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Leucites are tetrahedrally coordinated silicate framework structures with some of the silicon framework cations partially replaced by divalent or trivalent cations. These structures have general formulae A2BSi5O12 and ACSi2O6; where A is a monovalent alkali metal cation, B is a divalent cation, and C is a trivalent cation. In this paper, we report the Rietveld refinements of three more synthetic leucite analogues with stoichiometries of Cs2NiSi5O12, RbGaSi2O6, and CsGaSi2O6. Cs2NiSi5O12 is Ia$\bar{3}$d cubic and is isostructural with Cs2CuSi5O12. RbGaSi2O6 is I41/a tetragonal and is isostructural with KGaSi2O6. CsGaSi2O6 is $I\bar{4}3d$ cubic and is isostructural with RbBSi2O6.

Pure (BNT) and iron-doped bismuth sodium titanate (Fe-BNT) ceramics were produced according to the formula Bi0.5Na0.5Ti1−xFexO3−0.5x, where x = 0 to 0.1. The addition of Fe2O3 enables decreasing the sintering temperature to 900 °C in comparison with 1075 °C for pure BNT, whilst also achieving lower porosities and greater densities. This is attributed to oxygen vacancy generation arising from substitution of Fe3+ onto the Ti4+ site of the BNT perovskite structure, and the resulting increase in mass transport that this enables during sintering. X-ray diffraction (XRD) analysis of Fe-BNT samples shows single-phase BNT with no secondary phases for all studied Fe contents, confirming complete solid solution of Fe. Rietveld refinement of XRD data revealed a pseudocubic perovskite symmetry (Pm-3m), and unit cell lengths increased with increasing Fe content. Scanning electron microscopy (SEM) showed that average grain size increases with increasing Fe content from an average grain size of ~ 0.5 μm in (x = 0) pure BNT to ~ 5 μm in (x = 0.1) Fe-doped BNT. Increasing Fe content also led to decreasing porosity, with relative density increasing to a maximum > 97% of its theoretical value at x = 0.07 to 0.1. The addition of Fe to BNT ceramics significantly affects electrical properties, reducing the remnant polarization, coercive field, strain and desirable ferroelectric properties compared with those of pure densified BNT. At room temperature, a high relative permittivity (ɛ′) of 1050 (x = 0.07) at an applied frequency of 1 kHz and a lower loss factor (tanδ) of 0.006 (x = 0.1) at an applied frequency of 300 kHz were observed by comparison with pure BNT ceramics.

Leucites are silicate framework structures with some of the silicon framework cations partially replaced by divalent or trivalent cations. A monovalent extraframework alkali metal cation is also incorporated to balance the charges. We have previously reported Pbca leucite structures with the stoichiometries Cs2X2+Si5O12 (X = Mg, Mn, Co, Ni, Cu, Zn, Cd) and Rb2X2+Si5O12 (X = Mg, Mn, Ni, Cd). These orthorhombic leucite structures have all the silicon and non-silicon framework cations completely ordered onto separate crystallographic sites. This structure has five distinct Si sites and 1 X site; there are also two distinct sites for the extra-framework Cs or Rb. We have recently synthesised leucite analogues with two different extra-framework cations, these have the stoichiometry RbCsX2+Si5O12 (X = Mg, Ni, Cd). The initial Rietveld refinements assumed 50% Cs and 50% Rb on each of the two extra-framework cation sites. The refined structures for X = Ni and Cd have (within error limits) complete extra-framework cation site disorder. However, for X = Mg there is partial ordering of the extra-framework cation sites, the site occupancies are:- Cs1 0.37(3), Rb1 0.63(3), Cs2 0.63(3), Rb2 0.37(3).

