A decade of Density Functional Theory in Kenya

The African School Series on Electronic Structure Methods and Applications (ASESMA) has had a positive impact on growth of computational material science in Kenya, visibility of Kenyan universities and strong collaboration ties between Kenyan scientist and the rest of the world. Data retrieved from Scopus indicate that computational materials scientists in Kenya have published numerous articles, book chapters, and conference proceedings in peer-reviewed, high impact journals. The trend is likely to continue due to access to affordable computers, spread of electricity to remote areas, open-source softwares and cheaper internet

Ab-initio study of elastic and structural properties of layered nitride materials

Layered nitride materials in the form of Carbon nitride (C3N4) was speculated nearly 22 years ago. It has various structural forms ranging from layered graphitic to superhard structures. Using first principles calculations based on density functional theory, the structural and elastic properties of these phases are determined. Elastic constants, bulk and shear moduli of cubic phases are compared to that of diamond. From the work it is evident that, although the compressibility of some of the superhard phases may be better than diamond, the shear modulus indicates that C3N4 is not harder than diamond in contrast to what has been speculated ealier. The graphitic hexagonal, rhombohedral as well orthorhombic phases are soft as indicated by their bulk and shear moduli, which are similar to that of graphite. Other elastic properties such as the Young modulus and Poisson’s ratio as well as the Raman and infrared vibrational frequencies are also presented in this dissertatio

A computational study of layered and superhard carbon-nitrogen material

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. August 2014.The process of the computational discovery of materials for future technologies is a combination of numerical techniques and general scientific intuition to select elements and combine in order to form novel types of materials. Modern ab initio methods based on density functional theory are capable of predicting with a high level of accuracy the most stable ground state atomic configurations of any given material. Once the ground state configurations are established, the electronic, optical and mechanical properties of the novel bulk nitrides may be determined. Electronic properties of C3N4, CN2, SiN2, GeN2, C2N2(NH), Si2N2(NH), Ge2N2(NH) and Sn2N2(NH) are analysed by computing the Kohn-Sham band structures. The optical properties are investigated by calculating the real and the imaginary parts of the frequency-dependent dielectric constant. The mechanical properties are determined by calculating elastic constants, Young’s modulus, Poisson’s ratio, Vickers hardness, shear and bulk moduli

First-principles study of two-dimensional electron and hole gases at the head-to-head and tail-to-tail 180º domain walls in PbTiO3 ferroelectric thin films

We study from first principles the structural and electronic properties of head-to-head (HH) and tail-to-tail (TT) 180º domain walls in isolated free-standing PbTiO3 slabs. For sufficiently thick domains (n = 16 unit cells of PbTiO3), a transfer of charge from the free surfaces to the domain walls to form localized electron (in the HH) and hole (in the TT) gases in order to screen the bound polarization charges is observed. The electrostatic driving force behind this electronic reconstruction is clearly visible from the perfect match between the smoothed free charge densities and the bound charge distribution, computed from a finite difference of the polarization profile obtained after the relaxation of the lattice degrees of freedom. The domain wall widths, of around six unit cells, are larger than in the conventional 180? neutral configurations. Since no oxygen vacancies, defects, or dopant atoms are introduced in our simulations, all the previous physical quantities are the intrinsic limits of the system. Our results support the existence of an extra source of charge at the domain walls to explain the enhancement of the conductivity observed in some domains walls of prototypical, insulating in bulk, perovskite oxides.J.S. thanks the University of Cantabria for the scholarship funded by the Vice-rectorate for Internationalisation and the Theoretical Condensed Matter Group. J.J. and P.G.-F. acknowledge financial support from the Spanish Ministry of Economy and Competitiveness through the MINECO Grant No. FIS2015-64886-C5-2-P, and the Spanish Ministry of Science, Innovation and Universities through Grant No. PGC2018-096955-B-C41

First-principles calculations to investigate structural, elastic, electronic and thermodynamic properties of NbCoSn and VRhSn Half-Heusler compounds

In this study, we investigated the structural, elastic, electronic, and thermodynamic properties of NbCoSn and VRhSn HH compounds using the first-principles calculations as implemented in the density functional theory (DFT). The computed lattice constants of NbCoSn and VRhSn compounds were found to be consistent with the available theoretical as well as the experimental data. The compounds are mechanically stable since their elastic constants satisfy the Born-Huang criteria for cubic system stability. Due to the absence of imaginary phonons, NbCoSn is dynamically stable, whereas VRhSn is unstable. NbCoSn is harder than VRhSn HH because it has a higher Vicker’s hardness and shear modulus. Both compounds feature band gaps, indicating that they are semiconductors. When compared to NbCoSn HH compound, VRhSn has a narrow band gap. Furthermore, thermodynamic properties are computed and thoroughly explored. As a result of the findings, NbCoSn and VRhSn HH compounds are viable thermoelectric materials; however, doping and alloying could be employed to enhance the stability of VRhSn HH compound

Structural stability and electronic properties of AgInS

We employ state-of-the-art ab initio density functional theory techniques to investigate the structural, dynamical, mechanical stability and electronic properties of the ternary AgInS2 compounds under pressure. Using cohesive energy and enthalpy, we found that from the six potential phases explored, the chalcopyrite and the orthorhombic structures were very competitive as zero pressure phases. A pressure-induced phase transition occurs around 1.78 GPa from the low pressure chalcopyrite phase to a rhombohedral RH-AgInS2 phase. The pressure phase transition around 1.78 GPa is accompanied by notable changes in the volume and bulk modulus. The calculations of the phonon dispersions and elastic constants at different pressures showed that the chalcopyrite and the orthorhombic structures remained stable at all the selected pressure (0, 1.78 and 2.5 GPa), where detailed calculations were performed, while the rhombohedral structure is only stable from the transition pressure 1.78 GPa. Pressure effect on the bandgap is minimal due to the small range of pressure considered in this study. The meta-GGA MBJ functional predicts bandgaps which are in good agreement with available experimental values