Hydrocerussite-related minerals and materials: structural principles, chemical variations and infrared spectroscopy.

White lead or basic lead carbonate, 2PbCO3·Pb(OH)2, the synthetic analogue of hydrocerussite Pb3(OH)2(CO3)2, has been known since antiquity as the most frequently used white paint. A number of different minerals and synthetic materials compositionally and structurally related to hydrocerussite have been described within the last two decades. Herein, a review is given of general structural principles, chemical variations and IR spectra of the rapidly growing family of hydrocerussite-related minerals and synthetic materials. Only structures containing a hydroxo- and/or oxo-component, i.e. which are compositionally directly related with hydrocerussite and `white lead', are reviewed in detail. An essential structural feature of all the considered phases is the presence of electroneutral [PbCO3]0 cerussite-type layers or sheets. Various interleaved sheets can be incorporated between the cerussite-type sheets. Different sheets are stacked into two-dimensional blocks separated by the stereochemically active 6s2 lone electron pairs on Pb2+ cations. Minerals and synthetic materials described herein, together with a number of still hypothetical members, constitute a family of modular structures. Hydrocerussite, abellaite and grootfonteinite can be considered to constitute a merotype family of structures. The remaining hydrocerussite-related structures discussed are built on similar principles, but are more complex. Structural architectures of somersetite and slag phase from Lavrion, Attica, Greece, are unique for oxysalt mineral structures in general. Thus, the whole family of hydrocerussite-related phases can be denoted as a plesiotype family of modular structures. The crystal structures of hydrocerussite from Merehead quarry, Somerset, England, and of its synthetic analogue, both determined from single crystals, are reported here for the first time. The results of the infrared (IR) spectroscopy show that this method is useful for distinguishing several different minerals related to hydrocerussite and their synthetic analogues.

Uranyl Sulfate Nanotubules Templated by N-phenylglycine.

The synthesis, structure, and infrared spectroscopy properties of the new organically templated uranyl sulfate Na(phgH⁺)7[(UO2)6(SO4)10](H2O)3.5 (1), obtained at room temperature by evaporation from aqueous solution, are reported. Its structure contains unique uranyl sulfate [(UO2)6(SO4)10]8- nanotubules templated by protonated N-phenylglycine (C6H5NH2CH2COOH)⁺. Their internal diameter is 1.4 nm. Each of the nanotubules is built from uranyl sulfate rings sharing common SO4 tetrahedra. The template plays an important role in the formation of the complex structure of 1. The aromatic rings are stacked parallel to each other due to the effect of π-π interaction with their side chains extending into the gaps between the nanotubules.

The structure of denisovite, a fibrous nanocrystalline polytypic disordered 'very complex' silicate, studied by a synergistic multi-disciplinary approach employing methods of electron crystallography and X-ray powder diffraction.

Denisovite is a rare mineral occurring as aggregates of fibres typically 200-500 nm diameter. It was confirmed as a new mineral in 1984, but important facts about its chemical formula, lattice parameters, symmetry and structure have remained incompletely known since then. Recently obtained results from studies using microprobe analysis, X-ray powder diffraction (XRPD), electron crystallography, modelling and Rietveld refinement will be reported. The electron crystallography methods include transmission electron microscopy (TEM), selected-area electron diffraction (SAED), high-angle annular dark-field imaging (HAADF), high-resolution transmission electron microscopy (HRTEM), precession electron diffraction (PED) and electron diffraction tomography (EDT). A structural model of denisovite was developed from HAADF images and later completed on the basis of quasi-kinematic EDT data by ab initio structure solution using direct methods and least-squares refinement. The model was confirmed by Rietveld refinement. The lattice parameters are a = 31.024 (1), b = 19.554 (1) and c = 7.1441 (5) Å, ß = 95.99 (3)°, V = 4310.1 (5) Å3 and space group P12/a1. The structure consists of three topologically distinct dreier silicate chains, viz. two xonotlite-like dreier double chains, [Si6O17]10-, and a tubular loop-branched dreier triple chain, [Si12O30]12-. The silicate chains occur between three walls of edge-sharing (Ca,Na) octahedra. The chains of silicate tetrahedra and the octahedra walls extend parallel to the z axis and form a layer parallel to (100). Water molecules and K+ cations are located at the centre of the tubular silicate chain. The latter also occupy positions close to the centres of eight-membered rings in the silicate chains. The silicate chains are geometrically constrained by neighbouring octahedra walls and present an ambiguity with respect to their z position along these walls, with displacements between neighbouring layers being either Δz = c/4 or -c/4. Such behaviour is typical for polytypic sequences and leads to disorder along [100]. In fact, the diffraction pattern does not show any sharp reflections with l odd, but continuous diffuse streaks parallel to a* instead. Only reflections with l even are sharp. The diffuse scattering is caused by (100) nano-lamellae separated by stacking faults and twin boundaries. The structure can be described according to the order-disorder (OD) theory as a stacking of layers parallel to (100).

