Crystalline open-framework intermetallics have long attracted the attention of chemists, physicists, and materials scientists. The intriguing structures such materials exhibit are often intimately related to the unique physical properties they possess. The present work is focused on the preparation and characterization of open-framework intermetallic materials based on group 14 elements, in particular those crystallizing in clathrate and related structures such as the clathrate-II phases. Materials possessing the clathrate-II crystal structure have received increased attention in recent years, as a result of both the unique properties they exhibit as well as potential for use in technologically important applications such as thermoelectrics, photovoltaics, and optoelectronics. However, in comparison with other clathrate structure types, characterization of clathrate-II materials has in general been far less extensive. Moreover, many conceivable compositions have yet to be realized. The purpose of this work is to expand the current knowledge of the structural, chemical, and physical properties of these materials, while simultaneously exploring new compositions and synthetic routes to clathrate-II phases. One of the unique and promising aspects of clathrate-II materials is the ability to vary the guest concentration, which is shown to have significant implications for the structural and physical properties of NaxSi136 (0< x< 24) materials. It is demonstrated that new compositions can be explored by novel approaches to chemical design. Framework substitution in clathrate-II compounds is explored in an effort to assess possibilities for influencing the physical properties of these materials. A novel zeolite-like framework phase, Na1-xGe3+z, has been discovered, and is shown to be a new low-thermal conductivity crystalline solid, suggesting a new approach to the design of crystalline intermetallic materials with low thermal conductivity. New directions in synthesis of intermetallics are identified, with emphasis on unconventional preparative methods and the opportunities they offer. Processing of reactive precursors by spark plasma sintering is demonstrated as a new preparative tool for crystal growth, identifying the first method for preparation of clathrate-II Na24Si136single-crystals since the discovery of these compounds more than four decades ago.

Les alliages métalliques complexes (CMAs) sont des composés intermétalliques dont la structure cristallographique diffère de celle des alliages conventionnels par le nombre conséquent d'atomes dans la maille (jusqu'à plusieurs milliers d'atomes), généralement arrangés sous forme d'agrégats atomiques de haute symétrie. Ils sont prometteurs pour un certain nombre d'applications technologiques, en particulier les revêtements fonctionnels, en raison de leurs propriétés de surface uniques. Cette thèse a pour objectif, à la fois la détermination de la structure et des propriétés électroniques d’une surface d’un CMA de la famille des clathrates intermétalliques, et des propriétés de mouillage intrinsèques de plusieurs CMAs à base d’aluminium. Dans une première partie, nous nous sommes intéressés aux surfaces de bas indice (100) et (110) du clathrate Ba8Au5.25Ge40.75. Leurs structures atomiques et électroniques ont été déterminées en combinant des expériences de sciences des surfaces et des calculs basés sur la théorie de la fonctionnelle de la densité. La structure tridimensionnelle de Ba8Au5.25Ge40.75, formée d'un réseau de deux types de cages (structure hôte) à base de germanium et d’or, qui emprisonnent les atomes de Ba, induit une nanostructuration de la surface contrôlée par son orientation, puisque le type de cages préservées à la surface diffère pour les surfaces (100) et (110). Dans les deux cas, les atomes de Ba qui protrudent à la surface, ont un