The aim of this thesis was to improve the understanding of microstructure in single crystal ferroelectrics. This was achieved through macroscopic testing of Lead Magnesium Niobate – Lead Titanate (PMN-PT) and microscopic observations of Barium Titanate (BT) single crystals. Multi-axial polarization rotation tests on PMN-PT showed a gradual increase in the change in dielectric displacement due to ferroelectric switching as the electric field is applied at increasing angles to the initial polarization direction. A relatively high remnant polarization for loading angle near to 90° suggested that PMN-PT is more polarizable in certain directions. Strains measured in two directions, parallel to the electric field and perpendicular to the electric field, showed a noticeable variation on two opposite faces of the specimen suggesting an effect of local domain configurations on macroscopic behaviour. A micromechanical model gave an insight into the switching systems operating in the crystal during the polarization rotation test. Domain structure in BT was mapped using synchrotron X-ray reflection topography. By making use of the angular separation of the diffracted reflections and specimen rocking, different domain types could be unambiguously identified, along with the relative tilts between adjacent domains. Fine needle domains (width ≈ 10μm) were successfully mapped providing a composite topograph directly comparable with optical micrograph. The domain structure was confirmed using other techniques such as piezoresponse force microscopy and atomic force microscopy/scanning electron microscopy and optical observations on the etched crystal. Results show that combined use of multiple techniques is necessary to gain a consistent interpretation of the microstructure. Finally, domain evolution in BT under compressive mechanical loading was observed in-situ using optical and X-ray diffraction techniques providing a series of images that show ferroelastic transition. The domain configurations influence the switching behaviour and constitutive models that can account for such effects need to be developed. Quantitative and qualitative data presented in this thesis can assist model development and validation.

<div>Understanding grain growth in dielectric ceramics is essential to controlling the electrical and mechanical properties necessary to produce ceramic capacitors and sensors. The effect of alloying barium titanate with strontium titanate on the equilibrium crystal shape was investigated in order to determine possible impacts on grain growth. The equilibrium crystal shape was studied through three experimental methods to identify possible changes in grain boundary energy or anisotropy with changing composition.</div><div>The first method was by imaging intergranular pores to observe faceting behavior and relative interfacial energies. Intergranular pores were reconstructed to determine the relative surface energies of the identified facets. The second method was to perform atomic force microscopy on surface facets to collect topography data. The topography data was combined with orientation data obtained by EBSD analysis from the same region, and used to calculate the normal vector of the surface facets. These datasets were plotted in a stereographic projection to study the faceting anisotropy. The third method involved collecting EBSD orientation data and images of surface faceting behavior. The surface faceting behavior of each grain was categorized by type of facet and plotted on a stereographic projection at the corresponding orientation. This allowed for the analysis of faceting transitions and the differentiation of faceted and continuous regions of the equilibrium crystal shape. The analysis of faceting behavior across compositions has implications on grain growth of the barium titanate/strontium titanate system.</div>

Orientador: José de los Santos Guerra / Resumo: O titanato de bário (BaTiO3, BT) é um material ferroelétrico com diversas aplicações, dentro do mercado de componentes eletrônicos, devido às excelentes propriedades físicas que manifesta, em relação a outros sistemas ferroelétricos, continuando a atrair durante várias décadas o interesse científico e comercial. Dentre o grande número de trabalhos sobre o sistema BT reportados na literatura muitos são associados à vasta gama de aplicações para a indústria eletro-eletrônica, dentre as quais se destacam o uso em dispositivos para capacitores, termistores, entre outros. A dopagem do BaTiO3 com íons doadores implica na mudança de algumas propriedades físicas, que conduzem a comportamentos