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Orientadores: Eduardo Granado Monteiro da Silva, Tulio Costa Rizuti da Rocha Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin Resumo: Estados eletrônicos ordenados desempenham um papel importante em vários fenômenos físicos em sólidos. Seu possível acoplamento com a rede cristalina ainda é ilusório e pode trazer informações relevantes sobre a origem dos efeitos quânticos coletivos na Matéria Condensada. A ludwigita homometálica de valência mista, M3O2BO3 (M= Co, Fe), com quatro sítios cristalográficos M distintos cercados por octaedros de oxigênio é um caso representativo em que o ordenamento de carga presumivelmente não é acompanhada por uma transição de fase estrutural devido à presença de mais de um sítio inequivalente para os íons de valência mista na fase de carga desordenada. No CuSb2O6 com estrutura trirutilo, um ordenamento orbital do tipo 3z2-r2 é proposto para explicar seu antiferromagnetismo unidimensional, onde a presença de octaedros de oxigênio quase regulares promovem uma competição entre dois níveis de energia 3d eg (ou seja, 3z2-r2 e x2-y2). Medidas de Raman também indicam a possibilidade de excitações orbitais dispersivas coletivas. Neste trabalho, as duas famílias estruturais de materiais selecionadas, as ludwigitas e os trirutilos, foram investigadas. O objetivo é a caracterização do comportamento de ordenamento de cargas em ludwigitas homometálicas em função da temperatura e do padrão de ordenamento orbital no óxido de Cu com estrutura trirutilo. Foi utilizada uma coleção de medidas macroscópicas juntamente com sondas espectroscópicas e estruturais, onde métodos computacionais são aplicados para suportar os resultados experimentais. Confirmamos a estreita competição entre 3d estados eg em CuSb2O6 e que a ordem orbital é composta por múltiplos tipos de orbitais, onde o padrão quadruplica a rede unitária ao longo da direção [110]. O padrão de ordem de carga da ludwigita de Co é bem definido em temperaturas mais baixas, mas atinge uma fase desordenada com o aquecimento. Duas transições de fase de ordenação de carga são observadas bem acima da temperatura ambiente sem modificação de grupo espacial. Finalmente, uma transição de fase de alta temperatura não reportada anteriormente em T~500 K é revelada na ludwigita de ferro Fe3O2BO3. Os resultados apresentados aqui trazem novos insights sobre a física do ordenamento de carga e ordenamento orbital em óxidos de metais de transição Abstract: Ordered electronic states play a major role in a number of physical phenomena in solids. Their possible coupling with the underlying crystal lattice is still elusive and can bring relevant new information on the origin of collective quantum effects in Condensed Matter. The mixed-valent homometallic ludwigite, M3O2BO3 (M= Co, Fe), with four distinct M crystallographic sites surrounded by oxygen octahedra is a representative case where charge ordering is presumably not accompanied by a structural phase transition due to the presence of more than one inequivalent site for the mixed-valence ions in the charge-disordered phase. In CuSb2O6 with trirutile structure, a 3z2-r2-type orbital ordered pattern is proposed to explain its one-dimensional antiferromagnetism, where the presence of almost regular oxygen octahedra promotes a competition between two 3d eg energy levels (namely, 3z2-r2 and x2-y2). Raman measurements also indicate the possibility of collective dispersive orbital excitations. In this work, two selected structural families of materials, namely ludwigites and trirutiles were investigated. The goal is the characterization of the charge ordering behavior in homometallic ludwigites as a function of temperature and the orbital ordering pattern in the Cu oxide with trirutile structure. A collection of macroscopic measurements together with spectroscopic and structural probes were used, where computational methods are applied to support experimental results. We confirm the close competition between 3d eg states in CuSb2O6 and that the orbital ordering is composed of multiple orbital types leading to a fourfold increment of the unit cell along the [110] direction. The charge ordering pattern of the Co ludwigite is well defined at lower temperatures, but reaches a disordered phase upon warming. Two charge ordering phase transitions are observed well above room temperature with no space group modification. Finally, a previously unreported high-temperature phase transition at T~500 K is revealed in the iron ludwigite Fe3O2BO3. The results presented here bring new insights on the physics of charge and orbital ordering in transition-metal oxides Doutorado Física Doutor em Ciências CNPQ 142555/2018-5