European journal of chemistry 12(1), 60 - 63 (2021). doi:10.5155/eurjchem.12.1.60-63.2089, The synthetic leucite silicate framework mineral analogues Cs$_{2}$XSi$_{5}$O$_{12}$ (X = Cu, Cd, Zn) were prepared by high-temperature solid-state synthesis. The results of Rietveld refinement, using 18 keV synchrotron X-ray powder diffraction data collected at low temperatures (8K X = Cu, Zn; 10K X = Cd) show that the title compounds crystallize in the space group Pbca and are isostructural with the ambient temperature structures of these analogues. The structures consist of tetrahedrally coordinated SiO$_{4}$ and XO$_{4}$ sharing corners to form a partially substituted silicate framework. Extraframework Cs cations sit in channels in the framework. All atoms occupy the 8c general position for this space group. In these refined structures, silicon and X atoms are ordered onto separate tetrahedrally coordinated sites (T-sites)., Published by Univ., Tucker, Ga.

The leucite tectosilicate mineral analogues K2 X 2+Si5O12 (X = Fe2+, Co, Zn) and Rb2 X 2+Si5O12 (X = Mn) have been synthesized at elevated temperatures both dry at atmospheric pressure and at controlled water vapour pressure; for X = Co and Zn both dry and hydrothermally synthesized samples are available. Rietveld refinement of X-ray data for hydrothermal K2 X 2+Si5O12 (X = Fe2+, Co, Zn) samples shows that they crystallize in the monoclinic space group P21/c and have tetrahedral cations (Si and X) ordered onto distinct framework sites [cf. hydrothermal K2MgSi5O12; Bell et al. (1994a), Acta Cryst. B50, 560–566]. Dry-synthesized K2 X 2+Si5O12 (X = Co, Zn) and Rb2 X 2+Si5O12 (X = Mn) samples crystallize in the cubic space group Ia{\overline 3}d and with Si and X cations disordered in the tetrahedral framework sites as typified by dry K2MgSi5O12. Both structure types have tetrahedrally coordinated SiO4 and XO4 sharing corners to form a partially substituted silicate framework. Extraframework K+ and Rb+ cations occupy large channels in the framework. Structural data for the ordered samples show that mean tetrahedral Si—O and X—O bond lengths cover the ranges 1.60 Å (Si—O) to 2.24 Å (Fe2+—O) and show an inverse relationship with the intertetrahedral angles (T—O—T) which range from 144.7° (Si—O—Si) to 124.6° (Si—O—Fe2+). For the compositions with both disordered and ordered tetrahedral cation structures (K2MgSi5O12, K2CoSi5O12, K2ZnSi5O12, Rb2MnSi5O12 and Cs2CuSi5O12 leucites) the disordered polymorphs always have larger unit-cell volumes, larger intertetrahedral T—O—T angles and smaller mean T—O distances than their isochemical ordered polymorphs. The ordered samples clearly have more flexible frameworks than the disordered structures which allow the former to undergo a greater degree of tetrahedral collapse around the interframework cavity cations. Multivariant linear regression has been used to develop equations to predict intertetrahedral T—O—T angle variation depending on the independent variables Si—O and X—O bond lengths, cavity cation ideal radius, intratetrahedral (O—T—O) angle variance, and X cation electronegativity.

Rietveld refinements show that the crystal structures of synthetic leucite silicate framework mineral analogues Rb2 XSi5O12 (X = Ni, Mn) are isostructural with the Pbca cation-ordered structure of Cs2CdSi5O12., The synthetic leucite silicate framework mineral analogues Rb2 XSi5O12 {X = Ni [dirubidium nickel(II) penta­silicate] and Mn [dirubidium manganese(II) penta­silicate]} have been prepared by high-temperature solid-state synthesis. The results of Rietveld refinements, using X-ray powder diffraction data collected using Cu Kα X-rays, show that the title compounds crystallize in the space group Pbca and adopt the cation-ordered structure of Cs2CdSi5O12 and other leucites. The structures consist of tetra­hedral SiO4 and XO4 units sharing corners to form a partially substituted silicate framework. Extraframework Rb+ cations sit in channels in the framework. All atoms occupy the 8c general position for this space group. In these refined structures, silicon and X atoms are ordered onto separate tetra­hedrally coordinated sites (T-sites). However, the Ni displacement parameter and the Ni—O bond lengths suggest that for the X = Ni sample, there may actually be some T-site cation disorder.

