A series of Pt(S₂N₂)(P(OR)[subscript n]R′[subscript(3-n)])₂ complexes were prepared and analysed using ³¹P NMR and IR spectroscopy, elemental analysis and X-ray crystallography. Similarly, a series of Pt(SeSN₂)(P(OR)[subscript n]R′[subscript(3-n)])₂ complexes were also prepared and analysed. The ¹J[subscript(Pt-P)] coupling constants and Pt-P bond lengths for these complexes are influenced by the oxygen content of their phosphorus ligands. The ³¹P NMR spectra for a series of [Pt(S3N)(P(OR)[subscript n]R′[subscript(3-n)])₂][BF₄] complexes are also reported. Planar [S₂N₂H]⁻ complexes were prepared and the X-ray crystal structure of [Pd(S₂N₂H)(bipy)][Cl] is reported. The X-ray structures of MX₂(P(OR)[subscript n]R′[subscript(3-n)])₂ are reported and compared with the previously reported analogues. The magnitude of the ¹J[subscript(Pt-P)] varies linearly with the Pt-P bond length (l[subscript(Pt-P)] = 2.421 – J/24255) for the 12 platinum-containing complexes. This correlation is compared to that of a larger series of complexes. A series of M(ndsdsd₂ (ndsdsd = bis[(nitrilo(diphenyl)-λ⁶-sulfanyl)](diphenyl)-λ⁶-sulfanediimide (Ph₂S(=N-(Ph₂)S≡N)₂)) complexes were prepared and characterised using elemental analysis and multinuclear NMR and IR spectroscopy where appropriate. The X-ray crystal structures of five examples are reported.

Neste trabalho foram estudados os fenômenos fotoinduzidos apresentados pelos vidros calcogenetos de composições: Ga IND.10Ge IND.25S IND.65 e Ga IND.5Ge IND.25As IND.5S IND.65. Estes vidros ao serem iluminados com luz que possui energia próxima a do bandgap apresentam vários fenômenos fotoinduzidos. Sendo assim um estudo sistemático em função da potência, tempo de iluminação e do comprimento de onda foi efetuado. Para energias acima da banda proibida ocorre a fotoexpansão, variação do índice de refração e fotoclareamento. Por outro lado, para energias abaixo da banda proibida não foi observada nenhuma variação do volume; no entanto foi detectado um fotoescurecimento. Estes fenômenos mostraram ser irreversíveis, visto que ao serem levados a tratamento térmico a temperatura próxima a temperatura de transição vítrea, os vidros não recuperaram sua forma original. Várias técnicas experimentais foram usadas tais como: microscopia de força atômica, perfilometria, espectro de absorção óptica, espectroscopia na região do infravermelho, espalhamento Raman, EDX, EXAFS e RBS para caracterização dos fenômenos fotoinduzidos e elucidar o mecanismo envolvido. Os resultados obtidos a partir de EXAFS, Infravermelho, RBS e EDX indicaram a incorporação de oxigênio na superfície dos vidros quebrando assim as ligações Ge-S. Foi observado que a magnitude da fotoexpansão depende da atmosfera usada durante a irradiação. Um estudo comparativo entre o vidro e o filme depositado foi realizado. Foi mostrado através de espalhamento Raman e Infravermelho que os tipos de ligações presente no filme depositado é diferente daquela do vidro original. Os mesmos fenômenos fotoinduzidos foram também detectados nos filmes depositados, os quais foram estudados e caracterizados também de maneira sistemática. Como aplicação destes fenômenos fotoinduzidos, a fotoexpansão foi usada para a produção de redes de difração. ) As medidas de eficiência de difração e as imagens de microscopia de força atômica demonstraram que a fotoexpansão cria uma rede de relevo na superfície do vidro / We report the photoinduced phenomena observed in two chalcogenide glass compositions: Ga IOGe2SS6eSGasGe2sAssS6s. These glasses present several photoinduced phenomena when exposed to light having energy comparable to bandgap energy. Systematic study has been carried out in function of power density, exposure time and wavelength. Samples exposed to energy above the bandgap the photoexpansion, photorefraction and photobleaching has been observed. Otherwise, to energy below the bandgap a photodarkening was detected without volume variation. These phenomena are irreversible since the surface does not restore the original structure when annealing to the glass transition temperature. To Characterize and understand the mechanism processes of the photoexpansíon effect, atomíc force mícroscopy, perfilometry, absorption spectra, ínfrared absorption, Raman, EDX, EXAFS and RBS has been used before and after illumination of the glass samples. The EXAFS, infrared, RBS and EDX data showed that íllumínatíon leads to an introduction of oxygen ín the glass structure breaking Ge-S intermolecular bonds followed by the formatíon of Ge-O bonds. We observed that the magnitude of the photoínduced expansíon of the GaGeS glass ís strongly dependent on the atmosphere used. Comparative study has been performed between glass and filmo Infrared and Raman data shown a different bonding behavior ín the film prepared prevíously from the glass. Under irradíation, the same photoínduced phenomenon already detected on the glass samples are observed on the thin film and also characterized systematically. As applícation of the photoinduced phenomenon, photoexpansíon effect has been used to produce díffraction gratíngs. Atomic mícroscopy images and diffractíon efficiency data indícate that photoexpansíon leads to relíef gratíng on the glass surface