Magnetoelectric (ME) multiferroic materials, which present simultaneously two coupled properties between ferromagnetism and ferroelectricity, have attracted much attention recently, not only owing to their application perspectives, e.g., next-generation magnetic RAM, but also for the rich physics associated with the understanding of this coupling. Inverse trirutiles are of particular interest here since ME properties have been reported in this family of compounds. This manuscript presents the study of inverse trirutile Mn2TeO6 and its Cr-substitution series Mn2-xCrxTeO6. Mn2TeO6 and Cr-substituted series were prepared by solid state reaction at relatively low-temperature (< 700°C). Thanks to an extensive use of different techniques performed in a large temperature range (1.5K to 700°C), encompassing synchrotron, neutron and electron diffraction experiments combined with physical properties measurements, the very complex behaviour of Mn2TeO6 was revealed. A structural transition at 400°C from tetragonal (P42/mnm) to monoclinic (P21/c) is observed first, and related to a cooperative Jahn-Teller effect. Further cooling the sample, a hysteretic structural transition is observed spanning more than 50K, which leads to the coexistence of two monoclinic phases. A series of magnetic transitions are also observed between 48K and 22K, with magnetization, heat capacity measurement and neutron diffraction. Cr-substituted (x ≥ 0.15) samples crystallize in the tetragonal phase, implying the suppression of the cooperative Jahn-Teller effect, and involving a simpler, though short–range, magnetic order.; Les matériaux multiferroïques magnétoélectriques (ME) présentant simultanément des propriétés couplées de ferromagnétisme et de ferroélectricité suscitent beaucoup d’attention, non seulement pour leurs applications (comme la RAM magnétique de nouvelle génération), mais aussi pour la compréhension de la physique relative à ce couplage. Le but de cette thèse était de découvrir et de caractériser de nouveaux composés potentiellement multiferroïques, d’où le choix des trirutiles inverses. Ce manuscrit présente l'étude détaillée du trirutile inverse Mn2TeO6 et quelques résultats sur la série substituée au chrome Mn2-xCrxTeO6. Les composés Mn2TeO6 et substitués sont préparés par réaction à l'état solide à relativement basse température (

V tem magistrskem delu sem raziskovala fotokatalitske lastnosti Fe2TeO6 in Fe2-ZTeVZO6 (0z0.7). Za ta sistem sem se odločila, ker ima rutilu sorodno kristalno strukturo, ki predstavlja trikratno osnovno celico rutila, doslej najučinkovitejšega fotokatalizatorja. Fe2TeO6 sem pripravila s sintezo v trdnem, strukturo sem karakterizirala s XRD in SEM. Fotokatalitsko aktivnost sem spremljala z razgradnjo barvila metiloranž v odvisnosti od časa pod UV in vidno svetlobo. Da bi dosegla boljšo fotokatalitsko aktivnost sem se odločila, da v sistemu sistematično zamenjam Fe3+ ione z V3+. S sintezo v trdnem sem pripravila sisteme Fe2-ZTeVZO6 (0z0.7) in jih karakterizirala s XRD, SEM in TEM. Pripravljeni sistemi so bili dvofazni, s kristalno in steklasto amorfno fazo. S homogenizacijo prahov in daljšanjem reakcijskega časa ni bilo možno doseči enofaznega sistema. Kristalna faza je vsebovala vse tri kovinske ione, medtem ko je steklasta amorfna faza vsebovala le kovinski ion vanadija. Kristalna faza je imela strukturo trirutila, intenziteta vrhov v praškovnem difraktogramu je pokazala, da je prišlo do vključitve vanadijevih ionov, vendar ne po izovalentnem mehanizmu. Vključitev vanadijevih ionov v strukturo ni povečalo fotokatalitske aktivnosti pod UV svetlobo.

