Monoclinic hydroxyapatite

The existence of a monoclinic phase of hydroxyapatite, Ca(2)(PO(4))(4)OH, has been confirmed, by single-crystal structure analysis (weighted "reliability" factor = 3.9 percent on |F|(2)). The structure has space group P21/b, a = 9.4214(8) angstroms, b = 2a, c = 6.8814(7) angstroms, and gamma = 120 degrees , and is analogous to that of chlorapatite. The distortions from the hexagonal structure with which the monoclinic structure is pseudosymmetric are similar to those in chlorapatite, including enlargement of that triangular array of oxygen atoms in which the chlorine ion or, in hydroxyapatite, the hydroxyl hydrogen ion is approximately centered. The hydroxyapatite specimen was prepared by the conversion of a single crystal of chlorapatite in steam at 1200 degrees C, was mimetically twinned, and was approximately 37 percent monoclinic.     

In Situ Determination of the NiAs Phase of FeO at High Pressure and Temperature

In situ synchrotron x-ray diffraction measurements of FeO at high pressures and high temperatures revealed that the high-pressure phase of FeO has the NiAs structure (B8). The lattice parameters of this NiAs phase at 96 gigapascals and 800 kelvin are a = 2.574(2) angstroms and c = 5.172(4) angstroms (the number in parentheses is the error in the last digit). Metallic behavior of the high-pressure phase is consistent with a covalently and metallically bonded NiAs structure of FeO. Transition to the NiAs structure of FeO would enhance oxygen solubility in molten iron. This transition thus provides a physiochemical basis for the incorporation of oxygen into the Earth's core.     

In Situ X-ray Absorption Study of Surface Complexes: Selenium Oxyanions on agr-FeOOH

A novel application of x-ray absorption spectroscopy has provided structural information for ions sorbed at oxide-water interfaces. As an example, in situ extended x-ray absorption fine structure (EXAFS) measurements of adsorbed selenate and selenite ions at ah alpha-FeOOH(goethite)-water interface have been performed; these measurements show that selenate forms a weakly bonded, outer-sphere complex and that selenite forms a strongly bonded, inner-sphere complex. The selenite ion is bonded directly to the goethite surface in a bidentate fashion with two iron atoms 3.38 angstroms from the selenium atom. Adsorbed selenate has no iron atom in the second coordination shell of selenium, which indicates retention of its hydration sphere upon sorption. This method provides direct structural information for adsorbed species at solid-liquid interfaces.     

The structure of the c60 molecule: x-ray crystal structure determination of a twin at 110 k

Single-crystal x-ray diffraction methods were used to determine the crystal and molecular structure of C(60) buckminsterfullerene. At 110 kelvin C(60) is cubic, apparent Laue symmetry m3m, but it exhibits noncrystallographic systematic extinctions indicative of a twin in which I(hkl) and I(khl) are superimposed. In fact, C(60) crystallizes with four molecules in space group [See equation in the PDF file] of the cubic system (Laue symmetry m3) with lattice constant a = 14.052(5) angstroms (A) at 110 kelvin. The twin components are equal. A given component, which has crystallographically imposed symmetry [See equation in the PDF file] displays an ordered structure of a truncated icosahedron. The five independent C=C bonds that join C(6) rings average 1.355(9) A; the ten independent C-C bonds that join C(6) and C(5) rings average 1.467(21) A. The mean atom-to-atom diameter of the C(60) molecule is 7.065(3) A. The molecules are very tightly packed in the crystal structure, with intermolecular C...C distances as short as 3.131(7) A.     

A chromium terephthalate-based solid with unusually large pore volumes and surface area

We combined targeted chemistry and computational design to create a crystal structure for porous chromium terephthalate, MIL-101, with very large pore sizes and surface area. Its zeotype cubic structure has a giant cell volume (approximately 702,000 cubic angstroms), a hierarchy of extra-large pore sizes (approximately 30 to 34 angstroms), and a Langmuir surface area for N2 of approximately 5900 +/- 300 square meters per gram. Beside the usual properties of porous compounds, this solid has potential as a nanomold for monodisperse nanomaterials, as illustrated here by the incorporation of Keggin polyanions within the cages.     

