Submitted by Rosivalda Pereira (mrs.pereira@ufma.br) on 2017-06-06T19:20:38Z No. of bitstreams: 1 WanessaMelo.pdf: 3870140 bytes, checksum: 266e5a05b6c1164e224f978555ddc4d0 (MD5) / Made available in DSpace on 2017-06-06T19:20:38Z (GMT). No. of bitstreams: 1 WanessaMelo.pdf: 3870140 bytes, checksum: 266e5a05b6c1164e224f978555ddc4d0 (MD5) Previous issue date: 2008-05-20 / Fundação de Amparo à Pesquisa e ao Desenvolvimento Científico e Tecnológico do Maranhão (FAPEMA) / In this work we have studied possible phase transitions in Na2ThF6 single crystals using Di erential Scanning Calorimetry (DSC), Raman and Impedance spectroscopies and static simulations. The polarized Raman spectroscopy measurements were performed in the temperature range from 10 K up to 770 K and DSC measurements from 298 K up to 823 K. The results indicate that the Na2ThF6 single crystal are structurally stable in the temperature ranging from 50 K up to 770 K. Below 50 K, a mode with A1 symmetry disappear indicating a possible phase transition. Besides, the bandwidth of the main Raman modes increases with the temperature showing a behavior like Ahrrenius with activation energies of 0,17 eV and 0,16 eV. The Impedance spectroscopy measurements were performed in the temperature range between 298K and 773K in the clivage (311) crystalline plane. The results indicate an increase in the ionic conductivity at high temperatures showing a conduction mechanism with activation energy of 0,28 eV. The imaginary part of the dielectric constant and conductivity present an anomaly at around 735 K, possibly associated to a superionic behavior. The static simulations were performed using the GULP code in the hydrostatic pressure range between room pressure and 20 GPa, showing that the z coordinate of the Na+ ion tends to value 1=2 at around 6 GPa, indicating a possible structural phase transition induced by hydrostatic pressure. The ferroic characteristic of this transition was de ned according to the changes observed for the piezoelectric and compliance tensors. / Neste trabalho investigamos possíveis transições de fase no Na2ThF6, utilizando as técnicas de calorimetria diferencial de varredura, Espectroscopias Raman e de Impedância e simulações estáticas. As medidas de espectroscopia Raman polarizadas foram realizados na faixa de temperatura de 10K a 770K e as de calorimetria diferencial de varredura de 298 K a 823 K. Os resultados indicaram que o monocristal de Na2ThF6 é estável do ponto de vista estrutural entre 50 K e 770 K. Abaixo de 50 K, o desaparecimento de um modo na simetria y(zz)¹y indica, possivelmente, uma transição de fase ferróica. Além disso, a largura dos modos mais proeminentes observados têm um comportamento semelhante ao de Ahrenius, indicando energias de ativação de 0,17 eV e 0,16 eV. As medidas de espectroscopia de Impedância, realizadas no intervalo de temperatura entre 298 K e 773 K no plano de clivagem (311) da amostra, revelaram um aumento da condutividade em função da temperatura possuindo um processo condutivo termicamente ativado cuja energia de ativação é de 0,28 eV. Observou-se também uma anomalia na constante dielétrica imáginária e condutividade em 735 K. Esta anomalia é, provavelmente, consequência de um comportamento superiônico do material. As simulações estáticas realizadas através do programa GULP, da pressão ambiente à 20 GPa, evidenciaram que a coordenada z dos átomos de Na do Na2ThF6 migrou para uma posição 1/2 em 6 GPa, indicando deste modo uma possível transição de fase induzida por pressão hidrostática. O caráter ferróico dessa transição foi de nido pelas mudanças nos tensores de constante elástica e coeficiente piezoelétrico observadas na simulação em torno de 6 GPa .

Les travaux présents ici portent sur l’étude et la prospection de matériaux ferroiques. Cette étude consiste en une approche revêtant plusieurs aspects que sont la chimie du solide, la physique du solide et la science des matériaux. Deux parties sont développées, avec deux approches différentes. La première se concentre sur les liens entre composition/structure/propriétés dans des matériaux de la famille des TTB dérives de Ba2NdFeNb4O15 à travers l’étude de solutions solides à base de Li et l’étude de l’impact des différents paramètres de synthèse sur la nature de l’anomalie diélectrique mesurées sur pastilles densifiées. La mise en évidence de modulation structurale dans cette famille de matériaux semble être en lien direct avec l’observation des variations de propriétés diélectriques. L’utilisation de diffraction électronique notamment permet la mise en évidence de ces modulations structurales et leur évolution en température. Dans une seconde partie, l’approche consiste à utiliser le Spark Plasma Sintering (SPS) comme technique de densification pour des matériaux dits “fragiles” mais aussi d’explorer des propriétés diélectriques jusqu’alors inaccessibles, sur matériaux massifs. Le développement de la technique SPS à basse température permet ainsi non seulement de densifier à basse température des matériaux fragiles, mais aussi d’obtenir des phases inaccessibles dans des conditions de températures similaires par traitement thermique conventionnel. De même, l’obtention de céramique moléculaire de très haute densité a pu être réalisée, malgré des températures de décomposition extrêmement faible (100 ˚C). / The present work focuses on the prospection and understanding study of ferroic materials. It consists in a multiple aspect approach, including materials chemistry, materials physics and materials processing. Two parts compose this work, with two different approaches. The first one focuses on the links between composition, structure and properties in materials belonging to the TTB family, more specifically derived from Ba2NdFeNb4O15, through the exploration of Li containing solid solutions, and the impact of synthesis parameters on measured dielectric anomalies on dense samples. The observation of structural modulation in these materials seems to be closely related to the observation of dielectric anomalies variations. Using electron diffraction techniques allowed the evidence of such anomalies and the following of their thermal evolution. In a second part, the approach consists in using Spark Plasma Sintering (SPS) as a densification technique for so called “fragile” materials but also explore dielectric properties impossible to experimentally measure thus far. Developing low temperature SPS technique not only allows to densify ceramics at low temperatures fragile materials, but also to obtain inaccessible phases in similar temperature conditions using conventional thermal treatments. Moreover, sintering of molecular ceramic at very high density was possible, even if its decomposition temperature is extremely low (100 ˚C).

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