Magnetoelectric (ME) multiferroic materials, which present simultaneously two coupled properties between ferromagnetism and ferroelectricity, have attracted much attention recently, not only owing to their application perspectives, e.g., next-generation magnetic RAM, but also for the rich physics associated with the understanding of this coupling. Inverse trirutiles are of particular interest here since ME properties have been reported in this family of compounds. This manuscript presents the study of inverse trirutile Mn2TeO6 and its Cr-substitution series Mn2-xCrxTeO6. Mn2TeO6 and Cr-substituted series were prepared by solid state reaction at relatively low-temperature (< 700°C). Thanks to an extensive use of different techniques performed in a large temperature range (1.5K to 700°C), encompassing synchrotron, neutron and electron diffraction experiments combined with physical properties measurements, the very complex behaviour of Mn2TeO6 was revealed. A structural transition at 400°C from tetragonal (P42/mnm) to monoclinic (P21/c) is observed first, and related to a cooperative Jahn-Teller effect. Further cooling the sample, a hysteretic structural transition is observed spanning more than 50K, which leads to the coexistence of two monoclinic phases. A series of magnetic transitions are also observed between 48K and 22K, with magnetization, heat capacity measurement and neutron diffraction. Cr-substituted (x ≥ 0.15) samples crystallize in the tetragonal phase, implying the suppression of the cooperative Jahn-Teller effect, and involving a simpler, though short–range, magnetic order., Les matériaux multiferroïques magnétoélectriques (ME) présentant simultanément des propriétés couplées de ferromagnétisme et de ferroélectricité suscitent beaucoup d’attention, non seulement pour leurs applications (comme la RAM magnétique de nouvelle génération), mais aussi pour la compréhension de la physique relative à ce couplage. Le but de cette thèse était de découvrir et de caractériser de nouveaux composés potentiellement multiferroïques, d’où le choix des trirutiles inverses. Ce manuscrit présente l'étude détaillée du trirutile inverse Mn2TeO6 et quelques résultats sur la série substituée au chrome Mn2-xCrxTeO6. Les composés Mn2TeO6 et substitués sont préparés par réaction à l'état solide à relativement basse température (

Experimental work on the synthesis of the CoSb2O6 oxide and its CO2 sensing properties is presented here. The oxide was synthesized by a microwave-assisted colloidal method in presence of ethylenediamine after calcination at 600 °C. This CoSb2O6 oxide crystallized in a tetragonal structure with cell parameters and Å, and space group P42/mnm. To prove its physical, chemical and sensing properties, the oxide was subjected to a series of tests: Raman spectroscopy, Scanning Electron Microscopy (SEM) and impedance (Z) measurements. Microstructures, like columns, bars and hollow hemispheres, were observed. For the CO2 sensing test, a thick film of CoSb2O6 was used, measuring the impedance variations on the presence of air/CO2 flows (0.100 sccm/0.100 sccm) using AC (alternating current) signals in the frequency-range 0.1–100 kHz and low relative temperatures (250 and 300 °C). The CO2 sensing results were quite good.

Bystromite (MgSb2O6) nanorods were prepared using a colloidal method in the presence of ethylenediamine, after a calcination step at 800 °C in static air. From X-ray powder diffraction analyses, a trirutile-type structure with lattice parameters a = 4.64 Å and c = 9.25 Å and space group P42/mnm was identified. Using scanning electron microscopy (SEM), microrods with sizes from 0.2 to 1.6 μm were observed. Transmission electron microscopy (TEM) analyses revealed that the nanorods had a length of ~86 nm and a diameter ~23.8 nm. The gas-sensing properties of these nanostructures were tested using pellets elaborated with powders of the MgSb2O6 oxide (calcined at 800 °C) at temperatures 23, 150, 200, 250 and 300 °C. The pellets were exposed to different concentrations of carbon monoxide (CO) and propane (C3H8) at these temperatures. The results showed that the MgSb2O6 nanorods possess excellent stability and high sensitivity in these atmospheres.

International audience; We report on a detailed investigation of magnetic properties of CoTa2O6 using several techniques: neutron and X-ray diffraction, specific-heat, magnetic susceptibility, and magnetization measurements. The compound shows quasi-two-dimensional behavior due to its layered structure of alternating Co-O and Ta-O planes. We find that all magnetic moments lie entirely in the Co-O planes, along easy axes determined by the orientations of oxygen octahedra that surround the Co ions. The easy axes in successive magnetic planes have relative orientations that differ by 90°. Antiferromagnetic ordering is observed below 6.6 K, with propagation vectors (±1/4,1/4,1/4) associated to the two non-equivalent sets of Co2+ ions, whose magnetic moments are perpendicularly oriented.