Glasses with nominal molar composition 20B2O3 – (80-x)Bi2O3 – xFe2O3 (where x = 0–40) were successfully prepared by melt-quenching. These glasses were characterised by multiple techniques including density, X-ray diffraction (XRD), X-Ray fluorescence (XRF), Raman, FT-IR and Mossbauer spectroscopies, dilatometry and differential thermal analysis (DTA). Partial replacement of Bi2O3 by Fe2O3 leads to decreasing density and molar volume and a substantial increase in thermal stability, as measured by several parameters, with maximum improvements achieved when x = 10-20. These improvements are accompanied by modest increases in dilatometric softening point. FT-IR and Raman spectra confirm the presence of BO3 and BiO6 structural units in all glasses, with glass structure apparently little affected by Fe2O3. Mossbauer spectroscopy confirms that iron is present partly as 4-fold coordinated Fe3+ in all glasses, with some 5- and / or 6- coordinated Fe3+ sites also present.

Lanthanide orthoferrites have wide-ranging industrial uses including solar, catalytic and electronic applications. Here a series of lanthanide orthoferrite perovskites, LnFeO3 (Ln = La; Nd; Sm; Eu; Gd), prepared through a standard stoichiometric wet ball milling route using oxide precursors, has been studied. Characterisation through X-ray diffraction and X-ray fluorescence confirmed the synthesis of phase-pure or near-pure LnFeO3 compounds. 57Fe Mossbauer spectroscopy was performed over a temperature range of 10 K to 293 K to observe hyperfine structure and to enable calculation of the recoil-free fraction and Debye temperature (θD) of each orthoferrite. Debye temperatures (Ln = La 474 K; Nd 459 K; Sm 457 K; Eu 452 K; Gd 473 K) and recoil-free fractions (Ln = La 0.827; Nd 0.817; Sm 0.816; Eu 0.812; Gd 0.826) were approximated through minimising the difference in the temperature dependent experimental Centre Shift (CS) and theoretical Isomer Shift (IS), by allowing the Debye temperature and Isomer Shift values to vary. This method of minimising the difference between theoretical and actual values yields Debye temperatures consistent with results from other studies determined through thermal analysis methods. This displays the ability of variable-temperature Mossbauer spectroscopy to approximate Debye temperatures and recoil-free fractions, whilst observing temperature induced transitions over the temperature range observed. X-ray diffraction and Rietveld refinement show an inverse relationship between FeO6 octahedral volume and approximated Debye temperatures. Raman spectroscopy show an increase in the band positions attributed to soft modes of Ag symmetry, Ag(3) and Ag(5) from La to GdFeO3 corresponding to octahedral rotations and tilts in the [010] and [101] planes respectively.

The room-temperature crystal structure of the brownmillerite SrCaMnGaO5+δ (δ = 0.035) has been refined from neutron powder diffraction data; space group Ima2, a = 15.7817(6), b = 5.4925(2), c = 5.3196(2) Å. Mn and Ga occupy 99.0(2)% of the 6- and 4-coordinate sites, respectively. A combination of magnetometry, neutron diffraction and μSR spectroscopy has shown that the compound orders magnetically at 180 K, and that the low-temperature phase has a G-type antiferromagnetic structure, with an ordered magnetic moment of 3.30(2) μB per Mn at 2 K. Displaced hysteresis loops provide evidence that the atomic moment has an additional, glassy component. Magnetometry shows that significant short-range magnetic interactions persist above 180 K, and μSR that the spin fluctuations are thermally activated in this temperature region. The compound is an electrical insulator which at 159 K shows an unusually large magnetoresistance of 85% in 6T, increasing to 90% in 13 T. © 2002 Elsevier Science (USA).

The title compounds are prepared by melting and quenching of stoichiometric amounts of Rb2CO3, SiO2, and NiO or MnO (1573 K, 1.5 h for the Ni compound; 1673 K, 2 h for the Mn compound) followed by crystallization at 1193 K (12 h).

