Synthesis and structure-property relationships in selected metal fluorides
Abstract
There has been an increase in the interest in fluoride materials over the
last decade. This interest has focused on multiferroic materials and kagome
lattices, to name but a few areas. This thesis focuses on the synthesis and
crystallographic characterisation of selected transition metal fluorides and
oxyfluorides. Work is presented on the tetragonal tungsten bronze solid
solutions of K[subscript(x)]FeF₃, where x = 0.58 and x ≈ 0.5, and the copper analogue,
K₃Cu₃Fe₂F₁₅; the kagome structure of Cs₂ZrCu₃F₁₂; and hydrothermal reactions
using vanadium, manganese, or molybdenum as the transition metals in the
formation of new fluorides and oxyfluorides.
The tetragonal tungsten bronze compounds K[subscript(x)]FeF₃ (x = 0.58 and x ≈ 0.5)
are both tetragonal at 500 K. In the variant with the lower K-content, there is a
clear phase separation into two tetragonal phases even at this temperature. The
K₀.₅₈FeF₃ sample separates into two distinct phases below 340 K to possess one
tetragonal and one orthorhombic phase. Then at roughly 300 K, both samples
undergo a phase transition where the tetragonal phase in the P4/mbm space
group in K₀.₅₈FeF₃ changes to an orthorhombic phase with a larger unit cell; and
the tetragonal phase in P4₂bc for the K₀.₅FeF₃ sample changes to the same
orthorhombic model, whilst the P4/mbm model remains unchanged. The
evolution of the lattice parameters and phase fractions is studied in detail using
synchrotron powder X-ray diffraction (sPXRD).
The kagome structure investigated, Cs₂ZrCu₃F₁₂, possesses the “ideal”
kagome lattice at room temperature, but previous work has suggested that
there is a phase transition at 225 K. The two structures are determined by
single crystal X-ray diffraction at 300 K and 125 K. Variable temperature
sPXRD studies are performed between these two temperature ranges to
determine the phase evolution as a function of temperature. The structure
changes from a rhombohedral to a monoclinic phase at low temperature. This
is the result of the buckling of the kagome layers at the phase transition. The
Zr⁴⁺ ion changes from 6 to 7 coordinate and this is seen as the main driving
force for the distortion of the kagome layer from its “ideal” planar arrangement.
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The phase transition is first-order as seen from the electrical impedance
measurements.
The hydrothermal reactions presented reveal seven new materials and
their crystal structures. Sr₂V₂F₁₀·H₂O is new and found to be isostructural to
Sr₂Fe₂F₁₀·H₂O. BaVO₂F₃ is a cubic material that is potentially piezoelectric. Two
hybrid organic inorganic manganese compounds are reported. The ladder
structure (C₃N₂H₅)[Mn₂F₆(H₂O)₂] crystallises in a polar space group and shows
promise as a candidate for multiferroic studies. The second hybrid material,
(C₇NH₁₆)₂[MnF₅(H₂O)]·2H₂O, crystallises in a centrosymmetric space group.
The Mo hybrid materials are all centrosymmetric and possess isolated
molybdenum-centred monomeric or dimeric octahedral units.
Type
Thesis, PhD Doctor of Philosophy
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