Os calcogenetos têm atraído atenção devido à variedade de propriedades físicas e químicas que exibem, apontando para sua utilização em muitas aplicações tecnológicas, incluindo a possibilidade de se obter novos materiais bidimensionais. Os calcogenetos quaternários A2MIIMIV3Q8, onde A = K, Cs; MII = Mg, Zn, Cd, Hg; MIV = Ge, Sn; Q = S, Se, Te, possuem uma grande variabilidade de band gaps e portanto eles podem ser estudados para engenharia de band gap através de mudanças na composição química. Além disso, dois tipos de estruturas cristalinas são observados nessa família, um formado por empilhamento de camadas, e outro definido por uma rede tridimensional fechada. Assim, é importante entender os fatores que afetam a estabilidade de estruturas em camadas desses compostos complexos. Nesse trabalho, os materiais A2MIIMIV3Q8 são estudados com cálculos de teoria do funcional da densidade, usando funcionais de troca e correlação semi-local e híbrido, e correções de van der Waals. Os parâmetros de rede variam com a composição conforme o esperado com base no raio atômico. A redução do número atômico de um dos componentes, principalmente Q, aumenta a energia de coesão, devido à intensificação das interações iônicas. Os resultados de energia de ligação entre camadas demonstram a importância das interações de van der Waals, e os valores são similares aos reportados na literatura para vários materiais. Seguindo a tendência de funcionais semi-locais, os band gaps são subestimados, mas cálculos com o funcional híbrido fornecem valores mais apropriados. Os resultados mostram a diversidade de band gaps e uma correlação aproximadamente linear entre band gap e volume da célula unitária. O band gap é principalmente afetado pela mudança do calcogênio, em que o aumento do número atômico diminui o band gap, devido ao aumento da energia dos estados p de Q. As análises dos coeficientes de absorção óptica e elementos de matriz de transição mostram que não existe diferença significativa entre band gap fundamental e óptico nesses materiais. O estudo de estabilidade relativa das estruturas em 9 compostos, com diferentes A e Q, mostra que os raios atômicos têm um importante papel. A estrutura sem formação de camadas é favorecida comparada com as estruturas em camadas apenas na região de raios intermediários, o que é explicado com base na diminuição das tensões na estrutura e em interações coulombianas entre íons da rede. / Chalcogenides have attracted attention due to the variety of physical and chemical properties which they display, pointing to their use in many technological applications, including the possibility to obtain new bidimensional materials. The quaternary chalcogenides A2MIIMIV3Q8, where A = K, Cs; MII = Mg, Zn, Cd, Hg; MIV = Ge, Sn; Q = S, Se, Te, have a large variability of band gaps and therefore they can be studied for band gap engineering through changes in the chemical composition. Futhermore, two types of crystal structure are observed in this family, one formed by the stacking of layers, and the other defined by a closed three dimensional framework. Thus, it is important to understand the factors that affect the stability of layered structures of these complex compounds. Here, the materials A2MIIMIV3Q8 are studied with density functional theory calculations, using semi-local and hybrid exchange-correlation functionals, and van der Waals corrections. Lattice parameters vary with composition according to expected based on the atomic radius. The reduction of the atomic number of one of the components, mainly Q, increases the cohesive energy, due to the intensification of the ionic interactions. The results of interlayer binding energies demonstrate the importance of van der Waals interactions, and the values are simillar to those reported in the literature for several materials. Following the trend of semi-local functionals, band gaps are underestimated, but hybrid functional calculations provide more accurate values. The results show the diversity of band gaps and an approximate linear correlation between band gap and unit cell volume. The band gap is mainly affected by changing the chalcogen, in which the increase of the atomic number decreases the band gap, due to the increase in the energy of Q p states. The analysis of optical absorption coefficients and transition matrix elements show that there is no significative difference between fundamental and optical band gap in these materials. The study of relative stability of the structures in 9 compounds, with different A and Q, shows that the atomic radii have an important role. The structure without layer formation is favored compared with the layered structures only in the region of intermediate radii, which is explained based on the reduction of strain in the structure and coulomb interactions between ions in the framework.

