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<p> The alkali metal dichromates show extensive polymorphism. The crystal structures of the polymorphs α-Na2Cr2O7, β-Na2Cr2O7, β1-Rb2Cr2O7 and P21/c NaRbCr2O7 have been determined with x-ray methods. Crystal data were determined for β2-Rb2Cr2O7, P1 Cs2Cr2O7 and the P21/c NaCsCr2O7.</p> <p> The dichromate ions found in this work have been compared with the dichromate ions found in other crystal structure determinations. The anions are described in terms of the bridging oxygen angles b and the torsion angles α1 and α2. Many of the dichromate ions are close to having C2v symmetry with values for α1 and α2 close to zero and bridging angles of around 124°. But there is a number of dichromates with α1 = -α2 and 0°<|α|<60° for which the bridging angle varies between 131° to 141°.</p> <p> The structures determined in this work are discussed as part of a unified description of thortveitite like and dichromate like structures in terms of layers of Y2O7 anions. In terms of this description and Brown and Calvo's classification a structure is proposed for the β2Rb2Cr2O7, while for the structure of NaCsCr2O7 it is suggested that it is isostructural to that of P21/c NaRbCr2O7. The phase transition of α-Na2Cr2O7 to β-Na2Cr2O7 is considered and it is suggested that a twisting thermal mode plays an important role in this as well as in other transitions.</p> / Thesis / Doctor of Philosophy (PhD)

Les composés intermétalliques à base de terres rares et de métaux de transition présentent des propriétés magnétiques intéressantes pour les applications technologiques (aimants permanents, enregistrement magnétique à haute densité, réfrigération magnétique…).Ce travail est dédié à l'étude des propriétés structurales, magnétiques et magnétocaloriques des composés Pr2Fe17 dérivant de la structure Th2Zn17. Les propriétés magnétiques intrinsèques recherchées pour des propriétés performantes de ces composés sont améliorées sous l'effet de la substitution (fer par aluminium et praséodyme par dysprosium) et/ou l'insertion d'un élément léger (carbone). Les nanomatériaux intermétalliques sont élaborés par broyage à haute énergie suivi de recuit, ce qui pourrait conduire à des phases hors équilibre thermodynamique. L'homogénéité des alliages a été systématiquement analysée par diffraction des rayons X suivi de l'affinement Rietveld et par microscopie électronique en transmission. Les résultats ont montré qu'on peut former la phase désirée monophasée en faisant un broyage suivi d'un recuit de 30 min au lieu d'un recuit de 7 jours pour les composés massifs. Ceci représente un gain de temps non négligeable. D'après les mesures magnétiques effectuées, tous les composés possèdent une transition de phase de second ordre. Leur température de Curie augmente avec le taux d'Al et de Dy alors que la variation d'entropie diminue légèrement. Par ailleurs, l'insertion d'atomes interstitiels tels que le carbone a été réalisée. On remarque une nette amélioration de la température de transition. Il ressort de cette étude que ces composés présentent un grand intérêt dans la recherche de futurs matériaux magnétocaloriques pour la réfrigération magnétique à température ambiante / The intermetallic compounds based on rare earth and transition metals present interesting magnetic properties for technological applications (permanent magnets, high density magnetic recording, magnetic refrigeration ...).This work is dedicated to the study of structural, magnetic and magnetocaloric of Pr2Fe17 compounds Th2Zn17-type structure. The intrinsic magnetic properties of these compounds are improved due to the substitution (iron and aluminum by praseodymium by dysprosium) and / or the insertion of a light element (carbon). Intermetallic nanomaterials are prepared by high energy milling and subsequent annealing, which could