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim The effects of thermo-mechanical auxetic foam conversion parameters on the Young's modulus and Poisson's ratio of open-cell polyurethane foam are related to changes in chemical bonding and cell structure. Applied volumetric compression, conversion temperature, and duration are reported on foam Young's modulus, Poisson's ratio, and structural stability. Fourier transform infrared spectral analysis on samples converted at and above 160 °C strongly indicates a hydrogen bond interaction increase in urea groups (C=O---H-N) and an increase in hydrogen bonding population. Spectral changes inferred soft segment degradation following extensive heat exposure (200 °C, 180 min), specifically a shift and intensity change in CH2 and C-O-C polyol bands and a broad baseline increase between 3600 and 2400 cm−1. These changes are linked to (i) resistance to dimensional recovery over time and during re-heating; (ii) Poisson's ratio becoming negative, then zero in tension or marginally positive in compression; (iii) Young's Modulus reducing then increasing; (iv) mass loss, evidenced by thermogravimetric analysis increasing from 150 °C. The minimum mean values of Poisson's ratio of ≈−0.2 (to 10% compression/tension) are comparable to other studies. All samples that retain their imposed compression over time are isotropic, with near constant Young's moduli and Poisson's ratio (to 10% compression/tension).

In situ, high-temperature energy-dispersive X-ray powder diffraction (EDXRD) data have been collected on synthetic and a natural sample of mercury sulphide (HgS). These measurements were made between temperatures of 295 and 798 K. Synthetic samples of HgS were prepared by reaction between sulphide and mercury in aqueous solution. In a subsequently dried and aged synthetic HgS sample, heated in vacuo, there is a change from a poorly crystalline pseudocubic material into a well crystalline cubic material in the temperature region 583–623 K. At higher temperature (748 K), there is evidence for a partial phase transition to the high temperature hypercinnabar HgS structure. In a neoformed synthetic sample, heated in a sealed Ti container, the initial ‘pseudocubic’ metacinnabar phase partially transforms to a previously unknown phase (XHgS) in the temperature range 467–522 K. This phase disappears at 527 K, and the metacinnabar phase changes to a well crystalline cubic phase; cinnabar develops at 542 K. The proportion of cinnabar continues to increase up to 647 K. Both metacinnabar and cinnabar phases are retained on cooling. No phase transitions were observed for the natural cinnabar sample.

The crystal structures of the leucite analogues Cs2MgSi5O12, Cs2ZnSi5O12 and Rb2MgSi5O12 have been determined by synchrotron X-ray powder diffraction using Rietveld refinement in conjunction with 29Si 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 Cs2CdSi5O12, thus a framework with five ordered Si T sites and one ordered Cd T site was used as the starting model for refinement. 29Si MAS NMR shows five distinct Si T sites for Cs2MgSi5O12 and Rb2MgSi5O12, but six Si T sites for Cs2ZnSi5O12. The refined structures for Cs2MgSi5O12 and Rb2MgSi5O12 were determined with complete T-site ordering, but the refined structure for Cs2ZnSi5O12 was determined with partial disorder of Mg and Si over two of the T sites.

The chemical composition oft he 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 ofjohnbaumite and synthetic Sr-, Ba- and Pbarsenate 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 ofF and Pb (~0.9 and ~4.4 wt.% respectively) and hedyphane has the ideal composition (formula given above); the compositions of these coexisting phases define the two limbs ofa 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 Pbphosphate 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)].

The bacterium Geobacter sulfurreducens can produce nanoparticulate magnetite (Fe3O4) by the reduction of amorphous Fe(III) oxyhydroxide coupled to the oxidation of organic matter in the anoxic subsurface as an alternative to oxygen respiration. G. sulfurreducens can transfer electrons to solid Fe(III)-bearing minerals through either direct contact between the cell and the mineral surface or by using an electron shuttling compound. High resolution synchrotron X-ray powder diffraction has been used to study samples taken at different stages of this reaction. This shows that an initial amorphous phase first transforms to goethite (FeO(OH)), before undergoing a further transformation to magnetite. Magnetite is formed faster in the presence of the electron shuttling compound disodium anthraquinone 2,6 disulphonate.

