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Transition metal nanoclusters have attracted a significant attention in both theoretical and experimental studies due to their unique properties such as structural, electronic and magnetic. These properties are distinct, size dependent and ranges between those of bulk and single-particle species. Some of the systems sizes have been experimentally synthesized, which enables direct theory-experiment comparison. Other clusters that have been examined theoretically are of interest as models of larger systems. Often, the size dependence of their HOMO-LUMO (H-L) gap, optical properties, magnetic properties, etc., is of interest. In this study, we have performed a genetic algorithm search on the tight-binding interatomic potential energy surface (PES) for small TiN (N = 2–32) clusters. Lowest energy candidate clusters were further refined using density functional theory (DFT) calculations with the PBEsol exchange-correlation functional and evaluated with the PBEsol0 hybrid functional. The resulting clusters were analysed in terms of their structural features, growth mechanism and surface area. The results suggest a growth mechanism that is based on forming coordination centres by interpenetrating icosahedra, icositetrahedra and Frank–Kasper polyhedra. We identified centres of coordination, which act as centres of bulk nucleation in medium-sized clusters and determine the morphological features of the cluster. Molecular dynamics simulations were performed in order to investigate the impact of thermal agitation on the TiN (N = 7, 13, 17, 32, 57, 80 and 89). The calculations were carried out at 300 – 2400 K. The interatomic interactions for vacuum and inert gas environment were modelled using Gupta and Leonard-Jones potentials as implemented within the classical molecular dynamics simulation software DL_POLY. The total potential energy, radial distribution functions (RDF), diffusion coefficient, mean square displacement and density profiles were examined to study the structur, University of Limpopo (UL) University College London (UCL) Royal Society Newton’s Grant (NA140447) National Research Foundation (NRF) Titanium Centre of Competence (TiCoC)

In this work, we are studying the properties of CaS and CaO structures in both atomistic simulation and Density Functional Theory. Defects formation (vacancies, impurity and interstitial) will be mechanism studied by using atomistic simulation method. In this approach, Mott-Littleton method will be used since it is a good ap- proach of defects studies, and further explanation will be given on how the introduction of defects contribute on the stability of the bulk material. Diffusion of different atoms from one lattice site via interstitial path to vacancy lattice site, and how it segregates through the material, is also part of this study. The surface properties will be studied using both methods mentioned. Surface energies calculations of different surface layers (e.g. CaS (100), CaS (110), CaS (111), CaO (100), CaO (110) and CaO (111)) is the approach we used to determine the most stable surface. In atomistic simulation, we further studied how percentage coverage of atoms contributes on the stability of the surfaces. We further used Density Functional Theory to calculate surface energies of the above-mentioned surfaces. As in atomistic simulation method, we used surface ener- gies to determine the most stable surface. In DFT we used only the most stable surface of both CaS and CaO to study the adsorption of molecules, namely H2O, H2S, HS and S2 on CaO (100) and CaS (100). The most/least-adsorbed molecule on both surfaces is explained in this study.

The importance of gold for scientific uses is of fundamental importance to research and technology developments. The bulk gold shows reluctance to participate in chemical reactions, the effect which has been corrected by the change in the size towards nanoclusters. It is therefore imperative that the structure of gold nanomaterials is understood for better applications in catalysis and other developments. Molecular dynamics and the density functional theory have proven to be good tools in computational material science and have thus been used to greater lengths. Molecular dynamics simulations on different gold nanoclusters and nanotubes were successfully carried out at different thermodynamic conditions. The effect of size on the melting of materials was duly tested and our results to some extend agree with what has already been reported. Gold nanoclusters show melting below the bulk and the melting temperatures increase with cluster size. However, the Au55 cluster shows different results in that it melts above the bulk due to structural reconstruction. The structure of the clusters changes from spherical shapes to tetragonal or face centred cubic (fcc) structures. Gold nanotubes show no resistance to temperature and different configurations are obtained in different ensembles. Single wall nanotubes form spherical clusters in the NVT while the NPT conditions give patches of clusters at elevated temperatures. The multi wall nanotubes also form spherical clusters in the NVT but fcc structures are obtained in the NPT Berendsen ensemble towards melting. Ab initio calculations in DMOL3 code on different gold nanoclusters show the stability of the clusters to increase with size and the Au3 and Au8 clusters contain the most stable structures. The Au-Au bond length in the dimer was obtained to within reasonable agreement with experiments and other theoretical works. Doping of the clusters further improved their stability although different impurities give different observat, Mintek, and the National Research Foundation