anómalos em determinados parâmetros físicos, cuja natureza ainda não tem sido muito esclarecida, embora muitos trabalhos tenham se empenhado para tentar justificar tais efeitos. Neste contexto, neste trabalho, as propriedades físicas do sistema BaTiO3 serão investigadas considerando a adição de elementos doadores, nos sítios A e B da estrutura. Em particular, os íons de gadolínio (Gd3+) e nióbio (Nb5+) serão usados como modificadores da estrutura perovskita em substituição dos íons de bário (Ba2+) e titânio (Ti4+) nos sítios A e B, respectivamente, considerando as fórmulas químicas Ba1-xGdxTiO3 e BaTi1-x NbxO3 (sendo x = 0.001, 0.002, 0.003, 0.004 e 0.005). As propriedades estruturais e microestruturais foram investigadas à temperatura ambiente, enquanto as propriedades dielétricas e elétricas f... (Resumo completo, clicar acesso eletrônico abaixo) / Doutor

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No. of bitstreams: 1 oliveira_ma_dr_ilha.pdf: 7398841 bytes, checksum: acb7f174226b997e4c4c4192ff05e406 (MD5) Previous issue date: 2017-08-18 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O titanato de bário (BaTiO3, BT) é um material ferroelétrico com diversas aplicações, dentro do mercado de componentes eletrônicos, devido às excelentes propriedades físicas que manifesta, em relação a outros sistemas ferroelétricos, continuando a atrair durante várias décadas o interesse científico e comercial. Dentre o grande número de trabalhos sobre o sistema BT reportados na literatura muitos são associados à vasta gama de aplicações para a indústria eletro-eletrônica, dentre as quais se destacam o uso em dispositivos para capacitores, termistores, entre outros. A dopagem do BaTiO3 com íons doadores implica na mudança de algumas propriedades físicas, que conduzem a comportamentos anómalos em determinados parâmetros físicos, cuja natureza ainda não tem sido muito esclarecida, embora muitos trabalhos tenham se empenhado para tentar justificar tais efeitos. Neste contexto, neste trabalho, as propriedades físicas do sistema BaTiO3 serão investigadas considerando a adição de elementos doadores, nos sítios A e B da estrutura. Em particular, os íons de gadolínio (Gd3+) e nióbio (Nb5+) serão usados como modificadores da estrutura perovskita em substituição dos íons de bário (Ba2+) e titânio (Ti4+) nos sítios A e B, respectivamente, considerando as fórmulas químicas Ba1-xGdxTiO3 e BaTi1-x NbxO3 (sendo x = 0.001, 0.002, 0.003, 0.004 e 0.005). As propriedades estruturais e microestruturais foram investigadas à temperatura ambiente, enquanto as propriedades dielétricas e elétricas foram analisadas em uma ampla faixa de temperatura e frequência. Em particular, os efeitos condutivos proporcionados pela dopagem dos íons doadores em diferentes sítios cristalográficos, cujo estudo ainda não foi reportado na literatura, foram cuidadosamente investigados para melhor compreender os fatores que influenciam e determinam as propriedades semicondutoras de tais materiais, visando aprimorar no entendimento a nível microscópico e macroscópico dos fatores que influenciam e originam os mecanismos condutivos nestes materiais, fatores determinantes para uma posterior aplicação na indústria eletroeletrônica. Os materiais foram obtidos a partir do método de Pechini (ou Método dos Precursores Poliméricos), bem conhecido na literatura por ser um método muito eficiente, quando comparado com métodos convencionais para síntese de materiais policristalinos, pois apresenta inúmeras vantagens, tais como a síntese em baixas temperaturas, baixa contaminação, maior controle estequiométrico, alta homogeneização e possibilidade de obtenção de pós em escala nanométrica. / Barium titanate (BaTiO3, BT) is a ferroelectric material with several applications for the electronic components market, due to its excellent physical properties when compared to other ferroelectric systems. So that, BT continues up today attracting the scientific and commercial interest for several decades. Among the large number of papers on the BT system reported in the literature, many are associated with the wide range of applications in the electric and electronic industry, where the use in capacitors, thermistors and other devices stands out. By doping BaTiO3 with donor ions implies in the change of some physical properties leading to abnormal behaviors in certain physical parameters. The nature of these changes is not yet very clear, although many papers have been reported trying to justify such effects. In this context, this work aims the investigation of the physical properties of the BaTiO3 system, considering