Crystal Structure, Bonding, and Phase Transition of the Superconducting Na2CsC60 Fulleride

The crystal structure of superconducting Na(2)CsC(60) was studied by high-resolution powder neutron diffraction between 1.6 and 425 K. Contrary to the literature, the structure at low temperatures is primitive cubic [See equation in the PDF file], isostructural with pristine C(60). Anticlockwise rotation of the C(60) units by 98 degrees about [111] allows simultaneous optimization of C(60)-C(60) and alkali-fulleride interactions. Optimal Na(+)-C(60)(3-) coordination is achieved with each sodium ion located above one hexagon face and three hexagon-hexagon fusions of neighboring fulleride ions (coordination number 12). Reduction of the C(60) molecule lengthens the hexagon-hexagon fusions and shortens the pentagon-hexagon fusions (to approximately 1.43 angstroms). On heating, Na(2)CsC(60) undergoes a phase transition to a face-centered-cubic [See equation in the PDF file] phase, best modeled as containing quasi-spherical C(60)(3-) ions. The modified structure and intermolecular potential provide an additional dimension to the behavior of superconducting fullerides and should sensitively affect their electronic and conducting properties.     

Antiprismatic Coordination about Xenon: The Structure of Nitrosonium Octafluoroxenate(VI)

The structure of nitrosonium octafluoroxenate(VI), 2NOF . XeF(6), has been determined by means of single-crystal x-ray counter methods (R-index = 0.046, weighted R-index = 0.042). The space group is Pnma, with a = 8.914(10) angstroms, b = 5.945(10) angstroms, and c = 12.83(2) angstroms (the numbers in parentheses are the standard deviations to the least significant digit or digits); the calculated density (rho) is 3.354 grams per cubic centimeter, and there are four formula units per unit cell. The material consists of well-separated NO(+) and (XeF(8))(2-) ions; the structural formula is thus (NO)(2) (XeF(8)). The anion configuration is that of a slightly distorted Archimedean antiprism. The observed distortion appears incompatible with a lone-pair repulsion model. Xenon-fluorine bond lengths of 1.971(7), 1.946(5), 1.958(7), 2.052(5), and 2.099(5) angstroms were found.     

铝同晶替代对水铁矿向赤铁矿转化的影响

为明确铝(Al)等异质离子对水铁矿转化过程的影响,以现代光谱学技术(粉末X射线衍射和X射线吸收光谱)为主要手段,研究铝同晶替代对水铁矿向赤铁矿转化过程的影响。研究结果表明,铝替代水铁矿向赤铁矿转化的过程中存在着颗粒组装以及Ostwald熟化过程。一方面,Al~(3+)同晶替代会降低水铁矿向赤铁矿的转化速率;另一方面,Al~(3+)会促进水铁矿结构中的共边[FeO_6]八面体向共面[FeO_6]八面体转化,导致水铁矿向赤铁矿转化。     

Garnet-like structures of high-pressure cadmium germanate and calcium germanate

Crystals of CdGeO(3) grown at a pressure of 65 kilobars are tetragonal and have an ordered, garnet-like crystal structure with cadmium occupying the dodecahedral and octahedral sites, and germanium the octahedral and tetrahedral sites. The crystal structure (a = 12.406 +/- 1 angstroms, c = 12.256 +/- 1 angstroms, and space group 14,/a) has been refined by least-squares analysis to an R (discrepancy index) of 0.073. Two high-pressure phases of CaGeO(3) were synthesized, one isotypic with tetragonal CdGeO(3) (a = 12.514 +/- 3 angstroms, c = 12.358 +/- 3 angstroms), and the other isotypic with perovskite.     

Carbon: a suggested new hexagonal crystal form

Evidence for a new polymorphic form of carbon, similar to but distinct from chaoite, has been discovered. It is trigonal, with a(0) = 5.33 angstroms and c(0) = 12.24 angstroms. It is produced along with chaoite on the surfaces of graphitic carbons under free-vaporization conditions at low pressures, with temperatures above approximately 2550 degrees K.     