Lead Oxychloride Borates Obtained under Extreme Conditions.

[Pb10O4]Pb2(B2O5)Cl12 (1) and [Pb18O12]Pb(BO2OH)2Cl10 (2) were obtained via high-temperature high-pressure experiments. [O12Pb18](12+) and [O4Pb10](12+) oxocentered structural units of different dimensionality are excised from the ideal [OPb] layer in tetragonal α-PbO. 2 is formed with an excess of lead oxide component, and 1 is formed with an excess of borate and halide reagents. The structure of 2 can be visualized as the incorporation of {Pb(10)Cl4(BO2OH)2} clusters into alternating PbO and chloride layers, with the existence of square vacancies in both. However, the structure of 1 is described as the intrusion of [O4Pb10](12+) tetramers linked by disordered Pb(B2O5) groups into a halogen three-dimensional matrix. The structure of 2 contains 10 symmetrically independent Pb positions. The 6s(2) lone electron pair is stereochemically active on Pb(1)-Pb(9) atoms, whereas it is inert on Pb(10). All of the Pb coordinations in the structure of 2, in accordance with ECCv (volume eccentricity) parameters and the density of states (DOS), can be subdivided into three groups. The current study is the first attempt to analyze this unusual behavior in structurally complex oxyhalide material with the rare case of Pb(2+) cations, demonstrating both stereochemically active and inactive behavior of the lone pair via charge and first-principle calculations.

Highly distorted uranyl ion coordination and one/two-dimensional structural relationship in the Ba2[UO2(TO4)2] (T = P, As) system: an experimental and computational study.

Uranium compounds α-Ba2[UO2(PO4)2] (1), ß-Ba2[UO2(PO4)2] (2), and Ba2[UO2(AsO4)2] (3) were synthesized by H3BO3/B2O3 flux reactions, though boron is not incorporated into the structures. Phases 1 and 2 are topologically identical, but 1 is heavily distorted with respect to 2. An unusual UO7 pentagonal bipyramid occurs in 1, exhibiting a highly distorted equatorial configuration and significant bending of the uranyl group, due to edge-sharing with one neighboring PO4(3-) tetrahedron. Compound 2 contains more normal square bipyramids that share corners with four neighboring PO4(3-) tetrahedra, but the uranyl cation UO2(2+) is tilted relative to the equatorial plane. Experimental evidence as well as density functional theory (DFT) calculations suggest that 1 is more stable than 2. In theory, 1 and 2 can interconvert by forming/releasing the shared edge between the uranyl polyhedron and the phosphate tetrahedron. Similar fundamental building blocks in ß-Ba2[UO2(PO4)2] and Ba2[UO2(AsO4)2] indicate a possible evolution of uranyl-based structures from chain to layer type and formation of an accretional series.

Unusual structure, bonding and properties in a californium borate.

The participation of the valence orbitals of actinides in bonding has been debated for decades. Recent experimental and computational investigations demonstrated the involvement of 6p, 6d and/or 5f orbitals in bonding. However, structural and spectroscopic data, as well as theory, indicate a decrease in covalency across the actinide series, and the evidence points to highly ionic, lanthanide-like bonding for late actinides. Here we show that chemical differentiation between californium and lanthanides can be achieved by using ligands that are both highly polarizable and substantially rearrange on complexation. A ligand that suits both of these desired properties is polyborate. We demonstrate that the 5f, 6d and 7p orbitals are all involved in bonding in a Cf(III) borate, and that large crystal-field effects are present. Synthetic, structural and spectroscopic data are complemented by quantum mechanical calculations to support these observations.

Novel fundamental building blocks and site dependent isomorphism in the first actinide borophosphates.

Three novel uranyl borophosphates, Ag2(NH4)3[(UO2)2{B3O(PO4)4(PO4H)2}]H2O (AgNBPU-1), Ag(2-x)(NH4)3[(UO2)2{B2P5O(20-x)(OH)x}] (x = 1.26) (AgNBPU-2), and Ag(2-x)(NH4)3[(UO2)2{B2P(5-y)AsyO(20-x)(OH)x}] (x = 1.43, y = 2.24) (AgNBPU-3), have been prepared by the H3BO3-NH4H2PO4/NH4H2AsO4 flux method. The structure of AgNBPU-1 has an unprecedented fundamental building block (FBB), composed of three BO4 and six PO4 tetrahedra which can be written as 9□:[Φ] □<3□>□|□<3□>□|□<3□>□|. Two Ag atoms are linearly coordinated; the coordination of a third one is T-shaped. AgNBPU-2 and AgNBPU-3 are isostructural and possess a FBB of two BO4 and five TO4 (T = P, As) tetrahedra (7□:□<4□>□|□). AgNBPU-3 is a solid solution with some PO4 tetrahedra of the AgNBPU-2 end-member being substituted by AsO4. Only two out of the three independent P positions are partially occupied by As, resulting in site dependent isomorphism. The three compounds represent the first actinide borophosphates.

Synthesis of divalent europium borate via in situ reductive techniques.

A new divalent europium borate, Eu[B8O11(OH)4], was synthesized by two different in situ reductive methodologies starting with a trivalent europium starting material in a molten boric acid flux. The two in situ reductive techniques employed were the use of HI as a source of H2 gas and the use of a Zn amalgam as a reductive, reactive surface. While both of these are known reductive techniques, the title compound was synthesized in both air and water which demonstrates that strict anaerobic conditions need not be employed in conjunction with these reductive methodologies. Herein, we report on the structure, spectroscopy, and synthetic methodologies relevant to Eu[B8O11(OH)4]. We also report on a europium doping study of the isostructural compound Sr[B8O11(OH)4] where the amount of doped Eu(2+) ranges from 2.5 to 11%.

Structural changes within the alkaline earth uranyl phosphites.

Three new alkaline earth (AE) uranyl phosphites and one barium uranium(IV) phosphate were synthesized under hydrothermal conditions. The carbonate salts of the AE's were employed both as cation sources and as pH regulators. Despite having very similar formulas and uranyl building units, the Ca(2+), Sr(2+) and Ba(2+) uranyl phosphites have three different extended networks. The calcium compound also contains a Ca(2+)/UO2(2+) mixed cation position. The four structure types will be presented herein with the AE cations generating distinct structural transformations.

High structural complexity of potassium uranyl borates derived from high-temperature/high-pressure reactions.

Three new potassium uranyl borates, K12[(UO2)19(UO4)(B2O5)2(BO3)6(BO2OH)O10] ·nH2O (TPKBUO-1), K4[(UO2)5(BO3)2O4]·H2O (TPKBUO-2), and K15[(UO2)18(BO3)7O15] (TPKBUO-3), were synthesized under high-temperature/high-pressure conditions. In all three compounds, the U/B ratio exceeds 1. Boron exhibits BO3 coordination only, which is different from other uranyl borates prepared at room temperature or under mild hydrothermal conditions. A rare uranium(VI) tetraoxide core UO4O2, which is coordinated by two BO3 groups, is observed in the structure of TPKBUO-1. Both structures of TPKBUO-1 and TPKBUO-3 contain three different coordination environments of uranium, namely, UO4O2, UO2O4, and UO2O5 and UO2O4, UO2O5, and UO2O6 bipyramids in TPKBUO-1 and TPKBUO-3, respectively.

