A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, October, 2015. / Harvesting renewable energy and the miniaturisation of electronic components are among the major challenges of the 21st century. Transition metal chalcogenides (TMC) have interesting properties that are promising in meeting these challenges. It is therefore important to conduct a systematic theoretical study of the structural, electronic and optical properties of the transition metal chalcogenides as possible components of low dimensional transistors or as solar-energy harvesters. In this work, we present the detailed theoretical investigation of the structural, electronic and optical properties of transition metal chalcogenides MyXz, (where M = Hf, Zr, Tc or Re), (X = S, Se and Te), y and z are integers. The structural properties of TMCs were studied using energy-volume relationship (equation of states (EOS)), equilibrium structural lattice parameters, formation and cohesive energies were extracted from the EOS. Mechanical stability test based on elastic constants and phonon dispersion relation were carried out to determine the strengths of the TMCs against mechanical distortions. The most stable structural congurations were used to investigate electronic properties through partial density of states (PDOS) and band structure analysis. Optical properties (absorption coe cients, refractivity, re ectivity) of some of the TMCs were then computed. Our computations of the structural, electronic and the optical properties were based on density functional theory (DFT). Projector-Augmented wave (PAW) was used to mimic electron-ion interactions and generalised-gradient approximation was used in the exchange correlation functional. Van der Waal's correction terms proposed by Grimme (DFT-D2), Lundqvist and Langreth (vdW-DF) and Tkatchenko- Sche er (vdW-TS) were used to account for long range dispersion forces in addition to PBE and its modi ed version for solids PBEsol. Optical properties were investigated at the many body (GW) and Bethe-Salpeter equation (BSE) levels of approximations. Our results obtained are discussed within the theoretical frame work and compared with experimental and previous theoretical results where available.

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Thesis (Ph. D.)--Lehigh University, 2004. / Includes bibliographical references and vita.

A thesis submitted to the Faculty of Science, in fulfilment of the requirements for the degree of Doctor of Philosophy. University of the Witwatersrand, Johannesburg May 23, 2016 / Amongst the major technological challenges of the twenty rst century is the harvesting of renewable energy sources. We studied the solar cell performance of the ternary compounds AgAlX2 (X = S, Se and Te) and AgInS2 as promising materials for meeting this challenge. Structural, electronic and optical properties of the compounds were investigated by means of the density functional theory and many body perturbation theory. Using cohesive energy and enthalpy, we found that among six potential phases of AgAlX2 and AgInS2, the chalcopyrite and the orthorhombic structures were very competitive as zero pressure phases. We predicted a low pressure-induced phase transition from the chalcopyrite phase to a rhombohedral phase. For the chalcopyrite phase, we found that the tetragonal distortion and anion displacement were the cause of the crystal eld splitting. The bandgaps from the general gradient approximation PBEsol were underestimated when compared to experiment and accurate bandgaps were obtained from the hybrid functioanl HSE06, the meta-general gradient approximation MBJ and GW approximation. Optical absorption from the Bethe-Selpeter equation indicated the presence of bound exciton in AgAlX2. We estimated the solar cell performance of the compounds using the Shockley and Queisser model and the spectroscopy limited maximum e ciency approach. We found that apart from AgAlS2, the estimated theoretical e ciency of the other compounds was greater that 13 %.

There is evidence for the existence of non-classical bonding in several binary antimonides, selenides, and tellurides. Owing to such non-classical bonding, some of these solid materials exhibit exciting semiconducting and thermoelectric properties, which make them attractive from a technological view point. However, lack of efficiency is a serious limitation in most of those thermoelectrics. It is very crucial, hence, to find new materials with superior properties and understand the structure and bonding in such materials, in order to facilitate the fine-tuning of the physical properties. With this expectation, several quaternary barium copper chalcogenides are synthesized and characterized in the present study. The chalcogen elements, selenium tellurium, are used in various ratios, in order to understand and tune the binding interactions. Extensive single crystal x-ray diffraction studies are expected to reveal the minute details of the bonding interactions together with electronic structure calculation and physical property measurements. In addition, characterization techniques such as powder x-ray diffraction, electron microscopy, differential scanning calorimetry, thermopower and conductivity measurements are utilized. The ternary and quaternary chalcogenides, Ba₂Cu₄₋ₓSeyTe₅₋y were synthesized from the elements in stoichiometric ratios at 700°C, followed by annealing at 600°C. The ternary telluride Ba₂Cu₄₋ₓTe₅ crystallizes in a new structure type, space group C2/c, with lattice dimensions of a = 9.4428(6) Å, b = 9.3289(6) Å, c = 13.3028(8) Å, β = 101.635(1)°, V = 1147.8(1) Å3, for x = 0.75(1) (Z = 4). The corresponding selenide-telluride adopts another new, but strongly related, structure type, space group P4₁2₁2, with a = 6.5418(3) Å, c = 25.782(2) Å, V = 1103.3(1) Å3, for Ba₂Cu₃.₂₆₍₂₎Se₀.₇₂₉₍₈₎Te₄.₂₇₁ (Z = 4). Between 0.13 and 1.0 Te per formula unit can be replaced with Se, while the Cu content appears to vary only within 0.67 ≤ x ≤ 0.81 for Ba₂Cu₄₋ₓSeyTe₅₋y. Despite crystallizing in different crystal systems, the telluride and the selenide-telluride exhibit topologically equivalent structure motifs, namely chains of Cu(Se,Te)₄ tetrahedra with a Cu atom cis/trans chain as well as an almost linear Te atom chain. All these chalcogenides - as far as measured - are p-doped semiconductors, as determined by Seebeck coefficient and electrical conductivity measurements. Two new orthorhombic chalcogenides, Ba₂Cu₆₋ₓSeyTe₅₋y and Ba₂Cu₆₋ₓSyTe₅₋y were synthesized at 800°C. They are isostructural and crystallize in a new structure type, with space group Pbam. Ba₂Cu₆₋ₓSyTe₅₋y, with a = 9.6560(6) Å, b = 14.0533(9) Å, c = 4.3524(3) Å and Ba₂Cu₅.₆₄Se₁.₀₉Te₃.₉₁with a = 9.7048(6) Å, b = 14.1853(9) Å, c = 4.3840(3) Å. They have Cu6 units extending along c-axis, and two such units are interconnected by S or Se atoms along a-axis. These compounds are nonmetallic with low Seebeck coefficients. Two more new quaternary chalcogenides were uncovered, BaCu₅.₉₂₆₍₁₅₎SeTe₆ and BaCu₅.₇₂₍₁₆₎Se₀.₄₆₄₍₁₅₎Te₆.₅₃₆ with a = 6.9680(2) Å and a = 6.9888(4) Å, respectively, in space group Pm̅3. These compounds have basic Cu₈Te₁₂ frameworks, which can be an important feature for thermoelectric materials. Ba occupies the void. One Cu atom from each cage cluster of eight such cages forms a Cu₈ cube with Se atom occupying it. BaCu₅.₉SeTe₆ was experimentally determined to be p-type doped semiconductor with moderate Seebeck coefficient value.