rôle primordial pour la stabilité de surface : ils assurent un transfert de charge qui sature les liaisons pendantes des atomes de germanium en surface. Dans une seconde partie, les propriétés intrinsèques de mouillage de plusieurs CMAs à base d’aluminium, ont été déterminées par une approche couplant des mesures de microscopie et des calculs ab initio. Expérimentalement, les angles de contact de gouttes de plomb (métal sonde) sur plusieurs surfaces de CMAs ont été systématiquement mesurés. Les angles précédents étant fonction, entre autres, de l’énergie interfaciale, des calculs d'énergie interfaciale ont été menés, d’une part avec un substrat d’un métal simple, Al(111), et d’autre part sur un substrat de CMA, Al13Co4(100). Les résultats obtenus mettent en évidence une forte influence de la structure de l’interface sur l’énergie interfaciale / Complex metallic alloys (CMAs) are intermetallic compounds possessing a large unit cell containing several tens to hundreds of atoms. Their structure can be described alternatively by the packing of highly symmetric atomic clusters. Clathrate (or cage) compounds are a new class of CMAs having a crystal structure described by a complex arrangement of covalently-bonded cages. The Ba8Au5.25Ge40.75 type-I clathrate is one such cage compound, whose bulk properties have been (and still are) extensively explored for thermoelectric applications. In fact, it is possible to tune the compound electronic structure by a fine control of its bulk composition. Regarding the properties of the Ba8Au5.25Ge40.75 surface, information remains scarce if not inexistent. However, it is known that the surfaces of CMAs often exhibit interesting surface properties. To this end, we have studied two low-index surfaces: BaAuGe(100) and BaAuGe(110) by a combination of experimental (XPS; LEED; STM) and computational (DFT) methods. Experimental results show no evidence for surface segregation and LEED patterns are consistent with (1x1) bulk terminations with no surface reconstruction. The interplay between the 3D nano-caged structure and 2D surfaces is investigated. We demonstrate that the surface structures of the two surfaces considered preserve the bulk structure cages in addition to an ordered arrangement of surface Ba atoms. The two surfaces are formed by a breakage of highly directional covalent bonds present within the framework, hence leading to destabilizing dangling bonds. Ab initio calculations show that the surface structure is stabilized through electron charge transfer from protruding Ba to surface Ge and Au atoms, saturating the dangling bonds. This charge-balance mechanism lifts the possible surface reconstruction envisaged. We reveal how the surface nanostructuration is surface orientation dependent. The results indicate that the surface electronic structure of BaAuGe(110) is impacted by the Au surface concentration. The surface models for BaAuGe(100) and BaAuGe(110) present a metallic character and low work function values, useful for further applications. Such structurally complex surfaces may also be used as templates for novel nanoscale architectures. Further in this work, we also applied the state-of-the-art surface science techniques to investigate the wetting properties of Al-based CMAs. In these experiments, chemically inert Pb element was used as a metal probe. Systematic analysis is done to find the correlation between the wetting properties and the electronic structure properties of these CMAs. Interfacial energy calculations have been performed to model the Pb/CMA interface based on few approaches reported in literature. We have tested these approaches on a moiré patterned Pb(111)/Al(111) interface. This interface is found to be controlled by geometric factors. Hence, an acquired understanding was applied to Pb deposited on Al13Co4(100) (Al-rich side) interface

Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: The first part of this study describes the synthesis of new porous materials from basic building blocks. Five structurally related ligands namely: N,N'-bis(3-pyridylmethyl)-naphthalene diimide (L1), N,N'-bis(4-pyridylmethyl)-naphthalene diimide (L2), N,N'-bis(4-pyridylmethyl)- pyromellitic diimide (L3), N,N'-bis(3-pyridylmethyl)-pyromellitic diimide (L4) and 2-(pyridin-4- ylmethyl)-benzene tricarboxylic anhydride (L5) were synthesised. Ligands L1 and L2 were reacted with metal nitrates and carboxylates as co-ligands in a systematic manner with a view to obtaining potentially porous 3–D coordination polymers. Ten structurally diverse coordination polymers were obtained and they were characterised by single-crystal X-ray diffraction, powder X-ray diffraction and thermogravimetric analysis. Four of these compounds absorb moderate amounts of CO2 and, in addition, show sorption selectivity towards CO2 over N2. The reaction of L3 and L4 with transition metal halides yielded two 1–D chains, while the reaction of L5 with transition metal nitrates yielded seven coordination polymers of which four are 2–D and three are 1–D. Of the 2–D structures three are isostructural. The second part of this work describes a variable pressure study of a flexible metal-organic framework [Zn2(BDC)2(BPY)] (BPY = 4,4 -bipyridine and BDC = 1,4-benzene dicarboxylic acid). [Zn2(BDC)2(BPY)] is one of the few examples of a flexible metal-organic framework that undergoes phase transformations in response to gas pressure. The high pressure sorption recorded for this metal-organic framework displays two inflection steps in the pressure range 0 to 30 bar, possibly indicating two phase transformations. The gas-loaded structures for each phase transformation were determined by means of single-crystal X-ray diffraction. High-pressure differential scanning calorimetry was also carried out on the system in order to determine accurate gate-opening pressures, as well as the energies involved with each phase transformation. The results correlate with those obtained from single-crystal X-ray diffraction and high-pressure sorption. The final section reports the mechanochemical synthesis of two Werner complexes [NiCl2(4- PhPy)4] (1), [CoCl2(4-PhPy)4] (2) and their corresponding solid solution [Ni0.5Co0.5Cl2(4-PhPy)4] (3) (PhPy = phenyl pyridine). The solid solution could only be formed by mechanochemical synthesis and not by conventional solution crystallisation methods. The solid solution exhibits sorption properties that differ from those of the pure compounds. / AFRIKAANSE OPSOMMING: Die eerste deel van hierdie studie beskryf die sintese van nuwe poreuse stowwe uit basiese boublokke. Vyf struktureel verwante ligande naamlik: N,N'-bis(3-piridielmetiel)-naftaleen diimied (L1), N,N'-bis(4-piridielmetiel)-naftaleen diimied (L2), N,N'-bis(4-piridielmetiel)- piromellitien diimied (L3), N,N'-bis(3-piridielmetiel)-piromellitien diimied (L4) en 2-(piridiel-4- ielmetiel)benseen trianhidried (L5) is gesintetiseer. Ligande L1 en L2 is gereageer met metaal nitrate en karboksielsure as mede-ligande in 'n sistematiese wyse met 'n oog op die verkryging van potensieel poreuse 3–D koördinasie polimere. Tien struktureel diverse koördinasie polimere is verkry en hulle is gekarakteriseer deur enkel-kristal X-straal-diffraksie, poeier X-straal diffraksie en termo-analise (thermal analysis). Vier van hierdie verbindings het matige hoeveelhede CO2 geabsorbeer en, bykomend, wys sorpsie selektiwiteit van CO2 oor N2. Die reaksie van L3 en L4 met oorgangsmetaalhaliede het twee 1–D kettings gevorm, terwyl die reaksie van L5 met oorgangsmetaal nitrate sewe koördinasie polimere opgelewer het, waarvan vier 2–D en drie 1–D polimere is. Van die 2–D polimere het drie vergelykbare strukture. Die tweede deel van hierdie werk beskryf 'n veranderlike druk studie van 'n buigsame metaalorganiese raamwerk [Zn2(BDC)2(BPY)] (BPY = 4,4-bipiridien en BDC = 1,4-benseen dikarboksielsuur). [Zn2(BDC)2(BPY)] is een van die min voorbeelde van 'n buigsame metaalorganiese raamwerk wat fase transformasies (phase transformations) ondergaan in respons op ‘n verandering in gas druk. Die hoë-druk sorpsie aangeteken vir hierdie metaal-organiese raamwerk vertoon twee infleksie stappe in die gebestudeerde druk gebied (0 tot 30 bar), wat moontlik op twee fase transformasies dui. Die gas-gelaaide strukture vir elke fase transformasie is bepaal deur middel van enkel-kristal X-straal-diffraksie. Hoë-druk differensiële skandeer kalorimetrie (differential scanning calorimetry) is ook uitgevoer op die stelsel ten einde dié akkurate hekopenings druk, sowel as die energie betrokke by elke fase transformasie te bepaal. Die resultate stem ooreen met dié verkry vanaf enkel-kristal X-straal diffraksie en hoë-druk sorpsie. Die finale afdeling bespreek die meganochemiese sintese van twee Werner komplekse [NiCl2(4-PhPy)4] (1) en [COCl2(4-PhPy)4] (2) en hul ooreenstemmende vaste oplossing (solid solution) [Ni0.5Co0.5Cl2(4-PhPy)4] (3). Die vaste oplossing kan slegs gevorm word deur meganochemiese sintese en nie deur konvensionele oplossing kristallisasie metodes. Die vaste oplossing vertoon sorpsie eienskappe wat verskil van dié van die suiwer verbindings.