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 mo, the National Research Foundation, the Royal Society(U.K),the Council for Scientific and Industrial Research,and Eskom

Self-consistent electronic structure calculations have been performed on ordered cubic-based magnesium-lithium (Mgx-Li1−x) alloys spanning the concentration range 0 ≤ x ≤ 1, using an ab initio plane wave pseudopotential (PWP) method. The first principle pseudopotential planewave approach is used within the local density approximation (LDA) and generalized-gradient approximation (GGA)of the density functional theory (DFT) framework. We have calculated the binding energy curves and the systematic trends in various cohesive and elastic properties at zero temperature, as a function of Li concentration. The calculated equilibrium lattice parameters show a large deviation from Vegard’s rule in the Li-rich region whilst the bulk moduli decrease monotonically with increase in Li concentration. The heats of formation for different ground state superstructures predict that the DO3, B2 and DO22 structures would be the most stable at absolute zero amongst various phases having the Mg3Li, MgLi and MgLi3 compositions, respectively. This stability is reflected in the electronic density of states (DOS). Because of the special significance of the isotropic bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio for technological and engineering applications, we have also calculated these quantities from the elastic constants. The elastic constants indicate the softness of the material as more Li is added with the bcc-based phases becoming mechanically less stable for Li concentration less than 50%. Our results show good agreement within the estimated uncertainty with both experimental and previous theoretical results., The National Research Foundation (NRF), South Africa-Royal Society (RS), Great Britain collaboration and Council for the Scientific and Industrial Research (CSIR)

Molecular dynamics simulations have been carried out in order to examine the mechanism of diffusion of molecules in amorphous polymer matrix. PDMS model was folded in to a periodic cell, generated by rotational isomeric state (RIS) method at a prescribed temperature and density. Molecular dynamics was used to study transport properties of cyclic PDMS oligomers (hexa-methylcyclotrisiloxane (D3), octa-methylcyclotetrasiloxane (D4) and deca-methylcyclopentasiloxane (D5) using Dreiding and COMPASS force fields. Diffusion coefficients were calculated from the Einstein relation. Only D3 penetrant reached the long time limit from which the Einstein relation is satisfied. Analysis of displacement versus time for all the penetrants in PDMS matrix indicates that the penetrant motion is characterized by relatively long periods interspersed with fairly long and small jumps. Transport of solvent molecules occurs by jumps between individual sections of free volume (cavity/hole) through temporarily open channels., The National Research Foundation (NRF) and Eskom

Planewave pseudopotential calculations were conducted to predict the energetics and phase stability of Pt-Cr and Ru-Cr binary alloys. Validation of appropriate number of k-points and planewave energy cut-off was carried out for all studied systems. At the composition of A3B and AB3 (where A = Cr and B = Pt or Ru) phases, the heats of formation determined for five different structures, L12, A15, tP16, DOC and DO′ C are almost of the same magnitude and the relaxed structures show no rotation. We observed that the cubic L12 Pt3Cr is the most stable structure in agreement with the experiments. The results for PtCr3 indicate the negative heat of formation for the A15 phase whereas all the remaining studied phases have positive heats of formation. It is clear that the PtCr3 (A15) is the most stable structure. PtCr (L10) was found to be more stable compared with PtCr (B2) phase. The L12 Pt3Cr, A15 PtCr3 and L10 PtCr phases could be considered as possible coatings to cover the engines which are exposed to aggresive environments. The heats of formation of all studied compositions and phases of Ru-Cr systems are positive, these results suggest that, generally, studied Ru-Cr phases are not stable. The effect of pressure and doping were investigated on A15 RuCr3 structure which was reported to exist at a higher temperature. Elastic constants and moduli were investigated to determine the strength of the PtCr systems. The strength of PtCr L10 is greater than that of B2 phase. The ratio of shear to bulk modulus (G/B) has been used to predict the ductility or the brittleness of the material. It was found that Pt3Cr L12 is the most ductile phase among those considered in this study. The density of states were calculated to further analyze the stability of systems. The magnetic properties of Cr were studied using VASP which predicted an anti-ferromagnetic and a non-magnetic ground state for pure Cr. We have investigated the thermal stability at 0 GPa for different phases of Pt3Cr, Pt, The National Research Foundation,and the South African Gas Turbine Research Program