the addition of donor elements in the A- and B-sites of the structure. In particular, the gadolinium (Gd3+) and niobium (Nb5+) ions will be used as modifiers on the perovskite structure by substitution of barium (Ba+2) and titanium (Ti+4) ions in the A- and B-site, respectively, considering the Ba1-xGdxTiO3 and BaTi1-x NbxO3 ( where x = 0.001, 0.002, 0.003, 0.004 and 0.005). The structural and microstructural properties were investigated at room temperature, while the electrical and dielectric properties were analyzed in a wide range of temperature and frequency. Particularly, the conductive effects provided by the doping with donor ions in different crystallographic sites, whose study has not yet been reported in the literature, were carefully investigated to better understand the factors that influence and determine the semiconductor properties of such materials. The objective is to enhance the understanding of the factors that influence and originate the conductive mechanisms in these materials at the microscopic and macroscopic level, therefore determining factors for a subsequent application in the electronics industry. The materials were obtained from the Pechini method (known as the Polymeric Precursors Method). This method is well known in the literature because it is a very efficient method, when compared with conventional methods for synthesis of polycrystalline materials, because of the numerous advantages it presents, such as the synthesis at low temperatures, low contamination, greater stoichiometric control, high homogenization and possibility of obtaining nanoscale powders.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / The goals of the present work consisted in the production of nanoparticles of pure barium titanate and doped with Nd, by means of an unpublished route that uses humic substances as complexing agent of the metalic ions and in the investigation of the influence of the synthesis parameters on the structural properties, on the crystalization of the material and on the luminescent properties of doped samples. Barium titanate (BaTiO 3 ) has potential for application in electro-optical devices, sonars, infrared sensors and multilayer capacitors. Parameters such as concentration, pH and calcination temperature were investigated during production, with the purpose of developing a simple, efficient and low energy cost procedure. The synthesis route proposed in this work was very promising in the production of BaTiO 3 . The samples produced with concentration of 0.5 g/ml presented a single tetragonal phase after calcination at 1200 ºC/5h. The crystallite size was estimated by Scherrer equation at around 28 nm and the particles were spherical, rod and needle shaped. The samples of pH 7 and 10 presented a single phase at 1100 ºC/5h and the samples doped with Neodymium presented a single phase at 1200 ºC/5h and a crystallite size estimated at 22 nm. In addition, the luminescent properties of the doped samples were studied, which were presented in accordance with the neodymium emission bands in the BaTiO 3 matrix. / Os objetivos deste trabalho consistiram na produção de nanopartículas de titanato de bário (BaTiO3 ) puros e dopados com Nd 3+ por meio de uma rota inédita que utiliza as substâncias húmicas como agente complexador dos íons metálicos e na investigação da influência dos parâmetros de síntese sobre as propriedades estruturais, na cristalização do material e nas propriedades luminescentes das amostras dopadas. O titanato de bário (BaTiO 3 ) apresenta potencial para aplicação em dispositivos eletro-ópticos, sonares, sensores de infravermelho e capacitores multicamadas. Parâmetros como pH e temperatura de calcinação foram investigados durante a produção, com o propósito de desenvolver um procedimento simples, eficiente e de baixo custo para a produção do titanato de bário. A rota de síntese proposta neste trabalho mostrou-se bastante promissora na produção do BaTiO 3 . As amostras obtidas com concentração de 0,5 g/ml apresentaram fase única tetragonal após calcinação a 1200 ºC/5h. O tamanho de cristalito foi estimado através da equação de Scherrer em torno de 28 nm e as partículas apresentaram formato esférico, de bastonetes e de agulhas. Já as amostras de pH 7 e 10 apresentaram fase única a 1100 ºC/5h e as amostras dopadas com Neodímio apresentaram fase única em 1200 ºC/5h e tamanho de cristalito estimado em 22 nm. Além disso, foi estudada a propriedade luminescente das amostras dopadas, que se apresentaram em conformidade com as bandas de emissão do neodímio na matriz do BaTiO 3 relatadas na literatura.