A new family of lanthanide borate halides with unusual coordination and a new neodymium-containing cationic framework.

The reactions of Ln(2)O(3)/CeO(2)/Pr(6)O(11) (Ln = La-Nd, Sm), molten boric acid, and concentrated HBr or HI result in the formation of La[B(7)O(10)(OH)(3)(H(2)O)Br], Ln[B(6)O(9)(OH)(2)(H(2)O)(2)Br]·0.5H(2)O (Ln = Ce, Pr), Nd(2)[B(12)O(17.5)(OH)(5)(H(2)O)(4)Br(1.5)]Br(0.5)·H(2)O (NdBOBr), Sm(4)[B(18)O(25)(OH)(13)Br(3)], and Ln[B(7)O(11)(OH)(H(2)O)(3)I] (Ln = La-Nd, Sm). The lanthanide(III) centers in these compounds are found with 9-coordinate hula hoop or 10-coordinate capped triangular cupola geometries, where there are six approximately coplanar oxygen donors provided by the polyborate sheet. The sheets are formed into three-dimensional frameworks via BO(3) triangles that are roughly perpendicular to the layers. Additionally, a new cationic framework, NdBOBr, has been isolated. NdBOBr is unusual in that not only is it a cationic framework, but it is also the first trivalent f-element borate to have terminal halides bound exclusively to the base site of the hula hoop. The Ln[B(7)O(11)(OH)(H(2)O)(3)I] (Ln = La-Nd, Sm) structures require two corner-shared BO(3) units in order to tether the layers together because of the large size of the capping iodine atom.

From order to disorder and back again: in situ hydrothermal redox reactions of uranium phosphites and phosphates.

Five new uranium phosphites, phosphates, and mixed phosphate-phosphite compounds were hydrothermally synthesized using H(3)PO(3) as an initial reagent. These compounds are Cs(4)[(UO(2))(8)(HPO(4))(5)(HPO(3))(5)]·4H(2)O (1), Cs[U(IV)(PO(4))(H(1.5)PO(4))](2) (2), Cs(4)[U(IV)(6)(PO(4))(8)(HPO(4))(HPO(3))] (3), Cs(10)[U(IV)(10)(PO(4))(4)(HPO(4))(14)(HPO(3))(5)]·H(2)O (4), and Cs(3)[U(IV)(4)(PO(4))(3)(HPO(4))(5)] (5). The first contains uranium(VI) and the latter four uranium(IV). Of the U(IV) structures, two have extensive disordering among the cesium cation positions, one of which also contains disordering at some of the phosphate-phosphite positions. These intermediate compounds are bookended by nondisordered phases. The isolation of these transitional phases occurred at the higher of the pH conditions attempted here. Both the starting pH and the duration of the reactions have a strong influence on the products formed. Herein, we explore the second series of in situ hydrothermal redox reactions of uranyl nitrate with phosphorous acid and cesium carbonate. The isolation of these disordered crystalline products helps to illuminate the complex reaction pathways that can occur in hydrothermal syntheses.

Elucidation of tetraboric acid with a new borate fundamental building block in a chiral uranyl fluoroborate.

A new neutral borate species, H(2)B(4)O(7) (also known as tetraboric acid), with a one-dimensional chain structure, is found in the interlayer spacing in Rb(2)[(UO(2))(2)B(8)O(12)F(6)]·H(2)B(4)O(7) (RbUBOF-2) derived from boric acid flux reaction of uranyl(VI) nitrate with RbBF(4). This new form of tetraboric acid possesses a novel borate fundamental building block with the symbol 4Δ:<3Δ>Δ.

