A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Doctor of Philosophy School of Physics 2017 / Different experimental techniques have clearly demonstrated that the predominant intrinsic point defects in ionic barium fluoride are anion Frenkel pairs. Positron annihilation technique is utilized in obtaining Doppler broadening and positron lifetime spectra in the temperature range 300 - 900 K. Doppler broadening quantifies the defects whereas positron lifetime components elaborate on the nature of defects. Theoretical approach by density functional theory (DFT) and the generalized gradient approximation (GGA) in the calculation of electron-positron momentum density (or Doppler broadening) spectra at 0 K show that the positron annihilations decay predominantly with barium valence electrons, especially the 5p and 6s electrons and to a lesser extent with core electrons. These annihilations contribute towards the electron-positron momentum density. The annihilations with valence electrons partly contribute toward the short positron lifetime component. The positron-electron annihilations in barium atoms increase steadily with temperature. At 693 K, the annihilation fraction due to the Ba-atom when the anionic Frenkel is formed is found to be 84.44% compared to 15.56% for the fluorine atom. These annihiltions become part of a larger bulk positron-electron annihilations which form a short positron lifetime component. It is also noted that for F-divacancy at 693 K, the annihilation fraction due to 5p and 6s valence electrons in Ba increases by 2.13% to 86.57% indicating the role of defect clusters in the annihilation process. The long positron lifetime decreases in the temperature range from 500 ps at 300 K to 402 ps at 711 K, corresponds to a fractional increase of 22% in the temperature range 300 K to 693 K. The long positron lifetime component is attributed to a delocalized positronium which quickly annihilates through the pick-off (spin conversion) process. Pick-off process seems to be the dominant processes in the long positron lifetime component. The self-diffusion, at all temperature ranges, of cations Ba2+ in barium fluoride is several orders of magnitude smaller than that of F− which has a diffusion constant of 10−9 m2/s at 300 K. Therefore the contribution of cations in superionic conductivity in the temperature range can be ignored. This is also supported by the absence of third lifetime component which is an indication that only anionic vacancies, F−, are generated in the temperature range. The variation of the lattice constant with temperature as determined by X-ray diffraction becomes a major factor in the determination of S-parameters as a function of temperature hence it can reveal the critical temperature at which the formation of anion Frenkel defects commences before entering superionic region. The disordering of fluorine sublattice is found to deviate from linear behaviour at a temperature of 580 K (S-parameter of 0.50622 and lattice constant of 0.623 nm) without observing any appreciable superionic conductivity. X-ray diffraction technique provides a lattice constant of 0.625 nm at 693 K (corresponding to S-parameter of 0.50776) through which an appreciable small activity in conduction is first observed. This is demonstrated through the correlation between the lattice constants and conductivity values at elevated temparatures. This effectively means that lattice constant increases exponentially with temperature. Ilmenite (FeT iO3) which is an ionic conductor in which a permanent dipole moment can be formed by local changes in the environment of Ti4+ ion. It was used to test the validity the positron annihilation spectroscopy in a completely different environment of this corundum structure of space group R-3. The observed long positron lifetime components in comparison with theoretical calculations clearly show that these long positron lifetime components emanate from positron annihilations at metallic vacancies Fe2+. M¨ossbauer pressure effect confirms the increase of Fe3+ at high pressure. At ambient conditions (pressure and temperature), the ratio Fe3+/Fe2+ is small but gradually increase as the pressure increase. The relative intensity clearly shows a dramatic increase of the Fe3+ component with pressure. Further test was carried out using variable positron beam on a 100 keV Ar+ implanted LiF in the fluence range of 1012 − 1016 ions/m2. In the process of ion implantation on alkali halides, ion vacancies in the form of F centers are formed. Using the penetration depth profile, S-parameter at different incident positron beams from 0.03 to 25 keV energies identifies the concentration of defects. This identification was also confirmed by optical absorption which clearly identified the F-band at 242 nm and F2-band at 444 nm. / MT2018

