cl atoms instead of Br atoms into the structure causes a lowering of the symmetry from Cmcm to C2/m. [ABSTRACT FROM AUTHOR]

Dark-green platy crystals of the new compound Pb31O22Br10Cl8 (1) have been obtained by rapid quenching of a lead oxide halide melt. The structure of 1 (triclinic, P..., a = 12.1192(7) Å, b = 16.2489(10) Å, c = 18.3007(11) Å, α = 93.104(2)°, β = 95.809(2)°, γ = 111.252(1)°, V= 3325.4(3) ų, Z= 2) can be viewed as incorporation of [PbX6]4- halide units (X = Br, Cl) into the defect PbO matrix. The latter represents a two-dimensional [022- Pb30]16+ cationic layer of OPb4 tetrahedra that can be derived from the [OPb] tetrahedral layer observed in tetragonal PbO. The layer consists of 22 symmetrically inequivalent OPb4 tetrahedra and represents the topologically most complicated arrangement of tetrahedra known to date. [ABSTRACT FROM AUTHOR]

Examines the importance of uranyl minerals and inorganic compounds for long-term disposal of spent nuclear fuel and radioactive waste remediation. Flexibility of U-O-Mo linkages; Illustration of uranyl molybdatesstructural complexity and variability; Polymerization of polyhedra of high bond valence.

Novel Pb7O6Br2 (1) lead oxybromide was prepared from Pb oxybromide melt by the "rapid quenching" route. Bonding scheme, thermal expansion, and structural properties were studied. The structural features of this unexpectedly complex phase are described on the basis of lone electron pair stereochemical activity and Pb-Br versus Pb-O bonding scheme. The structure of 1 contains a number of cavities, which can be assigned to the self-containments of the lone electron pairs on Pb2+ cations. "Empty" Pb4 chains are observed in between of the folding sides of the adjacent strongly corrugated oxocentered [Pb7O6]2+ layers. Highly isotropic thermal expansion of 1 appeared to be unexpected. The possible explanations of such a behavior in 1 are given. The structure of 1 is an interesting example of tetrahedral framework with mixed chemical bonding and is the densest known among Pb oxyhalides with the density of 18.4 tetrahedra/1000 ų. Current study shows that oxocentered layers derivatives from α-PbO can be very flexible and form rather dense three-dimensional structural topologies. The properties and structure are compared to other phases crystallizing in the anhydrous PbO-PbX2 (X = F, Cl, Br, I) systems, illustrate the complexity of lead oxyhalides, and reveal new and general pathways for the targeted synthesis of new phases with the Pb-O units of desired dimensionality. The indirect gap value of ~2.04 eV obtained from generalized gradient approximation calculations demonstrates potentially good photocatalytic properties of 1. [ABSTRACT FROM AUTHOR]

The structure of a novel mixed-valent chromiumuranyl compound, (C3NH10)10[(UO2)13(Cr125+O42)(Cr6+O4)6(H2O)6]-(H2O)6 (1), obtained by the combination of a hydro-thermal method and evaporation from aqueous solutions with isopropylammonium, contains uranyl chromate hemispheres with lateral dimensions of 18.9 × 18.5 Å2 and a height of about 8 Å. The hemispheres are centered by a UO8 hexagonal bipyramid surrounded by six dimers of Cr5+O5 square pyramids, UO7 pentagonal bipyramids, and Cr6+O4 tetrahedra. The hemispheres are linked into two-dimensional layers so that two adjacent hemispheres are oriented in opposite directions relative to the plane of the layer. From a topological point of view, the hemispheres have the formula U21Cr23 and can be considered as derivatives of nanospherical cluster U26Cr36 composed of three-, four-, and five-membered rings. [ABSTRACT FROM AUTHOR]

Chemically induced polytypic phase transitions have been observed during experimental investigations of crystallization in the mixed uranyl sulfate-selenate Mg[(UO 2 )( T O 4 ) 2 (H 2 O)](H 2 O) 4 ( T = S, Se) system. Three different structure types form in the system, depending upon the Se:S ratio in the initial aqueous solution. The phases with the Se/(Se + S) ratios (in mol %) in the ranges 0-9, 16-47, and 58-100 crystallize in the space groups P 2 1 , Pmn 2 1 , and P 2 1 / c , respectively. The structures of the phases are based upon the same type of uranyl-based sulfate/selenate chains that, through hydrogen bonds, are linked into pseudolayers of the same topological type. The layers are linked into three-dimensional structures via interlayer Mg-centered octahedra. The three structure types contain the same layers but with different stacking sequences that can be conveniently described as belonging to the 1 M , 2 O , and 2 M polytypic modifications. The Se-for-S substitution demonstrates a strong selectivity with preferential incorporation of Se into less tightly bonded T 1 site. The larger ionic radius of Se 6+ relative to S 6+ induces rotation of ( T 1O 4 ) tetrahedra in the adjacent layers and reconstruction of the structure types. From the information-theoretic viewpoint, the intermediate Pmn 2 1 structure type is more complex than the monoclinic end-member structure types.