Preparation and characterisation of complex metal chalcogenides and halides.
thesisposted on 23.08.2013, 11:13 by Jennifer A. Armstrong
Various compounds with host structures based on geomaterials have been prepared using solid state methods (900-1200 0c), low pressure hydrothermal conditions (<220 °C, <1800 psi) or using purpose-built high pressure equipment <800°C, < 5500Opsi). Each material was characterized using various combinations of powder X-ray and neutron diffraction, single crystal X -ray diffraction, IR spectroscopy, Mossbauer spectroscopy and magnetometry. The iron end member, danalite, of the helvite family of minerals with the formula Feg[BeSi04]6S2 has been prepared for the first time. Isovalent substitution of the sulphide anion has allowed both selenide and telluride materials to be prepared. These compounds crystallize with the sodalite structure in the space group P-43n and exhibit an ordered framework constructed from alternating Be04 and Si04 tetrahedra containing a tetrahedral cluster of iron(II) ions around a central chalcogenide. Mossbauer measurements curiously show that, despite an elongated tetrahedral geometry of three oxide and one cha1cogenide around the iron ions, the valence and lattice contribution to the isomer shift oppose each other leading to zero quadrupole moment at room temperature for the sulphide material. Magnetic and polarized neutron experiments show that the interactions within the tetrahedral metal clusters of divalent ions are predominantly antiferromagnetic. In the sulphide, the clusters behave as totally isolated clusters showing no magnetic interaction across the cages. As the distance between the clusters shortens, as in the case of the selenide, threedimensional interactions between the clusters are observed. A systematic study of an extensive series of compounds in the helvite series, (Mn,Fe,Zn,Co)8[BeSi04]6X2;X = S, Se and Te has been carried out. All diffraction data of the compounds have been refmed using the Rietve1d method and very accurate structural parameters obtained. Correlations between parameters such as the beryllium and silicon tilt angles, framework bond angle and bond lengths as well as the IR absorptions have been drawn for the beryllosilicate sodalite frameworks. The theoretical maximum and minimum cell parameters for a sodalite framework, based on ordered Be04 and Si04 tetrahedra, has been determined.Transition metal boracites with the formula M3B7013X; M = Co, Zn, Mn, X = Cl, Br and I have been prepared using a borate flux. Their stoichiometries have been determined using the Mohr method and Rietveld refinement of powder X-ray diffraction data. In contrast to compounds prepared using hydrothermal methods, the halide site is not heavily substituted by hydroxide. A temperature dependant study of Mg3B7013CIo.78(OH)o.22 is also presented. Magnetic measurements have confirmed dual weak ferromagnetic and antiferromagnetic properties in the iodide materials A series of LnSF and Ln2AF4S2 materials have been synthesized using high temperature sealed tube methods. The missing member of the LnSF series, DySF, has been prepared for the first time. Precise structural data on the LnAF4S2 compounds where Ln = La, Ce, Dy and Er have been gleaned from the Rietveld refinement of powder diffraction data. . In contrast to previous studies, the lanthanide has been shown as preferentially distributed over the two LnlCa sites and the fluoride displaced from a special site to allow pseudo seven coordination for the smaller lanthanides. High-pressure methods have failed to produce the calciurnlgermanium based zeolites such as gismondine or the unusual mineral, bemalite. A new complex phosphate of the formula NaMllf;(P207 )2(P301O), containing the triphosphate unit, has been prepared at high pressure and the structure determined using single crystal methods. Using low pressure hydothermal methods, the full series of lanthanide hydroxide Ln(OH)3 have been prepared. Structure refinement has shown a contraction in the Ln-O bond distance as the series is transversed.