We have examined alkaline sulfidic (0.5–0.003 M Na 2 S), aqueous solutions of Hg(II)-S complexes (4–370 ppm Hg(II)) by Hg L III edge EXAFS spectroscopy at 296, 348 and 423 K. Data were collected using the ID26 High Brilliance X-ray Spectroscopy beamline at the ESRF. Analysis of these EXAFS spectra shows Hg coordinated by two S atoms at 2.30 A; multiple scattering analyses reveal a linear [–S–Hg–S–] arrangement in the solution complex. These results are in agreement with earlier results on more concentrated solutions of these complexes. There is also evidence in the data for polynuclear sulfide complexes at 296 K and 348 K for samples with the lowest sulfide concentrations although this is complicated by multiple scattering effects.

The focus of this study consists of examining how simultaneous SR-XRD and electrochemical measurements can provide information on the effectiveness of stabilization and storage treatments of copper artefacts in aqueous solution. The electrochemical cell used here was designed for in situ, time resolved SR-XRD studies of corrosion and inhibition studies on cultural heritage materials. Key objectives of the new cell were to monitor corrosion layers on alloys with realistic metallographic structures and to obtain co-incident, time resolved, electrochemical data such as reduction measurements, oxidation measurements and corrosion potential (Ecorr) measurements. Here we present some early results from the cell. Firstly, a correlated SR-XRD and corrosion potential (Ecorr) study of the reduction of nantokite during storage in sodium sesquicarbonate, which shows that the surface chemistry continues to change after Ecorr has stabilized. Secondly, the use of X-ray data to identify specific changes occurring as a function of potential in the forced reduction of a more complex system. Keywords: Spectroelectrochemistry, SR-XRD, Corrosion, Copper, Cultural heritage, Conservation

Transition metal oxoborates are of interest for magnetic and optical properties. Crystals of many M II 2 M III O 2 BO 3 and M II M III OBO 3 materials can be grown from borate fluxes. In the Fe II -Fe III -B-O system, flux growth results in Fe 3 O 2 BO 3 crystals, but solid state reaction at higher temperatures has yielded Fe 2 OBO 3 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 2+ ) 0.5 (Fe 3+ ) 0.5 valence is observed at the two crystallographically distinct sites in Fe 2 OBO 3 , indicating that charge ordering occurs.

Charge-ordering is an important phenomenon in conducting metal oxides: it leads to metal–insulator transitions1 in manganite perovskites (which show ‘colossal’ magnetoresistances), and the Verwey2 transition in magnetite (in which the material becomes insulating at low temperatures when the conduction electrons freeze into a regular array). Charge-ordered ‘stripes’ are found in some manganites3,4 and copper oxide superconductors5; in the latter case, dynamic fluctuations of the stripes have been proposed6 as a mechanism of high-temperature superconductivity. But an important unresolved issue is whether the charge-ordering in oxides is driven by electrostatic repulsions between the charges (Wigner crystallization7), or by the strains arising from electron–lattice interactions (such as Jahn–Teller distortions) involving different localized electronic states. Here we report measurements on iron oxoborate, Fe2OBO3, that support the electrostatic repulsion charge-ordering mechanism: the system adopts a charge-ordered state below 317 K, in which Fe2+ and Fe3+ ions are equally distributed over structurally distinct Fesites. In contrast, the isostructural manganese oxoborate, Mn2OBO3, has been previously shown8 to undergo charge-ordering through Jahn–Teller distortions. We therefore conclude that both mechanisms occur within the same structural arrangement.

The structure of a synthetic end-member wairakite (CaAl2Si4O12·2H2O) has been determined using Rietveld analysis of high-resolution, synchrotron 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 Al atoms [Q4(2Al) silicons]. Tetrahedral and cavity cation site disorder are coupled so that Al mainly occupies T2 sites, with Ca in M12A and M12B being balanced by Al in T12A and T12B; T11A and T11B sites appear to only contain Si, in agreement with the M11 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.

Sulphosalts in the system CuSbS2-CuBiS2(chalcostibite-emplectite) form a complete solid solution series. Seven compositions with the general formula Cu(SbxBi1–x)S2have been synthesized using dry methods at 310°C. All members of the series are orthorhombic (space groupPnma) and show smoothly increasingaandbcell parameters with substitution of Bi for Sb; theccell parameter increases up to 50% CuBiS2substitution and then becomes constant. DSC experiments on CuBiS2show 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% CuSbS2to 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 CuSbS2component was >30%. The local structure and co-ordination of Cu in the samples were studied by EXAFS analysis of the Cu-Kedge 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 CuSbS2tends 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 alongcappear to be controlled by the linkages between the (CuS3+ BiS2) sheets perpendicular tocbeing relatively inflexible.