lead to non-equilibrium phases. The homogeneity of these alloys was checked by X-ray diffraction and by transmission electron microscopy. The results have showed that single-phase can be obtained by milling and annealed during 30 min instead of 7 days annealing for bulk compounds. This represents a considerable saving of time. From the magnetic measurements, all compounds exhibit a second order phase transition. Their Curie temperature increases with the rate of Al and Dy as the entropy change slightly decreases. Moreover, the insertion of interstitial atoms such as carbon was achieved. We notice a significant improvement of the transition temperature. It appears from this study that these compounds are of great interest in the search for future magnetocaloric materials for magnetic refrigeration at room temperature

Submitted by Maike Costa (maiksebas@gmail.com) on 2017-06-21T12:11:13Z No. of bitstreams: 1 arquivototal.pdf: 5280829 bytes, checksum: 081debefa3f10398a1c9461855ba5b9d (MD5) / Made available in DSpace on 2017-06-21T12:11:14Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 5280829 bytes, checksum: 081debefa3f10398a1c9461855ba5b9d (MD5) Previous issue date: 2015-07-29 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Two series based on the structure of 2,5-diaryl-1,3,4-oxadiazole were synthesized, 2-aryl-5-methyl-1,3,4-oxadiazole (1a,h) and 2-aryl-5- trifluoromethyl-1,3,4-oxadiazole (2a,e), and their biological activity were investigated. These compounds had their chemical structures characterized with spectrometric methods such as IR, 1H and 13C NMR. To characterize the compound 2-(2-acetoxyphenyl)-5-methyl-1,3,4- oxadiazole, it was necessary the use of two-dimensional NMR techniques (COSY, HMQC and HMBC) and his structural arrangement was analyzed by the crystallographic X-ray technique. Mass spectrometric investigation, unprecedented for this class of compound, was also performed. All compounds were tested against eight strains of Staphylococcus aureus, Escherichia coli and also against strains of Aspegilles fumigatus, Aspegilles flavus, Candida albicans, Candida albicans and Candida tropicalis. The results showed that compounds 1b, 1c, 1e, 1g, 2a and 2c, produced inhibition on the growth of species of bacteria and fungi, where the MIC was set between 512 to 1224 mg mL-1. While the compounds 1d, 1e, 1f, 1h, 2a and 2b were inactive, the compounds 1d, 1e, 1f, 1h, 2a and 2b reported a broad spectrum. / Duas séries do heterocíclico 1,3,4-oxadiazol foram sintetizadas, a 2-aril-5-metil-1,3,4-oxadiazol (1a, h) e a 2-aril-5-trifluormetil-1,3,4- oxadiazol (2a, e), obtendo cinco moléculas inéditas (1a, 1e, 2a, 2c e 2e). Todas as moléculas obtidas foram caracterizados pelas técnicas espectroscópicas de 1D de RMN 1H e 13C, IV e realizado o estudo inédito de espectroscopia de Massas. Para a caracterização do composto 2-(2- acetoxifenil)-5-metil-1,3,4-oxadiazol se fez necessário a utilização das técnicas bidimensionais de RMN COSY, HMQC e HMBC, bem como seu arranjo estrutural foi analisado pela técnica cristalográfica de Raios-X. Os oxadiazóis obtidos foram avaliadas frente a oito cepas de Staphylococcus aureus e Escherichia coli e oito cepas, incluindo, Aspegilles fumigatus ATCC 16913, Aspegilles flavus, Candida albicans, Candida albicans e Candida tropicalis. Os ensaios para avaliações da atividade biológica dos produtos foram realizados pela técnica de microdiluição em meio líquido, onde foi determinada a concentração Inibitória Mínima (CIM). Os resultados mostraram que os compostos (1b), (1c), (1e), (1g), (2a) e (2c), produziram inibição sobre o crescimento de espécies de bactérias e de fungos, onde a CIM ficou estabelecida entre 512 a 1224 mg/ml. Enquanto que os compostos 1d, 1e, 1f, 1h, 2a e 2b se apresentaram inativos. Já os compostos 1d, 1e, 1f, 1h, 2a e 2b relataram um amplo espectro.