Integration of complex optical functionalities with high performance will lead to a huge development in the field of nanophotonics for a broad range of applications. Silicon photonics is currently the leading technology for the implementation of low-cost photonic integrated devices. The great potential of this technology relies on its compatibility with the mature silicon integrated circuits manufacturing based on complementary metal-oxide semiconductor (CMOS) processes widely used in microelectronic industry and the availability of high quality silicon-on-insulator wafers, an ideal platform for creating planar waveguide circuits that offers strong optical confinement due to the high index contrast between silicon (n=3.45) and silicon dioxide (n=1.45). In order to keep improving the performance of photonic devices on silicon, the integration of CMOS compatible materials with unique properties shows up as an excellent opportunity to overcome the current limitations in silicon while offering unprecedented and novel capabilities to the silicon platform. In this way, barium titantate (BaTiO3) stands out as one of the most disruptive candidates. The work developed in this thesis is essentially focused on the design, fabrication and characterization of an electro-optic modulator based on a hybrid BaTiO3 on silicon structure for the implementation of high performance electro-optic functionalities with beyond state-of-the art performance that currently cannot be afforded in silicon photonics technology. / La integración de funcionalidades ópticas con alto rendimiento llevará a un gran desarrollo en el campo de la nanofotónica para un amplio abanico de aplicaciones. Actualmente, la fotónica de silicio es la tecnología líder para la implementación de dispositivos fotónicos integrados a bajo coste. El gran potencial de esta tecnología reside en su compatibilidad con las maduras técnicas de fabricación de circuitos integrados de silicio basadas en los procesos "complementary metal-oxide semiconductor" (CMOS) ampliamente utilizados en la industria microelectrónica y la disponibilidad de disponer de obleas de silicio sobre aislante de alta calidad, una plataforma ideal para crear circuitos de guía de ondas planas que ofrecen un fuerte confinamiento óptico debido al alto contraste índices entre el silicio (n=3,45) y el dióxido de silicio (n=1,45). Para poder mejorar el rendimiento de dispositivos fotónicos en silicio, la integración de materiales con propiedades excepcionales y compatibles con los procesos de fabricación CMOS surge como una excelente oportunidad para superar las actuales limitaciones de la tecnología de silicio al mismo tiempo que ofrece oportunidades novedosas y sin precedentes en la plataforma de silicio. En este sentido, el material titanato de bario (BaTiO3) se postula como uno de los candidatos más prometedores. El trabajo desarrollado en esta tesis está esencialmente enfocado en el diseño, fabricación y caracterización de un modulador electro-óptico basado en una estructura híbrida de BaTiO3 en silicio para la implementación de funcionalidades electro-ópticas de alto rendimiento más allá del estado del arte de las que no se puede disponer actualmente en la tecnología de fotónica de silicio. / La integració de funcionalitats òptiques amb alt rendiment portarà a un gran desenvolupament en el camp de la nanofotònica per a un ampli ventall d'aplicacions. Actualment, la fotònica de silici és la tecnologia capdavantera per a la implementació de dispositius fotònics integrats a baix cost. El gran potencial d'aquesta tecnologia resideix en la seva compatibilitat amb les madures tècniques de fabricació de circuits integrats de silici basades en els processos "complementary metal-oxide semiconductor" (CMOS) amplament utilitzats en la indústria microelectrònica i la disponibilitat de disposar d'hòsties de silici sobre aïllant d'alta qualitat, una plataforma ideal per crear circuits de guia d'ones planes que ofereixen un fort confinament òptic a causa de l'alt contrast d'índexs entre el silici (n=3,45) i el diòxid de silici (n=1,45). Per poder millorar el rendiment de dispositius fotònics en silici, la integració de materials amb propietats excepcionals i compatibles amb els processos de fabricació CMOS sorgeix com una excel·lent oportunitat per superar les actuals limitacions de la tecnologia de silici al mateix temps que ofereix oportunitats noves i sense precedents en la plataforma de silici. En aquest sentit, el material titanat de bari (BaTiO3) es postula com un dels candidats més prometedors. El treball desenvolupat en aquesta tesi està essencialment enfocat en el disseny, fabricació i caracterització d'un modulador electro-òptic basat en una estructura híbrida de BaTiO3 en silici per a la implementació de funcionalitats electro-òptiques d'alt rendiment més enllà de l'estat de l'art de les quals no es pot disposar actualment a la tecnologia de fotònica de silici. / Castera Molada, P. (2017). Development of new photonic devices based on barium titanate in silicon [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/86197 / TESIS

This work is focused on the study of perovskite ferroelectric materials group and monitoring changes their dielectric parameters in temperature and frequency dependence Is described scrystallographic systems of barium titanate and their influence on material properties. The measured values are mathematically interpreted using the Curie – Weiss law and discussed about the influence of strontium titanite on important dielectric parameters.

<p>U poslednjih nekoliko decenija, feroelektrici su prepoznati kao dobri kandidati<br />za &scaron;irok spektar primene. Barijum titanat je jedan od najče&scaron;će istraživanih<br />perovskitnih materijala usled tipičnog feroelektričnog pona&scaron;anja na sobnoj<br />temperaturi, sa histerezisnom zavisnosti između polarizacije i električnog polja.<br />Specifična svojstva BaTiO₃ čine ovaj materijal veoma korisnim u proizvodnji<br />kondenzatora, memorija, senzora i dr. