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Title: Structural, magnetic and electron transport properties of mixed metal sulphides
Author: Engin, Tahir
ISNI:       0000 0001 3448 0450
Awarding Body: Heriot-Watt University
Current Institution: Heriot-Watt University
Date of Award: 2008
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The construction of cells that enables the electrical resistance of a material to be measured simultaneously with the collection of powder neutron diffraction data at a range of temperatures (4.2 :5 TIK:5 1000) is described. The cells have been used to investigate the origin of the anomalous electron-transport properties of the ternary sulphides, TICu6.ssS4, BaCu4S3 and CuFeS2. Data for TICu6.ssS4 reveal that the anomaly observed at 255 K is associated with a structural change from P41n to 141m on cooling due to a distortion involving a doubling of the e-axis of the unit cell. There,are .~/ two resistance anomalies observed in BaCu4S3. The first at 523 K is associated' with a phase transition from a two-phase mixture (Pllma + Cmem) to a Cmem phase, whilst the second anomaly can be correlated with the decomposition of the 'sample at 923 K. Variable temperature powder neutron diffraction reveals that CuFeS2 adopts a tetragonal antiferromagnetically ordered structure up to 748 K, above which it consists of a mixture of two phases until TN = 823 K, where a loss of magnetic order is coincident with a transformation to a cubic structure. Resistance anomalies are observed at each of those temperatures. A new family of vanadium-substituted chromium sulphides (VxCr2-xS3, 0 < x < 2) has been prepared and characterized by powder X-ray and neutron diffraction, SQUID magnetometry, electrical resistivity, and Seebeck coefficient measurements. Vanadium substitution leads to a single-phase region with a rhombohedral Cr2S3 structure over the composition range 0.0 < x:5 0.75, while at higher vanadium contents (1.6 :5 x < 2.0) a second single-phase region, in which materials adopt a cation-deficient Cr3S4 structure, is observed. Materials with the Cr2S3 structure all exhibit semiconducting behavior whilst those adopting Cr3S4-type structure exhibit metallic behavior. Materials in both single phase regions undergo a magnetic ordering transition at temperatures in the range 90 - 118 K. Materials with the Cr2S3 structure undergo a transition to a ferrimagnetic state, the nature of which has been determined by low-temperature neutron diffraction measurements. Structural studies of the mixed-metal sulphides, FexMnl_xS (0 :5 x :5 0.4) were performed using powder neutron diffraction. Rietveld refinements show that all samples adopt a cubic rock-salt structure (F'm3m) at room temperature. The Feo.2Mno.sS sample is essentially pure phase whilst materials with higher Fe content contain 3CFe7SS (21.3% in Feo.4Mno.6S). Low-temperature neutron diffraction reveals that all materials are magnetically ordered at temperatures in the range 180 - 185 K, below Which they show a structural distortion. The magnetic structure consists of ferromagnetically aligned layers, with antiferromagnetic coupling of adjacent layers. Electron-transport property measurements reveal that all the samples are Arrhenius semiconductors with activation energies that decrease with increasing Fe content.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available