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This thesis describes results obtained using high resolution electron microscopy, acoustic microscopy and chemical analysis to study the structure and properties of the superionic β-aluminas. The acoustic microscopy and chemical analysis results relate solely to sodium β- and β -alumina, which are used as the solid state electrolyte in the sodium/sulphur cell. The high resolution electron microscopy results cover sodium β- and β-alumina as well as a number of ion-exchanged β-aluminas. The β-alumina structure consists of spinel-like blocks separated by the so-called conduction planes. The conduction planes have a low density, and contain all the mobile cations. Lattice images of sodium β- and β-alumina, silver β-alumina, ammonium/hydronium β-alumina, gadolinium β -alumina and divalent and trivalent europium β -alumina are presented and discussed. A hitherto unreported long-period structure in sodium β-alumina is shown, as is superlattice ordering in the divalent and trivalent β-aluminas. Defects in these materials are also discussed. The most common damage mode in the β -aluminas, due to electron beam irradiation, is the loss of the mobile-ion containing planes, and the subsequent collapse and shear of the structure to form broad defect spinel blocks. It is shown that collapse vectors determined for sodium β-alumina can also be applied to ammonium/hydronium β-alumina. Two further damage modes observed in this β-alumina are also discussed. A damage mode has been observed in sodium B-alumina and silver β-alumina which involves the extrusion of material to the crystal surface. Electron diffraction patterns from the extruded material have been indexed. The acoustic microscope has been used to examine bulk sodium β/β-alumina electrolyte tube specimens. Images of rectangular features present in the tubes (approximately 40um in length) are presented and the possible nature of the features is discussed.

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Estudos experimentais de sistemas mistos contendo os fluoretos PbF2 e CdF2 vêm revelando o importante potencial tecnológico desses materiais. No entanto, o comportamento tanto estrutural quanto dinâmico desses materiais ainda não é completamente compreendido. A proposta desta tese é a contribuição no entendimento amplo desses sistemas utilizando uma simulação atomística feita por Dinâmica Molecular. Com uso de um potencial efetivo de pares promissor foi possível descrever as propriedades estruturais, dinâmicas e térmicas destes sistemas avaliando-se também a dependência da composição nestas propriedades. As simulações foram realizadas no ensemble NPT (número de partículas, pressão e temperatura constantes) permitindo acompanhar as variações de energia e volume do sistema durante os processos de aquecimento e resfriamento. Foram observadas as descontinuidades nas transições sólido-líquido e líquido-sólido durante estes processos. Esse ensemble aproxima a simulação da realidade experimental uma vez que em geral experimentos são realizados a pressão atmosférica. Na primeira parte dos resultados foram criadas soluções sólidas ideais construídas a partir da rede perfeita do PbF2 onde são escolhidos alguns dos íons de Pb que são substituídos aleatoriamente por íons de Cd. Os resultados da caracterização estrutural destes sistemas foram comparados com medidas de EXAFS mostrando excelente acordo e validando o potencial proposto. Uma vez que o potencial mostrou-se coerente na descrição das interações destes três íons, foram analisados os efeitos da temperatura nestes sistemas. Tanto o PbF2 quanto o CdF2 são sólidos condutores superiônicos onde os íons de flúor apresentam alta mobilidade iônica. O sistema misto composto por estes dois fluoretos, CdxPb1xF2, também é um material superiônico. As propriedades superiônicas do sistema binário foram obtidas pelas simulações mostrando excelente acordo com os dados experimentais. A fusão desses sistemas considerando-se diferentes concentrações dos fluoretos, apresentou um diagrama de fase com menor temperatura de fusão para as composições em torno de 35% de CdF2 , o que está em perfeito acordo com o observado experimentalmente. Sistemas