Crystal Structure and Optical Properties of Cd32S14(SC6H5)36. DMF4, a Cluster with a 15 Angstrom CdS Core

Recrystallization of the solid Cd(10)S(4)(SC(6)H(5))(12) from a solution of pyridine and N, N-di-methylformamide (DMF) results in the formation of the cluster Cd(32)S(14)(SC(6)H(5))(36)-DMF(4) as pale yellow cubes. The structure consists of an 82-atom CdS core that is a roughly spherical piece of the cubic sphalerite lattice approximately 12 angstroms in diameter. The four corners of the lattice are capped by hexagonal wurtzite-like CdS units, which results in an overall tetrahedral cluster approximately 15 angstroms in diameter. This cluster dissolves intact in tetrahydrofuran where its absorption spectrum reveals a sharp peak at 358 nanometers at room temperature and its emission spectra show a strong broad band at 500 nanometers.     

All-Carbon Molecules: Evidence for the Generation of Cyclo[18]carbon from a Stable Organic Precursor

The unambiguous structural characterization of a single-sized all-carbon molecule requires its chemical synthesis. For cyclo[18]carbon, ab initio calculations predict a relatively stable, cyclic D9h ground state geometry with alternating C-C (1.36 angstroms) and C identical withC (1.20 angstroms) bonds. The synthesis and x-ray crystal structure of a direct precursor to C(18) are described. The analysis of laser flash heating experiments on this precursor by time-of-flight mass spectroscopy shows a sequence of retro-Diels-Alder reactions leading to C(18) as the predominant fragmentation pattern. Structural evidence is provided for the generation of an all-carbon molecule from a well-characterized organic precursor.     

Electron small polarons and their mobility in iron (oxyhydr)oxide nanoparticles

Electron mobility within iron (oxyhydr)oxides enables charge transfer between widely separated surface sites. There is increasing evidence that this internal conduction influences the rates of interfacial reactions and the outcomes of redox-driven phase transformations of environmental interest. To determine the links between crystal structure and charge-transport efficiency, we used pump-probe spectroscopy to study the dynamics of electrons introduced into iron(III) (oxyhydr)oxide nanoparticles via ultrafast interfacial electron transfer. Using time-resolved x-ray spectroscopy and ab initio calculations, we observed the formation of reduced and structurally distorted metal sites consistent with small polarons. Comparisons between different phases (hematite, maghemite, and ferrihydrite) revealed that short-range structural topology, not long-range order, dominates the electron-hopping rate.     

High-Temperature XAS Study of Fe2SiO4 Liquid: Reduced Coordination of Ferrous Iron

X-ray absorption spectroscopy (XAS) of Fe(2+) in Fe(2)SiO(4) liquid at 1575 kelvin and 10(-4) gigapascal (1 bar) shows that the Fe(2+) -O bond length is 1.98 +/- 0.02 angstroms compared with approximately 2.22 angstroms in crystalline Fe(2)SiO(4) (fayalite) at the melting point (1478 kelvin), which indicates a decrease in average Fe(2+) coordination number from six in fayalite to four in the liquid. Anharmonicity in the liquid was accounted for using a data analysis procedure. This reduction in coordination number is similar to that observed on the melting of certain ionic salts. These results are used to develop a model of the medium-range structural environment of Fe(2+) in olivine-composition melts, which helps explain some of the properties of Fe(2)SiO(4) liquid, including density, viscosity, and the partitioning of iron and nickel between silicate melts and crystalline olivines. Some of the implications of this model for silicate melts in the Earth's crust and mantle are discussed.     

A Fluorite Isotype of SnO2 and a New Modification of TiO2: Implications for the Earth's Lower Mantle

