|dc.description.abstract||The structure and physical properties of a selection of mixed-metal
chalcogenides have been investigated. Materials covered in this study have
shown a range of unusual behaviours.
GeV4S8 has been successfully synthesized in order to investigate the
structural properties as a function of temperature. These data reveal that there
are three distinct polymorphs over the temperature range 7 ≤ T / K ≤ 32, of
which one had not been previously determined. Neutron diffraction
experiments reveal a series of peak splittings and broadenings upon cooling
to indicate a deviation from the face-centred cubic structure at room
temperature. A reduction in symmetry to R3m, at TS = 32K is observed.
Antiferromagnetic ordering is then associated with a second successive phase
transition at TN = 17 K to Cm, which is a sub-group of R3m. This monoclinic
phase has previously not been reported.
A new series has been prepared through substitution of chromium with
indium in Cr1-xInxGeTe3 (0 ≤ x ≤ 1) to investigate the effects of diluting the
magnetic cation on the structural, physical and magnetic properties. CrGeTe3
crystallises in space group R3ത with a three-layer repeating unit. It consists of
hexagonally close packed layers of Te2- ions, with Cr3+ ions and (Ge2)
dumbbell pairs occupying octahedral holes in only every other layer. The
unoccupied layers persist throughout the series; however cationic ordering
within the layers of Te2- ions results in different stacking sequences.
Cr1-xInxGeTe3, x = 0, 0.1, 0.2, 0.5, 0.6 samples have a range of ferromagnetism
that can be described using Curie-Weiss law, but they also have regions of
spin glass dominated behaviour below the ‘freeze’ temperature, Tf.
Semiconducting data showed samples did not exhibit thermally activated
conduction but were found to fit a variable-range-hopping model that
represents electrical conductivity in three-dimensions. At x = 0, Seebeck
coefficients are large and positive, however for x = 1, markedly different
behaviour is observed. InGeTe3 undergoes a change in the dominant charge
carriers from electrons to holes at 245 K.||en