essencialmente diferentes quanto à organização estrutural e homogeneidade, foram obtidos a partir da fusão seguida de diferentes taxas de resfriamento. O resfriamento do líquido feito com taxas rápidas levou a formação de vidros homogênos. Taxas mais lentas de resfriamento Levam a formação de vidros que passam a exibir regiões mais ricas em PbF2 e outras em CdF2 . Taxas de resfriamento ainda mais lentas levam a cristalização dos líquidos, a qual ocorre com tendência de separação de fases e geração de defeitos. Os vidros homogêneos foram aquecidos novamente e apresentaram separação de fase seguida da devitrificação. Essencialmente devido aos defeitos gerados durante os processos de vitrificação, cristalização e devitrificação os vidros e os cristais apresentam maiores mobilidades dos íons fluoretos que os sistemas ideais primeiramente avaliados. Os cristais com separação de fases apresentam também composições que possuem menores valores de temperatura de transição iônica-superiônica. O comportamento de máxima mobilidade em função da temperatura é observado para as composições em torno de 30% de CdF2 em acordo com dados experimentais. A simulação clássica realizada permitiu o acesso microscópico do sistema sugerindo uma descrição para esses sistemas que é de grande valia e nem sempre possível experimentalmente / Experimental studies of binary systems of PbF2 and CdF2 display important technological potential of these materials. Besides that, its structural and dynamical behavior is not well understood. The aim of this theses is to bring a contribution for understand these materials through molecular dynamics simulations. With an adequate interatomic pair potential it was possible to describe the structural, dynamical and thermal properties of the pure systems as well as for solid solutions. All simulations were performed in NPT ensemble (constant number of particle, pressure, and temperature) which allows observing and analyses the energy and volume variation during process of heating and cooling. Solid-liquid and liquid-solid transitions discontinuity were observed during these process. This ensemble resembles experimental reality, once in general experiments are done at atmospheric pressure. Initially, ideal solid-solutions were obtained from perfect PbF2 lattice, where Pb ions were randomly replaced by Cd ions. The molecular dynamics structural characterizations were compared with EXAFS results in excellent agreement, which validates the proposed interatomic potential. Further, thermal effects were also analyzed. Both PbF2 and CdF2 are super ionic solid conductors, where the F ions shows high ionic mobility, The binary system, CdxPb1xF2, is also a super ionic conductor. The super ionic properties for the binary system were simulated displaying excellent agréments with experimental available data. Melting temperature as a function of the CdF2 concentration shows a phase diagram in which the melting temperature has a minimum around 35% of CdF2 , in accord with experimental data. Using different cooling rates, cooling from the melt, result in systems with different structural organization and homogeneity. Cooling from the liquid, with fast cooling rates, results in homogeneous glass. Slow cooling rates produce glass in which there is regions rich in PbF2 and other rich in CdF2 . Slower cooling rates results in the crystallization of the liquid, which large tendency of phase separation and defects. Homogeneous glasses were re-heated resulting in a phase separated system followed by devitrification. Due to the presence of defects during the process of vitrification, crystallization and devitrification, glasses and crystals display greater mobility of F ions than the initial solid solution initially analyzed. The ionic-super ionic temperature occurs for smaller temperatures for some composition of the crystals with phase separation. Maximum mobility as a function of temperature was observed for composition around 30% CdF2 , in agreement with experiments. The classical simulation performed allows detailed microscopic description of the system in which, in general, are not possible experimentally