The intention of the current study is to investigate structure, ion transport and reactivity of various forms of the cobalt pentlandite, Co9S8, at different temperatures using atomistic simulation methods with the support of electronic structure calculations. The first interatomic potentials of Co9S8 were derived with input data as structure and elastic properties from experiment and electronic structure calculations respectively. The potentials were validated by running energy minimization and molecular dynamics calculations. Structure, elastic properties and phonon spectra were well reproduced, together with the complex high temperature transformations and melting of Co9S8 as deduced from crystal structure, radial distribution functions, density profiles and diffusion coefficients. Amongst the high symmetry surfaces {111}, {101} and {101} atomistic surface energy calculations proposed the {111} surface of Co9S8 as the most stable in agreement with experimental morphologies, and water adsorption energies on the such surfaces which mostly agreed with those from electronic structure calculations. The structural and ion transport variations with temperature were investigated and predicted surface melting at lower temperatures than the bulk. The effects of hydration on the surfaces at low and high temperatures were also studied. The structural and ion transport properties of Co9S8 nanoparticles of varying sizes, covered by high symmetry surfaces {111}, {101} and {100} were predicted using molecular dynamics method based on our derived interatomic potentials. The structural and ion transport properties of Co9S8 nanoparticles of varying sizes, covered by high symmetry surfaces {111}, {101} and {100} were predicted using molecular dynamics method based on our derived interatomic potentials. Generally for {111}, {101} nanoparticles, high temperature transitions were abrupt for smaller nanoparticles and these tended to disintegrate and form voids. However, for larger nanopa, Anglo Platinum, National Research Foundation (South Africa), and The Royal Society (UK)

We have derived a potential model for °uorapatite Ca10(PO4)6F2, ¯tted to structure, elastic constants and vibrational frequencies of the phosphate groups, which is compatible with existing calcite and °uorite potential mod- els. We then modelled the structure and stabilities of the dry and hydrated f0 0 0 1g, f1 0 1 0g, f1 0 1 1g, f1 1 2 0g, f1 0 1 3g and f1 1 2 1g surfaces, which calculations con¯rmed the experimental dominance of the f0 0 0 1g surface, which is prominently expressed in the calculated thermodynamic morphologies. The dehydrated morphology further shows the experimental f1 1 2 1g twinning plane, while the f1 0 1 0g cleavage plane is expressed in the hydrated morphology. Molecular adsorption of water has a stabilising e®ect on all six surfaces, where the surfaces generally show Langmuir be- haviour and the calculated hydration energies indicate physisorption (73 - 88 kJ mol¡1). The chains of °uoride ions surrounded by hexagonal calcium channels can become distorted in two major ways during relaxation: either by a shortening/lengthening of the FF distances, when the channel is perpendicular to the surface, or by distortion of the CaF bonds when the channel is parallel to the surface. Both distortions occur when the channel runs at an angle to the surface. Other relaxations include compression of the calcium sub-lattice and rotation of surface phosphate groups. We have modelled adsorption of a range of organic molecules onto dif- ferent °uorapatite surfaces, due to the importance of organic/ inorganic in- teractions in biological situations. We have selected organic molecules that represent a model for the carboxylic acids, alkyl hydroxamates and those 3 that contain both the aldehyde and hydroxyl functional groups. Adhesion of these organic molecules on the surfaces has shown strong interaction between the surface's Ca ions and the molecule's oxygens, more especially the car- bonyl oxygens than any other interactions. It was found that the number of in, National Research Foundation of South Africa (NRF), Council for Scientific and Industrial research (CSIR), and the University of the North

We present first principles structural, electronic and optical properties investigation of PdS, which are carried out using density functional theory under plane wave pseudopotential method within the local density approximation. We used ultrasoft- pseudopotentials to carry out our calculations. Calculated lattice parameters of the system show excellent agreement with the experimental values. The lattice parameters were observed to decrease linearly with increasing pressure. The density of states and optical properties of PdS have been computed under hydrostatic pressure. The actual size of the band gap remains constant with increasing pressure, whilst the peaks just below and above the Fermi energy moves to the left and to the right respectively. We also investigated the effect of compositional variation on our reflectance by calculating the reflectivity of Pd4-xPtxS4 and Pd4-xNixS4. Since we have different positions for the same concentration, we used the heats of formation to determine the most stable structures and these structures were used to study the effect of compositional variation on our reflectance spectrum. We studied the equation of state (EOS), structure under hydrostatic pressure, and deduced the bulk modulus. It is important to study these properties under such extreme conditions of pressure and temperature as they tend to occur below the earth's surface. Investigation of stability and mechanical properties of binary and ternary compounds from PtS to PdS have been carried out, were the presence of the miscibility gap is still uncertain. We investigate stability of these compounds by studying the heats of formation, elasticity and electronic properties. Our results show no miscibility gap but continuum solid solution between these compounds. A shift of the Fermi energy towards the conduction band is observed at a 50% concentration of Pd and Pt. All the information obtained on PdS is intended to assist in fitting interatomic potentials to enable studi

