A.M.T. Bell

Time-resolved synchrotron powder X-ray diffraction study of magnetite formation by the Fe(III)-reducing bacterium Geobacter sulfurreducens

Abstract The Fe(III)-reducing bacterium Geobacter sulfurreducens produces nanometer-sized magnetite by transferring electrons from organic matter or hydrogen to ferrihydrite, resulting in the reductive transformation of the ferrihydrite to magnetite, and the generation of ATP for growth. Electron transfer can occur by direct contact between the cell surface and the mineral or via a soluble “electron shuttle,” for example a quinone-containing humic species. The minerals produced at different stages of ferrihydrite reduction during two experiments, one with and one without the humic analog anthraquinone-2, 6-disulphonate (AQDS), were measured using high-resolution synchrotron powder X-ray diffraction. Amorphous 2-line ferrihydrite converts to goethite, then to a mixture of goethite and magnetite, and finally to magnetite. Samples with and without AQDS showed the same general mineralogical trends, and the rate of reaction was faster in the presence of AQDS. In addition, two transient minerals structurally similar to goethite and magnetite were observed to form as intermediates between ferrihydrite and goethite and goethite and magnetite, but only in samples produced in the absence of the electron shuttle. These transient minerals were named proto-goethite and proto-magnetite. Proto-goethite has a shorter c-axis [4.467(20) Å] than crystalline goethite, a function of size (<2 nm) where quantum properties prevail. Proto-magnetite is identified by long tetrahedral (2.113 Å) and short octahedral (1.943 Å) Fe-O bonds compared to stoichiometric magnetite, possibly indicative of a coordination crossover caused by charge density [Fe(II)] migration to tetrahedral sites. Fe(II) in solution or sorbed to the mineral surface is considered to be the catalyst causing the mineral transformations. The Fe(II) is thought to form predominantly from the reductive dissolution of 2-line ferrihydrite by G. sulfurreducens.

Synthesis, structure and properties of a semivalent iron oxoborate, Fe2OBO3

Transition metal oxoborates are of interest for magnetic and optical properties. Crystals of many M n II n 2 nM n III n nO n 2 nBO n 3 n and M n II n nM n III n nOBO n 3 n materials can be grown from borate fluxes. In the Fe n II n n-Fe n III n n-B-O system, flux growth results in Fe n 3 nO n 2 nBO n 3 n crystals, but solid state reaction at higher temperatures has yielded Fe n 2 nOBO n 3 n as a polycrystalline powder. This has been characterised by synchrotron and neutron diffraction, electron microscopy, Mossbauer spectroscopy, and conductivity and magnetic measurements. Two notable transitions occur, a broad semiconductor-semiconductor change accompanied by a structural transition at 317 K, and L-type ferrimagnetic order below a Curie temperature of 155 K. An average (Fe n 2+ n) n 0.5 n(Fe n 3+ n) n 0.5 n valence is observed at the two crystallographically distinct sites in Fe n 2 nOBO n 3 n, indicating that charge ordering occurs.

The crystal chemistry of the solid solution series between chalcostibite (CuSbS2) and emplectite (CuBiS2)

Abstract Sulphosalts in the system CuSbS2−CuBiS2 (chalcostibite-emplectite) form a complete solid solution series. Seven compositions with the general formula Cu(SbxBi1-x)S 2 have been synthesized using dry methods at 310°C. All members of the series are orthorhombic (space group Prima) and show smoothly increasing a and b cell parameters with substitution of Bi for Sb; the c cell parameter increases up to 50% CuBiS2 substitution and then becomes constant. DSC experiments on CuBiS2 show an endothermic heat effect (2.45 kJ/mol.) at 472°C due to the breakdown reaction to Cu3BiS3 (wittichenite) plus Bi2S3 (bismuthinite). With the addition of 10% CuSbS2 to CuBiS2, the decomposition temperature increases and the endothermic peak is broadened but the energy remains essentially the same (2.53 kJ/mol.). No evidence of this decomposition was observed when the amount of the CuSbS2 component was >30%. The local structure and co-ordination of Cu in the samples were studied by EXAFS analysis of the Cu-K edge but no significant variation occurs in the local Cu environment. The Debye-Waller factor for the first shell of S atoms surrounding Cu in end member CuSbS2 tends to be slightly smaller than for the intermediate solid solutions, suggesting that the tetrahedral Cu environments in the intermediate composition samples is somewhat more disordered than in the end-member. The low expansion characteristics along c appear to be controlled by the linkages between the (CuS3 + BiS2) sheets perpendicular to c being relatively inflexible.