Sr2TiO(Si2O7), the Sr analogue of the mineral fresnoite Ba2TiO(Si2O7), is prepared by quenching a stoichiometric melt of SrCO3, TiO2, and SiO2 followed by annealing at 1323 K (7 d).

Lattice parameter of metastable high-pressure phase of zinc oxide, rock-salt ZnO was measured in the 10-300 K temperature range using synchrotron X-ray powder diffraction. No phase transition was observed down to 10 K. The lattice parameter of rock-salt ZnO was found to increase from 4.266 {\AA} in the 10-80 K range up to 4.2752(3) {\AA} at 298 K, while the volume thermal expansion coefficient increases from slight negative values below 40 K up to 4.77\times10^-5 K^-1 at 298 K., Comment: 4 pages, 3 figures, 1 table

Structures were detd. by the Rietveld method for synthetic analogs of the natural mineral leucite (KAlSi2O6) studied by high-resoln. synchrotron x-ray powder diffractometry. MAS-NMR and electron diffraction were also used to provide structural information. The samples were prepd. by heating a mixt. of appropriate amts. of high-purity amorphous alkali metal carbonate and divalent metal oxide in a Pt crucible and then quenching in liq. N to form a glass. The powd. glass was heated in air at ambient pressure for dry synthesis and was heated under pressure with water in a hydrothermal synthesis. The crystal structure of dry synthesized Cs2CuSi5O12 is Ia3d cubic. Hydrothermally synthesized Rb2MgSi5O12, Rb2CdSi5O12, and Cs2MnSi5O12 analogs have the Cs2CdSi5O12 Pbca structure. Hydrothermally synthesized Cs2CuSi5O12, Cs2ZnSi5O12, and Cs2MgSi5O12 have a previously unknown P42/ncm structure. [on SciFinder(R)]

The crystal structure of barium oxalate hydrate, BaC204.2H20, was reinvestigated using a synchrotron X-ray powder pattern and a single crystal X-ray data set. The compound is monoclinic, a = 7.538(l), b = 11.062(l), c = 7.105(1) angstroms, beta = 105.42(l)“, space group P21/c, with Z = 4. The precision of the structure arrived at with the synchrotron X-ray powder pattern is not as high as that obtained with the single crystal X-ray data. The barium atoms are bonded to the. oxalate ions and to the water molecules with nine barium oxygen- bonds in the range 2.71(l)-2.97(l)angstroms. The structure has hydrogen bonds in the range 2.72(2)-2.91(2)angstroms, and the oxalate ion has only a minor deviation from planarity.

The P\ to Pile phase transition in Cui. ΛZnxWO4 has been investigated by observing structural changes across the solid solution. The crystal structures of samples with x = 0.85, 0.55 and 0.30 have been refined by the Rietveld method from high-resolution synchrotron X-ray diffraction data. The Cu/Zn and W atoms are in distorted octahedral coordination. The microscopic order parameter (defined in terms of changes in Cu/Zn-O bond lengths) behaves in a similar manner to the previously-recorded macroscopic spontaneous strain. Rigid-unit-mode analysis shows that the transition does not involve distortions of a framework of rigid octahedra. Rather, the co-operative Jahn-Teller distortion of the CuO6 octahedra is identified as the driving mechanism, with WO6 octahedral distortions occurring as a secondary process.