Os calcogenetos têm atraído atenção devido à variedade de propriedades físicas e químicas que exibem, apontando para sua utilização em muitas aplicações tecnológicas, incluindo a possibilidade de se obter novos materiais bidimensionais. Os calcogenetos quaternários A2MIIMIV3Q8, onde A = K, Cs; MII = Mg, Zn, Cd, Hg; MIV = Ge, Sn; Q = S, Se, Te, possuem uma grande variabilidade de band gaps e portanto eles podem ser estudados para engenharia de band gap através de mudanças na composição química. Além disso, dois tipos de estruturas cristalinas são observados nessa família, um formado por empilhamento de camadas, e outro definido por uma rede tridimensional fechada. Assim, é importante entender os fatores que afetam a estabilidade de estruturas em camadas desses compostos complexos. Nesse trabalho, os materiais A2MIIMIV3Q8 são estudados com cálculos de teoria do funcional da densidade, usando funcionais de troca e correlação semi-local e híbrido, e correções de van der Waals. Os parâmetros de rede variam com a composição conforme o esperado com base no raio atômico. A redução do número atômico de um dos componentes, principalmente Q, aumenta a energia de coesão, devido à intensificação das interações iônicas. Os resultados de energia de ligação entre camadas demonstram a importância das interações de van der Waals, e os valores são similares aos reportados na literatura para vários materiais. Seguindo a tendência de funcionais semi-locais, os band gaps são subestimados, mas cálculos com o funcional híbrido fornecem valores mais apropriados. Os resultados mostram a diversidade de band gaps e uma correlação aproximadamente linear entre band gap e volume da célula unitária. O band gap é principalmente afetado pela mudança do calcogênio, em que o aumento do número atômico diminui o band gap, devido ao aumento da energia dos estados p de Q. As análises dos coeficientes de absorção óptica e elementos de matriz de transição mostram que não existe diferença significativa entre band gap fundamental e óptico nesses materiais. O estudo de estabilidade relativa das estruturas em 9 compostos, com diferentes A e Q, mostra que os raios atômicos têm um importante papel. A estrutura sem formação de camadas é favorecida comparada com as estruturas em camadas apenas na região de raios intermediários, o que é explicado com base na diminuição das tensões na estrutura e em interações coulombianas entre íons da rede. / Chalcogenides have attracted attention due to the variety of physical and chemical properties which they display, pointing to their use in many technological applications, including the possibility to obtain new bidimensional materials. The quaternary chalcogenides A2MIIMIV3Q8, where A = K, Cs; MII = Mg, Zn, Cd, Hg; MIV = Ge, Sn; Q = S, Se, Te, have a large variability of band gaps and therefore they can be studied for band gap engineering through changes in the chemical composition. Futhermore, two types of crystal structure are observed in this family, one formed by the stacking of layers, and the other defined by a closed three dimensional framework. Thus, it is important to understand the factors that affect the stability of layered structures of these complex compounds. Here, the materials A2MIIMIV3Q8 are studied with density functional theory calculations, using semi-local and hybrid exchange-correlation functionals, and van der Waals corrections. Lattice parameters vary with composition according to expected based on the atomic radius. The reduction of the atomic number of one of the components, mainly Q, increases the cohesive energy, due to the intensification of the ionic interactions. The results of interlayer binding energies demonstrate the importance of van der Waals interactions, and the values are simillar to those reported in the literature for several materials. Following the trend of semi-local functionals, band gaps are underestimated, but hybrid functional calculations provide more accurate values. The results show the diversity of band gaps and an approximate linear correlation between band gap and unit cell volume. The band gap is mainly affected by changing the chalcogen, in which the increase of the atomic number decreases the band gap, due to the increase in the energy of Q p states. The analysis of optical absorption coefficients and transition matrix elements show that there is no significative difference between fundamental and optical band gap in these materials. The study of relative stability of the structures in 9 compounds, with different A and Q, shows that the atomic radii have an important role. The structure without layer formation is favored compared with the layered structures only in the region of intermediate radii, which is explained based on the reduction of strain in the structure and coulomb interactions between ions in the framework.