Polymorphism in cyclohexanol

The crystal structures and phase behaviour of phase II and the metastable phases III and III of cyclohexanol, C(6)H(11)OH, have been determined using high-resolution neutron powder, synchrotron X-ray powder and single-crystal X-ray diffraction techniques. Cyclohexanol-II is formed by a transition from the plastic phase I cubic structure at 265 K and crystallizes in a tetragonal structure, space group P42(1)c (Z = 1), in which the molecules are arranged in a hydrogen-bonded tetrameric ring motif. The structures of phases III and III are monoclinic, space groups P2(1)/c (Z = 3) and Pc (Z = 2), respectively, and are characterized by the formation of hydrogen-bonded molecular chains with a threefold-helical and wave-like nature, respectively. Phase III crystallizes at 195 K from a sample of phase I that is supercooled to ca 100 K. Alternatively, phase III may be grown via phase III, the latter transforming from supercooled phase I at ca 200 K. Phase III is particularly unstable and is metastable with respect to both I and II. Its growth is realised only under very restricted conditions, thus making its characterization especially challenging. The cyclohexanol molecules adopt a chair conformation in all three phases with the hydroxyl groups in an equatorial orientation. No evidence was found indicating hydroxyl groups adopting an axial orientation, contrary to the majority of spectroscopic literature on solid-state cyclohexanol; however, the H atom of the equatorial OH groups is found to adopt both in-plane and out-of-plane orientations.

Structures of synthetic K2MgSi5O12 leucites by integrated X-ray powder diffraction, electron diffraction and 29Si MAS NMR methods

The structures of disordered and ordered varieties of the title compound have been determined using integrated TEM, MAS NMR and Rietveld analysis of synchrotron X-ray powder diffraction data. Both samples have a leucite-like framework topology. The dry-synthesized sample is cubic, la3d [a= 13.4190 (1) ,~, V= 2416.33 (5) A 3] with disordered Mg and Si in tetrahedral framework sites. The hydrothermally synthesized analogue is monoclinic, P21/c [a = 13.168 (5), b = 13.652 (1), c = 13.072 (5)/~,/3 = 91.69 °, V = 2348 (2) A3], and has a fully ordered framework with four K, ten Si and two Mg sites per 24 O atoms (one quarter of the unit cell). Two of these Si sites are linked to Si tetrahedra only [Q4(4Si)], while the other eight Si sites have one Mg and three Si tetrahedra as next-nearest neighbours [Q4(3Si, IMg)]. Q4(4Si) and Mg tetrahedra

Synchrotron X-ray absorption spectroscopy and X-ray powder diffraction studies of the structure of johnbaumite [Ca10(AsO4)6(OH,F)2] and synthetic Pb-, Sr- and Ba-arsenate apatites and some comments on the crystal chemistry of the apatite structure type in general

Abstract The chemical composition of the natural arsenate-apatite mineral johnbaumite [nominally Ca10(AsO4)6(OH)2] and its alteration product hedyphane [Ca4Pb6(AsO4)6Cl2] have been determined by electron microprobe analysis and the structures of johnbaumite and synthetic Sr-, Ba- and Pb-arsenate apatites have been studied by As K-edge X-ray absorption spectroscopy and synchrotron X-ray powder diffraction. All samples belong to the holosymmetric apatite space group P63/m with As5+ substituted for P5+ in the tetrahedral structural site. Johnbaumite contains small amounts of F and Pb (~0.9 and ~4.4 wt.% respectively) and hedyphane has the ideal composition (formula given above); the compositions ofthese coexisting phases define the two limbs of a solvus occurring between Ca- and Pb-arsenate apatite end members. The unit-cell parameters and cation−oxygen bond lengths for the arsenate apatites studied are discussed alongside published data for end-member Ca-, Sr-, Ba- and Pb phosphate apatite analogues with (OH), F, Cl or Br as the anions at the centres of the channels in the apatite structure. This discussion rationalizes the relationships between the two structural sites A(1) and A(2) occupied by divalent cations in terms of the size of the A−O polyhedra and the distortion of the A(1)−O polyhedron as measured by the metaprism twist angle [O(1)−A(1)−O(2) projected onto (001)].