There is evidence for the existence of non-classical bonding in several binary antimonides, selenides, and tellurides. Owing to such non-classical bonding, some of these solid materials exhibit exciting semiconducting and thermoelectric properties, which make them attractive from a technological view point. However, lack of efficiency is a serious limitation in most of those thermoelectrics. It is very crucial, hence, to find new materials with superior properties and understand the structure and bonding in such materials, in order to facilitate the fine-tuning of the physical properties. With this expectation, several quaternary barium copper chalcogenides are synthesized and characterized in the present study. The chalcogen elements, selenium tellurium, are used in various ratios, in order to understand and tune the binding interactions. Extensive single crystal x-ray diffraction studies are expected to reveal the minute details of the bonding interactions together with electronic structure calculation and physical property measurements. In addition, characterization techniques such as powder x-ray diffraction, electron microscopy, differential scanning calorimetry, thermopower and conductivity measurements are utilized. The ternary and quaternary chalcogenides, Ba₂Cu₄₋ₓSeyTe₅₋y were synthesized from the elements in stoichiometric ratios at 700°C, followed by annealing at 600°C. The ternary telluride Ba₂Cu₄₋ₓTe₅ crystallizes in a new structure type, space group C2/c, with lattice dimensions of a = 9.4428(6) Å, b = 9.3289(6) Å, c = 13.3028(8) Å, β = 101.635(1)°, V = 1147.8(1) Å3, for x = 0.75(1) (Z = 4). The corresponding selenide-telluride adopts another new, but strongly related, structure type, space group P4₁2₁2, with a = 6.5418(3) Å, c = 25.782(2) Å, V = 1103.3(1) Å3, for Ba₂Cu₃.₂₆₍₂₎Se₀.₇₂₉₍₈₎Te₄.₂₇₁ (Z = 4). Between 0.13 and 1.0 Te per formula unit can be replaced with Se, while the Cu content appears to vary only within 0.67 ≤ x ≤ 0.81 for Ba₂Cu₄₋ₓSeyTe₅₋y. Despite crystallizing in different crystal systems, the telluride and the selenide-telluride exhibit topologically equivalent structure motifs, namely chains of Cu(Se,Te)₄ tetrahedra with a Cu atom cis/trans chain as well as an almost linear Te atom chain. All these chalcogenides - as far as measured - are p-doped semiconductors, as determined by Seebeck coefficient and electrical conductivity measurements. Two new orthorhombic chalcogenides, Ba₂Cu₆₋ₓSeyTe₅₋y and Ba₂Cu₆₋ₓSyTe₅₋y were synthesized at 800°C. They are isostructural and crystallize in a new structure type, with space group Pbam. Ba₂Cu₆₋ₓSyTe₅₋y, with a = 9.6560(6) Å, b = 14.0533(9) Å, c = 4.3524(3) Å and Ba₂Cu₅.₆₄Se₁.₀₉Te₃.₉₁with a = 9.7048(6) Å, b = 14.1853(9) Å, c = 4.3840(3) Å. They have Cu6 units extending along c-axis, and two such units are interconnected by S or Se atoms along a-axis. These compounds are nonmetallic with low Seebeck coefficients. Two more new quaternary chalcogenides were uncovered, BaCu₅.₉₂₆₍₁₅₎SeTe₆ and BaCu₅.₇₂₍₁₆₎Se₀.₄₆₄₍₁₅₎Te₆.₅₃₆ with a = 6.9680(2) Å and a = 6.9888(4) Å, respectively, in space group Pm̅3. These compounds have basic Cu₈Te₁₂ frameworks, which can be an important feature for thermoelectric materials. Ba occupies the void. One Cu atom from each cage cluster of eight such cages forms a Cu₈ cube with Se atom occupying it. BaCu₅.₉SeTe₆ was experimentally determined to be p-type doped semiconductor with moderate Seebeck coefficient value.

Thesis (Ph.D.)--Auburn University, 2007. / Abstract. Vita. Includes bibliographic references.

Thesis (Ph.D.)--Victoria University of Wellington, 2009. / Includes bibliographical references.

Thesis (M.S.)--Boise State University, 2009. / Includes abstract. Includes bibliographical references (leaves 85-86).