The existence of a cubic fluorite-type SnO(2) and a hexagonal TiO(2) (which may be related to the fluorite structure) have been demonstrated by an in situ x-ray diffraction study in which a diamond-anvil pressure cell was used after the samples had been heated by a continuous yttrium-aluminum-garnet laser. At room temperature, the lattice parameter for SnO(2) (fluorite) is a = 4.925 +/- 0.005 angstroms and those for TiO(2) (fluorite-related) are a = 9.22 +/- 0.01 angstroms and c = 5.685 +/- 0.006 angstroms at about 250 kilobars. The volume change associated with the transition from rutile to fluorite (or related structure) is about -8 percent for SnO(2) and -10.5 percent for TiO(2) at transition. Upon release of pressure, both the fluorite-type SnO(2) and the TiO(2) reverted to the alpha-PbO(2) structure at room temperature. The hypothesis that the earth's lower mantle is composed of oxide phases might be feasible if it were possible for SiO(2) to possess the fluorite structure or its related forms at high pressure, as shown for SnO(2) and TiO(2) in this study. The oxide hypothesis proposed here differs from that postulated by Birch in that the primary coordination of silicon is 6 for Birch's hypothesis and 8 for the hypothesis presented here.     

Crystallographic and spectroscopic characterization of a nonheme Fe(IV)-O complex

Following the heme paradigm, it is often proposed that dioxygen activation by nonheme monoiron enzymes involves an iron(IV)=oxo intermediate that is responsible for the substrate oxidation step. Such a transient species has now been obtained from a synthetic complex with a nonheme macrocyclic ligand and characterized spectroscopically. Its high-resolution crystal structure reveals an iron-oxygen bond length of 1.646(3) angstroms, demonstrating that a terminal iron(IV)=oxo unit can exist in a nonporphyrin ligand environment and lending credence to proposed mechanisms of nonheme iron catalysis.     

Fractal surfaces of proteins

Fractal surfaces can be used to characterize the roughness or irregularity of protein surfaces. The degree of irregularity of a surface may be described by the fractal dimension D. For protein surfaces defined with probes in the range of 1.0 to 3.5 angstroms in radius, D is approximately 2.4 or intermediate between the value for a completely smooth surface (D = 2) and that for a completely space-filling surface (D = 3). Individual regions of proteins show considerable variation in D. These variations may be related to structural features such as active sites and subunit interfaces, suggesting that surface texture may be a factor influencing molecular interactions.     

Crystalline transfer RNA: the three-dimensional Patterson function at 12-angstrom resolution

An orthorhombic form of crystalline formylmethionine transter RNA has been obtained which contains one molecule as the asymmetric unit of the unit cell. Three-dimensional x-ray diffraction data have been collected up to a resolution of 12 angstroms, and from this a Patterson function has been calculated. The function contains an elongated ridge of interatomic vectors parallel to the c-axis of the crystal. Analysis of the function suggests that the molecules are elogated and dimerized in an overlapping antiparrael fashion along the c-axis. The dimer has a length near 109 angstroms and a width of 35 angstroms in one direction. The individual molecular length is approximately 80 angstroms with an irregular cross section measuring 25 by 35 angstrms.     

New phases of c60 synthesized at high pressure

The fullerene C(60) can be converted into two different structures by high pressure and temperature. They are metastable and revert to pristine C(60) on reheating to 300 degrees C at ambient pressure. For synthesis temperatures between 300 degrees and 400 degrees C and pressures of 5 gigapascals, a nominal face-centered-cubic structure is produced with a lattice parameter a(o) = 13.6 angstroms. When treated at 500 degrees to 800 degrees C at the same pressure, C(60) transforms into a rhombohedral structure with hexagonal lattice parameters of a(o) = 9.22 angstroms and c(o) = 24.6 angstroms. The intermolecular distance is small enough that a chemical bond can form, in accord with the reduced solubility of the pressure-induced phases. Infrared, Raman, and nuclear magnetic resonance studies show a drastic reduction of icosahedral symmetry, as might occur if the C(60) molecules are linked.     

Was there really an Archean phosphate crisis?

During the Archean, massive amounts of iron were deposited in the form of banded iron formations. It has been suggested that sedimenting particles of ferric oxyhydroxide may have stripped dissolved phosphate from the oceans, causing a reduction in phytoplankton productivity. However, that model does not take into account the high concentration of dissolved silica that was present in seawater at that time. We show experimentally that silica effectively competes with phosphate for sorption sites on ferrihydrite particles. Furthermore, coprecipitation of silica with ferrihydrite reduces particle reactivity toward phosphate. Hence, Archean oceans probably contained considerably more phosphate than previously predicted.