Nowadays the ferroelectrics containing of several feroically active sublattices are very attractive, because interactions between these sublattices can caused novel phenomena. Antiferroelectrics, ferrielectrics and multiferoics belong to these materials. In this work new crystalline materials MNP2X6 (M = Cu, Ag; N=In, Cr, Bi; X=S, Se) were investigated, which have ferrielectric and multiferoic properties. The dielectric and electric properties of above mentioned materials have been investigated by broadband dielectric spectroscopy methods, which allows to analyze the collective processes related to order – disorder and displacive phase transitions, ions migration and freezing of dipoles (glassy state) in wide temperature (25 K - 500 K) and frequency (10-5 HZ - 3 GHz) ranges. By substitution or doping it becomes possible to tailor the ferroelectric materials to different properties. In this work is determined that the substitution of 10% Cu ions by Ag ions shifts the phase transition temperature of CuInP2S6 crystal toward lower temperatures while the addition of In ions shifts the phase transitions temperature toward the higher ones. The phase transition temperature difference is about 50 K for mentioned crystals. If the ferroelectric crystal CuInP2S6 is mixed with the antiferroelectric CuCrP2S6 the dipole glass phase occupies the middle of the phase diagram. The distribution of relaxation times has been calculated from the broadband dielectric spectra of dipolar glasses. The... [to full text] / Šiai dienai ypač populiarūs ferroelektrikai susidedantys iš kelių feroiškai aktyvių subgardelių, kurių persitvarkymas fazinio virsmo temperatūroje atskleidžia naujų, dar neaprašytų reiškinių. Prie šių medžiagų priskiriami antiferoelektrikai, ferielektrikai ir multiferoikai. Šiame darbe buvo tiriama nauja medžiagų šeimos MNP2X6 (M = Cu, Ag; N=In, Cr, Bi; X=S, Se ), kurios pasižymi ferielektrinėmis bei multiferoinėmis savybėmis, ir kurių dielektrines ir elektrines savybes galima efektyviai keisti įvedant priemaišas. Minėtų medžiagų dielektrinės ir elektrinės savybės buvo tiriamos dielektrinės spektroskopijos metodais, kurie leidžia tirti kristalų kolektyvinius reiškinius susijusius su tvarkos – netvarkos bei poslinkio tipo faziniais virsmais, jonų migracija bei dipolių užšalimu (stiklėjimu) plačiame dažnių (10-5 Hz iki 3 GHz) bei temperatūrų (25 K iki 500 K) intervaluose. Įvedus 10% Ag jonų vietoj Cu jonų ferielektriniame kristale CuInP2S6 fazinio virsmo temperatūra pasislenka į žemesnias temperatūras, o padidinus indžio koncentraciją fazinio virsmo temperatūra pasislenka į aukštesnes temperatūras. Minėtų kristalų fazinių virsmų temperatūrų skirtumas 50 K. Sumaišius skirtingomis proporcijomis feroelektriką (CuInP2S6) su antiferoelektriku (CuCrP2S6) stebima dipolinio stiklo fazė. Iš dielektrinių matavimų stiklo fazėje buvo paskaičiuota relaksacijos trukmių pasiskirstymo funkcija, kurios aprašymas dvigubos potencialinės duobės modeliu leido susieti mikroskopinius kristalo... [toliau žr. visą tekstą]

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Estudos experimentais de sistemas mistos contendo os fluoretos PbF2 e CdF2 vêm revelando o importante potencial tecnológico desses materiais. No entanto, o comportamento tanto estrutural quanto dinâmico desses materiais ainda não é completamente compreendido. A proposta desta tese é a contribuição no entendimento amplo desses sistemas utilizando uma simulação atomística feita por Dinâmica Molecular. Com uso de um potencial efetivo de pares promissor foi possível descrever as propriedades estruturais, dinâmicas e térmicas destes sistemas avaliando-se também a dependência da composição nestas propriedades. As simulações foram realizadas no ensemble NPT (número de partículas, pressão e temperatura constantes) permitindo acompanhar as variações de energia e volume do sistema durante os processos de aquecimento e resfriamento. Foram observadas as descontinuidades nas transições sólido-líquido e líquido-sólido durante estes processos. Esse ensemble aproxima a simulação da realidade experimental uma vez que em geral experimentos são realizados a pressão atmosférica. Na primeira parte dos resultados foram criadas soluções sólidas ideais construídas a partir da rede perfeita do PbF2 onde são escolhidos alguns dos íons de Pb que são substituídos aleatoriamente por íons de Cd. Os resultados da caracterização estrutural destes sistemas foram comparados com medidas de EXAFS mostrando excelente acordo e validando o potencial proposto. Uma vez que o potencial mostrou-se coerente na descrição das interações destes três íons, foram analisados os efeitos da temperatura nestes sistemas. Tanto o PbF2 quanto o CdF2 são sólidos condutores superiônicos onde os íons de flúor apresentam alta mobilidade iônica. O sistema misto composto por estes dois fluoretos, CdxPb1xF2, também é um material superiônico. As propriedades