A new lithium zinc orthophosphate, δI-LiZnPO4, was synthesized from aqueous solution. Information from powder diffraction, using synchrotron radiation, and 31P MAS NMR spectroscopy allowed determination of the structure. The structure is orthorhombic (a = 10.0193(1), b = 4.9657(1), c = 6.6746(1) A space group Pna21 , No. 33, Z = 4. The structure may be seen as built from corner-sharing ZnO4- and PO4- tetrahedra forming a cristobalite-type framework structure. The lithium ions are 4-coordinated and situated in the 6-ring channels. The thermal transformations were investigated by thermogravimetric measurements (TG), differential thermal analysis (DTA), and high-temperature powder diffraction (HT-PXD). A transformation at ∼575°C is clearly visible on the Guinier Simon film (HT-PXD). This transformation is characterized by some disappearing lines in the powder pattern. The high-temperature polymorph of δI-LiZnPO4 is named δII-LiZnPO4. Phase transformations at ∼730 and δ1003°C are observed by HT-PXD and DTA, respectively. These observations are designated to the transformation to β-LiZnPO4 and from β- to γ-LiZnPO4.

The fresnoite-type compound Sr2TiO(Si2O7), distrontium oxidotitanium disilicate, has been prepared by high-temperature solid-state synthesis. The results of a Rietveld refinement study, based on high-resolution synchrotron X-ray powder diffraction data, show that the title compound crystallizes in the space group P4bm and adopts the structure of other fresnoite-type mineral samples with general formula A2 TiO(Si2O7) (A = alkaline earth metal cation). The structure consists of titanosilicate layers composed of corner-sharing SiO4 tetra-hedra (forming Si2O7 disilicate units) and TiO5 square-based pyramids. These layers extend parallel to the ab plane and are stacked along the c axis. Layers of distorted SrO6 octa-hedra lie between the titanosilicate layers. The Sr(2+) ion, the SiO4 tetra-hedron and the bridging O atom of the disilicate unit are located on mirror planes whereas the TiO5 square-based pyramid is located on a fourfold rotation axis.

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

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

High resoln. synchrotron x-ray powder diffraction, in conjunction with magic angle spinning 29Si NMR and electron diffraction, were used to det. structures of synthetic analogs based on the 3 dimensional silicate framework of the natural mineral leucite (KAlSi2O6). These structures were refined by the Rietveld method. The structure detns. of dry and hydrothermal synthesized K2MgSi5O12 analogs illustrate the techniques used, and the structural differences between analogs are discussed in terms of cation sizes and synthetic conditions. [on SciFinder(R)]

It is shown by magnetometry and μSR spectroscopy that short-range magnetic interactions between the Mn cations in the nonmetallic K2NiF4-like phase La0.8Sr1.2Mn0.6Rh0.4O4 become significant below ∼200 K. Negative magnetoresistance (ρ/ρ(0) ∼ 0.5 in 14 T at 108 K) is apparent below this temperature. Neutron diffraction has shown that an applied magnetic field of 5 T is sufficient to induce saturated (3.38(7)μB per Mn) long-range ferromagnetic ordering of the atomic moments at 2 K, and that the induced ordering persists up to a temperature of 50 K in 5 T. Spin glass behavior is observed below 20 K in the absence of an applied field. The induced magnetic ordering is attributed to the subtle changes in band structure brought about by the external field, and to the controlling influence of Rh3+ over the relative strength of competing magnetic exchange interactions.

Multi heavy atom cluster compounds like K2PtBr6 offer a way forward to solve, de novo, unknown protein structures by powder diffraction involving dispersive (measured at two X-ray wavelengths) and isomorphous intensity differences as a complement to micro-crystallography or by using both approaches in combination. Towards this end, using the ESRF high resolution synchrotron X-ray powder diffraction beamline ID31, we have recorded high quality protein powder diffractograms at the platinum LIII and bromine K absorption edges, as well as reference wavelengths, for K2PtBr6 bound to lysozyme. These experiments were conducted at 80K to protect the sample against X-radiation damage as much as possible and also to fix the K2PtBr6 bound state, which seemed to show instability at room temperature. With multiple powder pattern analysis we extracted intensities and showed the PtBr6 bound in lysozyme using ‘omit electron\ud density maps’. In addition the wavelength dispersive Fourier\ud around the Br K edge shows up the bromine signal at\ud PtBr6 binding site 1 in one of the six samples tested. To\ud better understand the chemical properties of this heavy\ud atom compound we have elucidated the detailed binding\ud behaviour using single crystal analyses with time-resolved\ud freeze quenching after soak times of 10, 90 and 170 minutes.\ud Whilst the quick soaking of 10 to 30 minutes, used\ud at ESRF ID31 shows clear binding there is increasing\ud binding strength with increasing soak time. Thus, these\ud time-resolved analytical chemistry results show that further\ud heavy atom signal optimizations are possible. Prospects\ud for extending our approach to the yet larger isomorphous\ud and wavelength dispersive signal case of Ta6Br12 bound to\ud lysozyme are also described.