Thesis (Ph.D. (Engineering mechanics)) --University of Limpopo, 2004 / This thesis explores the important issues associated with the insertion of Mg2+ and Li+ into the solid materials: molybdenum sulphide and titanium disulphide. This process, which is also known as intercalation, is driven by charge transfer and is the basic cell reaction of advanced batteries. We perform a systematic computational investigation of the new Chevrel phase, MgxMo6S8 for 0 ≤ x ≤ 2, a candidate for high energy density cathode in prototype rechargeable magnesium (Mg) battery systems. Mg2+ intercalation property of the Mo6S8 Chevrel phase compound and accompanied structural changes were evaluated. We conduct our study within the framework of both the local-density functional theory and the generalised gradient approximation techniques. Analysis of the calculated energetics for different magnesium positions and composition suggest a triclinic structure of MgxMo6S8 (x = 1 and 2). The results compare favourably with experimental data. Band-structure calculations imply the existence of an energy gap located ~1 eV above the Fermi level, which is a characteristic feature of the electronic structure of the Chevrel compounds. Calculations of electronic charge density suggest a charge transfer from Mg to the Mo6S8 cluster, which has a significant effect on the Mo-Mo bond length. There is relatively no theoretical work, in particular ab initio pseudopotential calculations, reported in literature on structural stability, cations "site energy" calculations, and pressure work. Structures obtained on the basis from experimental studies of other ternary molybdenum sulphides are examined with respect to pressure-induced structural transformation. We report the first bulk and linear moduli of the new Chevrel phase structures. This thesis also studies the reaction between lithium and titanium disulfide, which is the perfect intercalation reaction, with the product having the same structure over the range of reaction 0  x  1 in LixTiS2. Calculated lattice parameters, bulk moduli, linear moduli, elastic constants, density of states, and Mulliken populations are reported. Our calculations confirm that there is a single phase present with an expansion of the crystalline lattice as is typical for a solid solution, about 10% perpendicular to the basal plane layers. A slight expansion of the lattice in the basal plane is also observed due to the electron density increasing on the sulfur ions. Details on the correlation between the electronic structure and the energetic (i.e. the thermodynamics) of intercalation are obtained by establishing the connection between the charge transfer and lithium intercalation into TiS2. The theoretical determination of the densities of states for the pure TiS2 and Li1TiS2 confirms a charge transfer. Lithium charge is donated to the S (3p) and Ti (3d) orbitals. Comparison with experiment shows that the calculated optical properties for energies below 12 eV agrees well with reflectivity spectra. The structural and electronic properties of the intercalation compound LixTiS2, for x = 1/4, 3/4, and 1, are also investigated. This study indicates that the following physical changes in LixTiS2 are induced by intercalation: (1) the crystal expands uniaxially in the c-direction, (2) no staging is observed. We also focus on the intercalation voltage where the variation of the cell potential with the degree of discharge for LiTiS2 is calculated. Our results show that it can be predicted with these well-developed total energy methods. The detailed understanding of the electronic structure of the intercalation compounds provided by this method gives an approach to the interpretation of the voltage composition profiles of electrode materials, and may now clearly be used routinely to determine the contributions of the anode and cathode processes to the cell voltage. Hence becoming an important tool in the selection and design of new systems. Keywords Magnesium rechargeable battery; Chevrel, Lithium batteries; Li and Mg-ion insertion; TiS2; Mo6S8; Charge transfer; reflectivity, intercalation, elastic constants, voltage, EOS, Moduli. / the National Research Foundation, the Royal Society(U.K),the Council for Scientific and Industrial Research,and Eskom

Thermoelectric materials play an important role in energy conversion as they represent environmentally safe and solid state devices with a great potential towards enhancing their efficiency. The ability to generate electric power in a reliable way without using non-renewable resources motivates many experimentalists as well as computational physicists to search and design new thermoelectric materials. Several classes of materials have been identified as good candidates for high efficient thermoelectrics because of their inherently low thermal conductivity. The complex study of the crystal and electronic structures of such materials helps to reveal hidden properties and give fundamental understanding, necessary for the development of a new generation of thermoelectrics. In the current thesis, ab-initio computational methods along with experimental observations are applied to investigate several material classes suitable for thermoelectric applications. One example are Bi-Sb bismuth rich alloys, for which it is shown how structural anomalies affect the electronic structure and how inclusion of the Spin-Orbit coupling is necessary for this type of materials. Another example are bournonite materials whose low thermal conductivity is attributed to distortions and interactions associated with lone-electron s^2 pair distributions. In addition, it is shown how doping with similar atoms can affects electronic structure of these materials leading to changes in their transport properties. Clathrate materials from the less studied type II Sn class are also investigated with a detailed analysis for their structural stability, electronic properties and phonons. These systems are considered with partially substituted atoms on the framework and different guests inside. The effect upon insertion of Noble gases into the cage network is also investigated. In addition, the newly synthesized As based cationic material is also studied finding novel structure-property relations. Another class of materials, quaternary chalcogenides, have also been studied. Because of their inherently low thermal conductivity and semiconducting nature their transport properties may be optimized in a favorable way for thermoelectricity. The present work provides an in-depth study of structural and electronic properties of several classes of materials, which can be used by experimentalists for input and guidance in the laboratory.

