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|Title: ||Preparation and characterisation of complex metal chalcogenides and halides.|
|Authors: ||Armstrong, Jennifer A.|
|Issue Date: ||2004|
|Publisher: ||© Jennifer Ann Armstrong|
|Abstract: ||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.|
|Description: ||A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University|
|Appears in Collections:||PhD Theses (Chemistry)|
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