Structural and physical properties of selected complex chalcogenides
Stephenson, Alexandra Kate
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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) 6+ 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.