superiônicas do sistema binário foram obtidas pelas simulações mostrando excelente acordo com os dados experimentais. A fusão desses sistemas considerando-se diferentes concentrações dos fluoretos, apresentou um diagrama de fase com menor temperatura de fusão para as composições em torno de 35% de CdF2 , o que está em perfeito acordo com o observado experimentalmente. Sistemas essencialmente diferentes quanto à organização estrutural e homogeneidade, foram obtidos a partir da fusão seguida de diferentes taxas de resfriamento. O resfriamento do líquido feito com taxas rápidas levou a formação de vidros homogênos. Taxas mais lentas de resfriamento Levam a formação de vidros que passam a exibir regiões mais ricas em PbF2 e outras em CdF2 . Taxas de resfriamento ainda mais lentas levam a cristalização dos líquidos, a qual ocorre com tendência de separação de fases e geração de defeitos. Os vidros homogêneos foram aquecidos novamente e apresentaram separação de fase seguida da devitrificação. Essencialmente devido aos defeitos gerados durante os processos de vitrificação, cristalização e devitrificação os vidros e os cristais apresentam maiores mobilidades dos íons fluoretos que os sistemas ideais primeiramente avaliados. Os cristais com separação de fases apresentam também composições que possuem menores valores de temperatura de transição iônica-superiônica. O comportamento de máxima mobilidade em função da temperatura é observado para as composições em torno de 30% de CdF2 em acordo com dados experimentais. A simulação clássica realizada permitiu o acesso microscópico do sistema sugerindo uma descrição para esses sistemas que é de grande valia e nem sempre possível experimentalmente / Experimental studies of binary systems of PbF2 and CdF2 display important technological potential of these materials. Besides that, its structural and dynamical behavior is not well understood. The aim of this theses is to bring a contribution for understand these materials through molecular dynamics simulations. With an adequate interatomic pair potential it was possible to describe the structural, dynamical and thermal properties of the pure systems as well as for solid solutions. All simulations were performed in NPT ensemble (constant number of particle, pressure, and temperature) which allows observing and analyses the energy and volume variation during process of heating and cooling. Solid-liquid and liquid-solid transitions discontinuity were observed during these process. This ensemble resembles experimental reality, once in general experiments are done at atmospheric pressure. Initially, ideal solid-solutions were obtained from perfect PbF2 lattice, where Pb ions were randomly replaced by Cd ions. The molecular dynamics structural characterizations were compared with EXAFS results in excellent agreement, which validates the proposed interatomic potential. Further, thermal effects were also analyzed. Both PbF2 and CdF2 are super ionic solid conductors, where the F ions shows high ionic mobility, The binary system, CdxPb1xF2, is also a super ionic conductor. The super ionic properties for the binary system were simulated displaying excellent agréments with experimental available data. Melting temperature as a function of the CdF2 concentration shows a phase diagram in which the melting temperature has a minimum around 35% of CdF2 , in accord with experimental data. Using different cooling rates, cooling from the melt, result in systems with different structural organization and homogeneity. Cooling from the liquid, with fast cooling rates, results in homogeneous glass. Slow cooling rates produce glass in which there is regions rich in PbF2 and other rich in CdF2 . Slower cooling rates results in the crystallization of the liquid, which large tendency of phase separation and defects. Homogeneous glasses were re-heated resulting in a phase separated system followed by devitrification. Due to the presence of defects during the process of vitrification, crystallization and devitrification, glasses and crystals display greater mobility of F ions than the initial solid solution initially analyzed. The ionic-super ionic temperature occurs for smaller temperatures for some composition of the crystals with phase separation. Maximum mobility as a function of temperature was observed for composition around 30% CdF2 , in agreement with experiments. The classical simulation performed allows detailed microscopic description of the system in which, in general, are not possible experimentally