In this work, patterns in the behavior of different classes and types of thermoelectric materials are observed, and an alchemy that could help engineer a highly efficient thermoelectric is proposed. A method based on cross-correlation of Seebeck waveforms is also presented in order to capture physics of magnetic transition. The method is used to compute Curie temperature of LaCoO3 with an accuracy of 10K. In total, over 26 systems are analyzed, and 19 presented: Chalcogenides (PbSe, PbTe, Sb2Te3, Ag2Se), Skutterudites and Clathrates (CoSb3, SrFe4Sb12, Cd (CN)2, CdC, Ba8Ga16Si30*), Perovskites (SrTiO3, BaTiO3, LaCoO3, CaSiO3, Ce3InN*, YCoO3*), Half-Heuslers (ZrNiSn, NbFeSb, LiAlSi, CoSbTi, ScPtSb*, CaMgSi*), and an assorted class of thermoelectric materials (FeSi, FeSi2, ZnO, Ag QDSL*). Relaxation time is estimated from experimental conductance curve fits. A maximum upper bound of zT is evaluated for systems that have no experimental backing. In general, thermoelectric parameters (power factor, Seebeck coefficient and zT) are estimated for the aforementioned crystal structures. Strongly correlated systems are treated using LDAU and GGAU approximations. LDA/GGA/L(S)DA+U/GGA+U approach specific errors have also been highlighted. Densities of experimental results are estimated.

Thesis (Ph. D.)--University of Missouri-Columbia, 2000. / Typescript. Vita. Includes bibliographical references (leaves 139-143). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2000. / Typescript. Vita. Includes bibliographical references (leaves 139-143). Also available on the Internet.

This thesis deals with the synthesis and characterization of Ge-Te-Sb (GST) thin films. The films were deposited using a Pulsed Laser Deposition (PLD) method and mainly characterized with XRD, SEM, AFM and TEM. For amorphous and polycrystalline films, un-etched Si(100) was used. The amorphous films showed a similar crystallization behavior as films deposited with sputtering and evaporation techniques. When depositing GST on un-etched Si(100) substrates at elevated substrate temperatures (130-240°C), polycrystalline but highly textured films were obtained. The preferred growth orientation was either GST(111) or GST(0001) depending on if the films were cubic or hexagonal. Epitaxial films were prepared on crystalline substrates. On KCl(100), a mixed growth of hexagonal GST(0001) and cubic GST(100) was observed. The hexagonal phase dominates at low temperatures whereas the cubic phase dominates at high temperatures. The cubic phase is accompanied with a presumed GST(221) orientation when the film thickness exceeds ~70 nm. Epitaxial films were obtained with deposition rates as high as 250 nm/min. On BaF2(111), only (0001) oriented epitaxial hexagonal GST films are found, independent of substrate temperature, frequency or deposition background pressure. At high substrate temperatures there is a loss of Ge and Te which shifts the crystalline phase from Ge2Sb2Te5 towards GeSb2Te4. GST films deposited at room temperature on BaF2(111) were in an amorphous state, but after exposure to an annealing treatment they crystallize in an epitaxial cubic structure. Film deposition on pre-cleaned and buffered ammonium fluoride etched Si(111) show growth of epitaxial hexagonal GST, similar to that of the deposition on BaF2(111). When the Si-substrates were heated directly to the deposition temperature films of high crystal-line quality were obtained. An additional heat treatment of the Si-substrates prior to deposition deteriorated the crystal quality severely. The gained results show that PLD can be used as a method in order to obtain high quality epitaxial Ge-Sb-Te films from a compound target and using high deposition rates.