Effect of pH and reaction time on the structures of early lanthanide(III) borate perchlorates.

Reactions of LnCl(3)·6H(2)O (Ln = La-Nd, Sm, Eu), concentrated (11 M) perchloric acid, and molten boric acid result in the formation of four different compounds. These compounds are Ln[B(8)O(10)(OH)(6)(H(2)O)(ClO(4))]·0.5H(2)O (Ln = La-Nd, Sm), Pr[B(8)O(11)(OH)(4)(H(2)O)(ClO(4))], Ln[B(7)O(11)(OH)(H(2)O)(2)(ClO(4))] (Ln = Pr, Nd, Sm, and Eu), and Ce[B(8)O(11)(OH)(4)(H(2)O)(ClO(4))]. All Ln(III) cations are ten-coordinate with a capped triangular cupola geometry and contain an inner-sphere, monodentate perchlorate moiety. This geometry is obtained because of the coordination of the oxygen donors within the polyborate sheet which create triangular holes and provide residence for the lanthanide metal centers. Aside from Ln[B(8)O(10)(OH)(6)(H(2)O)(ClO(4))]·0.5H(2)O (Ln = La-Nd, Sm), which are two-dimensional sheet structures, all other compounds are three-dimensional frameworks in which the layers are tethered together by BO(3) units found roughly perpendicular to the sheets. Furthermore, a change in product is observed depending on the reaction duration while holding all other synthetic variables constant. This report also demonstrates that lanthanide borates can be prepared in extreme acidic conditions.

Cation-cation interactions between neptunyl(VI) units.

The boric acid flux reaction of NpO(2)(ClO(4))(2) with NaClO(4) affords Na[(NpO(2))(4)B(15)O(24)(OH)(5)(H(2)O)](ClO(4))·0.75H(2)O (NaNpBO-1). NaNpBO-1 possesses a layered structure consisting of double neptunyl(VI) borate sheets bridged by another Np(VI) site through cation-cation interactions. The sole presence of Np(VI) in NaNpBO-1 is supported by absorption and vibrational spectroscopy.

Polytypism and oxo-tungstate polyhedra polymerization in novel complex uranyl tungstates.

Three new uranyl tungstates, α-, ß-Cs(2)[(UO(2))(2)(W(2)O(9))], and Rb(6)[(UO(2))(7)(WO(5))(2)(W(3)O(13))O(2)], have been obtained by high temperature solid state reactions. All three compounds display novel structure topologies: α- and ß-Cs(2)[(UO(2))(2)(W(2)O(9))] are based upon layers with a new topology that can be related to the uranophane topology; Rb(6)[(UO(2))(7)(WO(5))(2)(W(3)O(13))O(2)] is a rare example of a non-molecular inorganic phase with layers containing oxo-tungstate trimers. The structural relationship between α- and ß-Cs(2)[(UO(2))(2)(W(2)O(9))] can be assigned to polytypism.

Effects of large halides on the structures of lanthanide(III) and plutonium(III) borates.

Reactions of LnBr(3) or LnOI with molten boric acid result in formation of Ln[B(5)O(8)(OH)(H(2)O)(2)Br] (Ln = La-Pr), Nd(4)[B(18)O(25)(OH)(13)Br(3)], or Ln[B(5)O(8)(OH)(H(2)O)(2)I] (Ln = La-Nd). Reaction of PuOI with molten boric acid yields Pu[B(7)O(11)(OH)(H(2)O)(2)I]. The Ln(III) and Pu(III) centers in these compounds are found as nine-coordinate hula-hoop or 10-coordinate capped triangular cupola geometries where there are six approximately coplanar oxygen donors provided by triangular holes in the polyborate sheets. The borate sheets are connected into three-dimensional networks by additional BO(3) triangles and/or BO(4) tetrahedra that are roughly perpendicular to the layers. The room-temperature absorption spectrum of single crystals of Pu[B(7)O(11)(OH)(H(2)O)(2)I] shows characteristic f-f transitions for Pu(III) that are essentially indistinguishable from Pu(III) in other compounds with alternative ligands and different coordination environments.

