Structural and physical properties of low-dimensional chalcogenides

Abstract

This thesis describes work on three families of low-dimensional chalcogenides. The Jamesonite-Benavidesite series has been investigated to observe the effects of chemical substitution as iron is replaced by manganese. A series of materials with the general formula Fe1‑xMnxPb4Sb6S14 (0 ≤ x ≤ 1) has been synthesised for analysis. Structural investigations of the highly anisotropic structure using powder neutron diffraction have been performed at temperatures from 2 K up to room temperature. The magnetism has been studied with low temperature powder neutron diffraction and SQUID magnetometry. Benavidesite (x = 1) has been shown to have antiferromagnetic order at 2 K with moments of 2.58(9) μB. The electrical transport properties indicate materials are p-type semiconductors with activation energies in the range 177.2 meV to 626 meV. Work on a cobalt Shandite series, Co3Sn2-xInxS2 (0 ≤ x ≤ 2), enables tuning of the Fermi level within narrow Co d-states. This results in a compositionally induced double metalto- semiconductor-to-metal transition and manipulation of the electronic and thermal transport properties. A narrow semiconducting region exists between compositions of 1 ≤ x ≤ 1.05. There are two main group sites in the structure: one in an intra-layer trigonal antiprismatic site and one in a kagome layer. Powder neutron diffraction has been used to investigate ordering of indium and tin across the two main-group sites. X-ray Photoelectron Spectroscopy and 119Sn Mössbauer spectroscopy have been used to determine the oxidation states of the elements in the material. Results suggest that cobalt, tin, indium and sulphur are all in the 0 oxidation state. The thermoelectric properties have been measured and the figure of merit for Co3Sn1.2In0.8S2 and Co3Sn1.15In0.85S2 (ZT = 0.28) are the highest reported for a sulphide at 425 K. The sulphide In2Sn3S7 has been prepared as a bulk polycrystalline phase for the first time. Powder neutron diffraction has been undertaken to study partial ordering of tin and indium over the MS6 (M = Sn, In) octahedral sites within the structure. Sites were observed to contain approximately two-thirds indium and one-third tin. The structural analogue Cr2Sn3Se7 has also been synthesised. The electrical resistivity, Seebeck coefficient and thermal conductivity have been measured for both samples. The previously unreported Cr2Sn3S7 phase (Space Group: P21/m, a = 11.2945(6) Å, b = 3.6456(2) Å, c = 12.2473(7) Å, β = 105.352(2) °) has been synthesised as well as a new ternary chromium tin selenide with the approximate stoichiometry Cr2Sn5Se11.