No description available.

6,6’-Dimethoxygossypol (DMG) is a natural product of the cotton variety Gossypium barbadense and a derivative of gossypol. Gossypol has been shown to form an abundant number of clathrates with a large variety of compounds. One of the primary reasons why gossypol can form clathrates has been because of its ability to from extensive hydrogen bonding networks due to its hydroxyl and aldehyde functional groups. Prior to this work, the only known solvate that DMG formed was with acetic acid. DMG has methoxy groups substituted at two hydroxyl positions, and consequently there is a decrease in its ability to form hydrogen bonds. Crystallization experiments were set up to see whether, like gossypol, DMG could form clathrates. The following results presented prove that DMG is capable of forming clathrates (S1 and S2) and two new polymorphs (P1 and P2) of DMG have been reported.

Thesis (MTech (Chemistry))-- Cape Peninsula University Technology, 2012 / The inclusion behaviour of a series of hydroxyl hosts with a variety of liquid guests has been investigated. The host 9-(4-methoxyphenyl)-9H-xanthen-9-ol (A1), C20H16O3, forms inclusion compounds with aniline (ANI), 3-picoline (3PIC), morpholine (MORPH), Nmethylacetamide (NMA) and N-methylformamide (NMF). Their structures have been elucidated and correlated with their thermal behaviour. The inclusion compounds A1ANI and A1MORPH were successfully solved in space group P21/c, whereas A13PIC was solved in 𝑃ī. Non-isothermal kinetics of desolvation were performed for A13PIC and A1MORPH. The packing of A13PIC and A1MORPH is characterized by (Host)(Guest) hydrogen bonds, whereas A1ANI is stabilised by (Host)(Host) hydrogen bonding. Three structures were obtained for the host A1 and the guest N-methylacetamide, with structural formulas of C20H16O3 C3H7NO (A1NMA), C20H16O3 2C3H7NO (A12NMA) and 2C20H16O3 2C3H7NO (2A12NMA). The packing of A1NMA, A12NMA and 2A12NMA are characterized by (Host)-OHO-(Guest) and (Guest)-NHO-(Guest) hydrogen bonds, which gave hydrogen bonding patterns of 𝐶2 2(7), 𝐶3 3(11) and 𝐶4 2(11) respectively. The hydrate A1NMFH2O was successfully solved in the triclinic space group 𝑃ī. The A1NMFH2O hydrogen bond pattern may be described according to Etter’s notation as 𝑅4 2(8) and 𝑅6 6(16). The host 9-(3-methoxyphenyl)-9H-xanthen-9-ol (A2), C20H16O3, forms inclusion compounds with morpholine (A2MORPH), N-methylacetamide (A2NMA) and N-methylformamide (A2NMF), with host-guest ratios 1:1. The crystal structure of the apohost was solved in Pbca with Z=8. The structures of A2MORPH and A2NMF were solved in 𝑃ī, whereas A2NMA was solved in P21/n. The packing of these structures is stabilised by (Host)(Guest) hydrogen bonds. The host 5-(4-methoxyphenyl)-5H-dibenzo[a,d]cyclohepten-5-ol (A26), C22H18O2, forms inclusion compounds with aniline (A26ANI) and morpholine (A26MORPH). A26MORPH and A26ANI crystallised in the space groups Pc and 𝑃ī respectively. The packing of these structures are characterized by (Host)-OHO-(Host) hydrogen bonding. A guest exchange reaction was performed. The host compounds 5-(4-chlorophenyl)-5H-dibenzo[a,d]cyclohepten-5-ol (C21H15OCl), 5-[3(trifluoromethyl)phenyl]-5H-dibenzo[a,d]cyclohepten-5-ol (C22H15OF3) and 5-(naphthalen-1-yl)-5H-dibenzo[a,d]cyclohepten-5-ol (C25H18O) form inclusion compounds with morpholine. All three structures were solved in 𝑃ī with the host molecules hydrogen bonded to the morpholine guests.

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.

Clathrates are a class of new materials that have an open-framework structure that allows guest atoms or molecules to be enclosed inside of their polyhedral framework. Varying the number, weight, and size of the guest species in a particular framework allows one to alter the physical properties of the clathrate. This relationship enables one to further the fundamental understanding of the physics and chemistry of the clathrate structure and use this knowledge to "tune" certain properties. This "tunability" of inorganic clathrates is of great interest as it allows one to optimize their physical properties; making them promising candidates for a range of applications such as thermoelectric, optoelectronic, and superconductivity. In this study, new synthesis methods of type I and II clathrates of group 14 are introduced, along with two new compositions of type I clathrates. A new synthesis method used to produce single crystal and microcrystalline Na8Si46 and Na24Si136 clathrates by the spark plasma sintering technique is introduced. Microcrystalline type I Na8Si46 and type II Na24Si136 are also selectively synthesized with no phase impurity of the other type using the low temperature ionic liquid method. In addition, the synthesis of microcrystalline Na8Ge3Si38 and single crystal Ba2Cs6Ga8Sn38 type I clathrates are presented for the first time.

The hydrogen molecule dynamics in tetrahydrofuran-H2-H2O clathrate hydrate has been studied by high-resolution inelastic neutron scattering and Raman light scattering. Several intense bands in the neutron spectrum are observed that are due to H2 molecule excitations. These are rotational transitions, center-of-mass translational transitions (rattling) of either para- or ortho-H2, and combinations of rotations and center-of-mass transitions. The rattling of the H2 molecule is a paradigmatic example of the motion of a quantum particle in a non-harmonic three-dimensional potential well. Both the H2 rotational transition and the fundamental of the rattling transition split into triplets. Raman spectra show a similar splitting of the S0(0) rotational transition, due to a significant anisotropy of the potential with respect to the orientation of the molecule in the cage. The comparison of our experimental values for the transition frequencies to a recent quantum mechanical calculation gives qualitative agreement, but shows some significant difference.