The crystal structure of a hydrothermally synthesized leucite analogue Cs2CuSi5O12 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 Cs2CdSi5O12, and exhibits five ordered Si sites and one ordered Cu tetrahedrally coordinated (T) site. This structure for Cs2CuSi5O12 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_12_12. The crystal structure of a dry-synthesized leucite analogue Cs2CuSi5O12 has also been refined; this has the Ia\bar 3d cubic pollucite structure with disordered T sites.

The apatite-type compound Ba5(AsO4)3Cl, pentabarium tris[arsenate(V)] chloride, has been synthesized by ion exchange at high temperature from a synthetic sample of mimetite (Pb5(AsO4)3Cl) with BaCO3 as a by-product. The results of the Rietveld refinement, based on high resolution synchrotron X-ray powder diffraction data, show that the title compound crystallizes in the same structure as other halogenoapatites with general formula A 5(YO4)3 X (A = divalent cation, Y = pentavalent cation, X = Cl, Br) in space group P63/m. The structure consists of isolated tetrahedral AsO4 3− anions (m symmetry), separated by two crystallographically independent Ba2+ cations that are located on mirror planes and threefold rotation axes, respectively. The Cl− anions are at the 2b sites (\overline{3} symmetry) and are located in the channels of the structure.

The crystal structures and phase behaviour of phase II and the metastable phases III′ and III of cyclohexanol, C6H11OH, 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 P\bar 42_1c (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 P21/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.

The adducts formed between various aryl-substituted nitrones and aryl isocyanates have been shown, using 15N n.m.r. spectroscopy and X-ray crystallography, to be substituted 1,2,4-oxadiazolidinones and not the previously reported 1,3,4-oxadiazolinones.

The nature of the nickel sulfides formed in low temperature\ud aqueous solutions is not well-understood. The material has some\ud intrinsic interest to mineralogy, geochemistry and materials science\ud as well as to biogeochemisty, especially as a possible catalyst\ud involved in the origin and early evolution of life.\ud We synthesized Ni sulfide under anoxic conditions at 25 C: (1)\ud chemically, by the addition of 50 mL of 0.1 M NiSO4Æ7H2O to\ud 100 mL of 0.05M Na2SÆ9H2O; (2) electrochemically, with a Ni\ud foil and H2S gas. At pH 6 5, millerite (b-NiS) was produced electrochemically\ud and NiS mixtures, including heazlewoodite (Ni3S2)\ud and polydymite (Ni3S4), were obtained chemically. At pH >11, a-\ud NiS was obtained from the chemical reaction. At pH 6–9, the\ud product produced only two broad peaks (d = ca. 2.7 and 1.8 )\ud with conventional and synchrotron XRPD which could be\ud assigned to a number of Ni sulfides. It has previously been\ud referred to as ‘‘amorphous NiS’’ Jeong and Manthiram, 2001.\ud Eight SAED reflections were collected which identified the material\ud as godlevskite, orthorhombic NiS. HRTEM shows that the\ud godlevskite particles are ca. 30 nm in diameter and plate-like.\ud SAXS analyses show that the material is 6–8.5 nm thick.\ud Godlevskite is structurally related to makinawite, tetragonal\ud FeS, and is found naturally in similar parageneses-associated with\ud the monosulfide solid solution products of high temperature nickel\ud ores. Mackinawite is the black FeS precipitate from the reaction\ud between Fe(II) and S(-II) in aqueous solution. It appears that,\ud geochemically, godlevskite is the Ni analogue of mackinawite.

The adducts formed between various aryl-substituted nitrones and aryl isocyanates have been shown, using 15N n.m.r. spectroscopy and X-ray crystallography, to be substituted 1,2,4-oxadiazolidinones and not the previously reported 1,3,4-oxadiazolinones.