In general, four main classes of technique have been employed in the current literature on nanoscaled energy materials: atomistic (static lattice), quantum mechanical (QM), global minimization (GM) and simulated amorphization and recrystallization (A+R) methods. A brief overview of two such methods, GM and simulated A+R, is given in this chapter. MnO2 seeds observed during nucleation and crystallization are discussed. The presence of the four to eight-membered ring structure alludes to similarity of structures whether spontaneously built bottom up from small clusters with a few atoms by GM or top down using A+R, which involves tens of thousands of atoms; this demonstrates agreement on predictive capabilities of evolutionary simulation methods at the nanoscale. Accordingly, at the nanoscale, the authors argue that the quenching of surface dipoles drives the evolution of particular polymorphs. This edition first published 2013 © 2013 John Wiley & Sons, Ltd.

Calculations were carried out on transition-metal sulphides (TMS) and transitionmetal arsenides (TMA), in both pyrite- and marcasite-type structures, using planewave (PW) pseudopotential methods within density functional theory (DFT) in the local density approximation (LDA). The structural, electronic and optical properties for both pyrite- and marcasite-type structures (naturally occurring and converted) have been investigated. The equilibrium lattice parameters were investigated and are in good agreement with the experimental values. The heats of formation calculations predict that the naturally occurring pyrite- and marcasite-type structures are more stable than the converted ones. In particular, the calculated pyrite-type RuS2 compares well to the experimental value (with energy difference of 0.381 eV/atom). The bulk modulus and elastic properties were calculated. The predicted anisotropic ratio shows that the naturally occurring pyrite- and marcasite-type structures are more stable than the converted ones. Moreover, the electronic density of states and band structure calculations reveal that most compositions shows semiconducting behaviour except for the converted pyritetype structures, i.e OsAs2 and RuAs2 where a metallic behaviour was observed. The electronic charge density and charge density difference show charge accumulation on bonding atoms, predicting the charge gain/ loss and nature of bonding to be covalent/ weak ionic between the atoms. Lastly, optical properties are computed at equilibrium and predict that naturally occurring structures have lower absorption and reflectivity than the converted structures. At different pressures ranging from -10 GPa to 10 GPa, the absorption and reflectivity spectra show a shift from the 0 GPa spectrum for all the structures, National Research Foundation

The structural and energetic properties of the Ti-Al and Pt-Ti alloys have been carried out using first-principles total energy calculation of the density functional theory. We found a good correlation between VASP and CASTEP calculations with the experimental data. The equilibrium lattice constants for both systems are in good agreement with the experimental values (within 3% agreement). Furthermore, the heats of formation were calculated in order to determine the relative structural stabilities of the Ti-Al and Pt-Ti alloys. We predict that the L10 TiAl is the most stable structure with the lowest heats of formation (more negative Hf) consistent with the experimental observations. The 50% composition of the PtTi SMA’s in particular B19/B19′ phases predict values to be closer to each other, with B19′ being the most stable phase. A comparison of the energy differences between different PtTi phases, yields the relative energies in the order B2>L10>B19>B19'. The elastic constants for B19/B19′ and L10 show the positive shear modulus while a negative shear modulus was observed for B2 phase (mechanical instability). Similarly, the phonon dispersions and the density of states for the B2, L10, B19 and B19′ PtTi shape memory alloys were calculated and are consistent with the heats of formation. The phonon dispersion curves revealed a softening of modes along high symmetry directions (M and R). This confirms that the B2 structure is less stable as compared to the other structures. The density of states for the 50% PtTi composition structures were also calculated and are consistent with the stability trend. Furthermore the transformation from B2-L10 was investigated using Bain’s path and the B2 and L10 phases were depicted at c/a=1 and c/a=√2 respectively., NRF & CSIR

The local environments of Cd and Pb cations in the system Cd1-xPbxF2 (x = 0.6) have been measured by means of EXAFS. A comparative analysis of the Fourier transforms of the experimental data provides evidence of the Pb-F bond length being longer than the Cd-F one. Furthermore, our studies show a loss of structure around the Pb(L(111)) edge with increasing temperature. This suggests that the temperature-induced vacancies on the anion sub-lattice are preferentially located adjacent to Pb ions as previously observed in PbSnF4 mixed-metal fluorides. The results obtained accord well with our computer simulation findings.