Systematic evolution from uranyl(VI) phosphites to uranium(IV) phosphates.

Six new uranium phosphites, phosphates, and mixed phosphate-phosphite compounds were hydrothermally synthesized, with an additional uranyl phosphite synthesized at room temperature. These compounds can contain U(VI) or U(IV), and two are mixed-valent U(VI)/U(IV) compounds. There appears to be a strong correlation between the starting pH and reaction duration and the products that form. In general, phosphites are more likely to form at shorter reaction times, while phosphates form at extended reaction times. Additionally, reduction of uranium from U(VI) to U(IV) happens much more readily at lower pH and can be slowed with an increase in the initial pH of the reaction mixture. Here we explore the in situ hydrothermal redox reactions of uranyl nitrate with phosphorous acid and alkali-metal carbonates. The resulting products reveal the evolution of compounds formed as these hydrothermal redox reactions proceed forward with time.

Differentiating between trivalent lanthanides and actinides.

The reactions of LnCl(3) with molten boric acid result in the formation of Ln[B(4)O(6)(OH)(2)Cl] (Ln = La-Nd), Ln(4)[B(18)O(25)(OH)(13)Cl(3)] (Ln = Sm, Eu), or Ln[B(6)O(9)(OH)(3)] (Ln = Y, Eu-Lu). The reactions of AnCl(3) (An = Pu, Am, Cm) with molten boric acid under the same conditions yield Pu[B(4)O(6)(OH)(2)Cl] and Pu(2)[B(13)O(19)(OH)(5)Cl(2)(H(2)O)(3)], Am[B(9)O(13)(OH)(4)]·H(2)O, or Cm(2)[B(14)O(20)(OH)(7)(H(2)O)(2)Cl]. These compounds possess three-dimensional network structures where rare earth borate layers are joined together by BO(3) and/or BO(4) groups. There is a shift from 10-coordinate Ln(3+) and An(3+) cations with capped triangular cupola geometries for the early members of both series to 9-coordinate hula-hoop geometries for the later elements. Cm(3+) is anomalous in that it contains both 9- and 10-coordinate metal ions. Despite these materials being synthesized under identical conditions, the two series do not parallel one another. Electronic structure calculations with multireference, CASSCF, and density functional theory (DFT) methods reveal the An 5f orbitals to be localized and predominately uninvolved in bonding. For the Pu(III) borates, a Pu 6p orbital is observed with delocalized electron density on basal oxygen atoms contrasting the Am(III) and Cm(III) borates, where a basal O 2p orbital delocalizes to the An 6d orbital. The electronic structure of the Ce(III) borate is similar to the Pu(III) complexes in that the Ce 4f orbital is localized and noninteracting, but the Ce 5p orbital shows no interaction with the coordinating ligands. Natural bond orbital and natural population analyses at the DFT level illustrate distinctive larger Pu 5f atomic occupancy relative to Am and Cm 5f, as well as unique involvement and occupancy of the An 6d orbitals.

Rich coordination of Nd3+ in Mg2Nd13(BO3)8(SiO4)4(OH)3, derived from high-pressure/high-temperature conditions.

A neodymium borosilicate, Mg(2)Nd(13)(BO(3))(8)(SiO(4))(4)(OH)(3) (MgNdBSi-1), was obtained from a high-temperature (1400 °C), solid-state reaction under high-pressure conditions (4.5 GPa). MgNdBSi-1 contains six different types of Nd(3+) coordination environments with three different ligands: BO(3), SiO(4), and OH groups. Mg(2+) cations are only bond to BO(3) groups and form porous two-dimensional layers based on 12-membered ring fragments. Surprisingly, the OH groups are retained at high temperature and reside at the center of Mg-BO(3) rings.