Browsing by Author "Mwanga, S. F."

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    Landau theoretical interpretation of domain and domain walls in ferroelectric ferroics
    (Asian Research Publishing Network, 2006) Rwenyagila, E. R.; Mwanga, S. F.
    In this paper some behaviors of domain and domain walls in perovskite ferroics were theoretically investigated. Landau type theory was used to examine the phenomenological properties of domains and domain walls in ferroelectric perovskite materials. Landau predictions on the prototype ferroelectric BaTiO3 showed that it has a stable tetragonal structure at room temperature. The number of domains in the three ferroelectric phases of BaTiO3 was also calculated. The results confirmed six, eight and twelve domain states in the tetragonal, rhombohedral and orthorhombic structures of BaTiO3, respectively. The respective spontaneous polarization and domain wall thickness in the stable tetragonal BaTiO3 have values of 0.26 Cm-2 and 1.63nm at room temperature. Analytical and numerical solutions for the relationship between temperature and domain wall thickness and for the spatial distribution of polarizations inside the domain wall are also determined and discussed.
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    Molecular clusters Cs3X3 and Cs4X4 (X = Br, I): Quantum chemical study of structure and thermodynamic properties
    (Taylor & Francis, 2016) Mwanga, S. F.; Pogrebnaya, T. P.; Pogrebnoi, A. M.
    The properties of trimer Cs3X3 and tetramer Cs4X4 (X = Br, I) molecules have been studied using DFT with B3LYP5 functional and MP2 and MP4 methods. Two equilibrium geometrical structures of trimers, hexagonal (D3 h) and “butterflyshaped” (Cs), and one for tetramers, distorted cubic (Td), are confirmed to exist; geometrical parameters and vibrational spectra are determined. The relative concentration of Cs3X3 isomers has been evaluated; the butterfly-shaped isomer dominates over hexagonal in saturated vapour in a broad temperature range. The dissociation reactions through different channels have been considered and enthalpies of formation ΔfH°(0) of clusters determined:−858 } 20 kJ mol−1 (Cs3Br3), −698 } 20 kJ mol−1 (Cs3I3), −1270 } 30 kJ mol−1 (Cs4Br4) and −1045 } 30 kJ mol−1 (Cs4I4). The Gibbs free energies ΔrG°(T) calculated for the dissociation reactions of trimer and tetramer molecules have indicated that these molecules are resistive in narrow temperature range only and decompose spontaneously with temperature increase with elimination of dimer molecules.
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    Structure and properties of molecular and ionic clusters in vapour over caesium fluoride
    (Taylor & Francis, 2015) Mwanga, S. F.; Pogrebnaya, T. P.; Pogrebnoi, A. M.
    The properties of neutral molecules Cs2F2, Cs3F3, and Cs4F4, and positive and negative cluster ions Cs2F+, CsF2−, Cs3F2+, Cs2F3−, Cs4F3+, and Cs5F4+ were studied by several of quantum chemical methods implementing density function theory and Möller–Plesset perturbation theory of second and fourth orders. For all species, the equilibrium geometrical structure and vibrational spectra were determined. Different isomers have been revealed for the trimer neutral molecule Cs3F3; pentaatomic, both positive and negative, Cs3F2+, Cs2F3−; and heptaatomic Cs4F3+ ions. The most abundant isomers in the saturated vapour were determined. Enthalpies of dissociation reactions and enthalpies of formation of the species were obtained.