Leucite-pollucite structure-type variability and the structure of a synthetic end-member calcium wairakite (CaAl2Si4O12 2H2O)

Abstract The structure of a synthetic end-member wairakite (CaAl2Si4O12·2H2O) has been determined using Rietveld analysis of high-resolution, syn&rotron X-ray powder diffraction data, and 29Si and 27Al magic angle spinning nuclear magnetic resonance spectroscopy. The framework in the synthetic sample is more disordered than that in natural wairakite. Ca is distributed over the cavity cation sites M2, M12A, M12B in the approximate proportions 0.8:0.1:0.1, respectively, with M11 being vacant. 29Si MAS NMR data are consistent with about 80% of the Si occupying tetrahedral T11 and T12 sites linked to two A1 atoms [Q4(2Al) silicons]. Tetrahedral and cavity cation site disorder are coupled so that A1 mainly occupies T2 sites, with Ca in M12A and M12B being balanced by A1 in T12A and T12B; T11A and T11B sites appear to only contain Si, in agreement with the Mll site being vacant. The crystal chemistries of the wide range of stoichiometries which crystallize with the leucite/pollucite structure-type are also reviewed, with particular attention being paid to the tetrahedral ordering configurations present in these phases, and the implications to crystallographic phase transitions.

Structural relations and tetrahedral ordering pattern of synthetic orthorhombic Cs2CdSi5O12 leucite: a combined synchrotron X-ray powder diffraction and multinuclear MAS NMR study

A previously unknown leucite-related structure has been determined for synthetic CszCdSisO~2. NMR spectroscopy shows that there are five distinct tetrahedral sites (T-sites) occupied by Si and one T-site occupied by Cd in the framework structure, while analysis of the synchrotron X-ray powder diffraction pattern establishes that this material is orthorhombic, Pbca [RI = 13.1%, R wp = 16.1%, Rex p = 13.1%; eight formula units per unit cell; unit-cell parameters a= 13.6714(1), b= 13.8240 (1), c= 13.8939 (1) ,~, V = 2625.83 (6) ,~3]. Tetrahedral cation ordering rates for Si and Cd are sufficiently high for both hydrothermally and dry-synthesized samples to be fully ordered. The symmetry relations between leucites with P2/c and Pbca structures are discussed and it is shown that such materials are related by a displacive phase transition, in which the

Revision of the structure of Cs2CuSi5O12 leucite as orthorhombic Pbca

The crystal structure of a hydrothermally synthesized leucite analogue Cs(2)CuSi(5)O(12) has been determined and refined using the Rietveld method from high-resolution synchrotron X-ray and neutron powder diffraction data. This structure is based on the topology and cation-ordering scheme of the Pbca leucite structure of Cs(2)CdSi(5)O(12), and exhibits five ordered Si sites and one ordered Cu tetrahedrally coordinated (T) site. This structure for Cs(2)CuSi(5)O(12) is topologically identical to other known leucite structures and is different from that originally proposed by Heinrich & Baerlocher [(1991), Acta Cryst. C47, 237-241] in the tetragonal space group P4(1)2(1)2. The crystal structure of a dry-synthesized leucite analogue Cs(2)CuSi(5)O(12) has also been refined; this has the Ia3d cubic pollucite structure with disordered T sites.

Crystal structures and cation ordering in Cs2MgSi5O12, Rb2MgSi5O12 and Cs2ZnSi5O12 leucites.

The crystal structures of the leucite analogues Cs(2)MgSi(5)O(12), Cs(2)ZnSi(5)O(12) and Rb(2)MgSi(5)O(12) have been determined by synchrotron X-ray powder diffraction using Rietveld refinement in conjunction with (29)Si MAS NMR spectroscopy. These leucites are framework structures with distinct tetrahedral sites (T sites) occupied by Si and a divalent cation (either Mg or Zn in these samples); there is also a monovalent extra-framework cation (either Cs or Rb in these samples). The refined crystal structures were based on the Pbca leucite structure of Cs(2)CdSi(5)O(12), thus a framework with five ordered Si T sites and one ordered Cd T site was used as the starting model for refinement. (29)Si MAS NMR shows five distinct Si T sites for Cs(2)MgSi(5)O(12) and Rb(2)MgSi(5)O(12), but six Si T sites for Cs(2)ZnSi(5)O(12). The refined structures for Cs(2)MgSi(5)O(12) and Rb(2)MgSi(5)O(12) were determined with complete T-site ordering, but the refined structure for Cs(2)ZnSi(5)O(12) was determined with partial disorder of Mg and Si over two of the T sites.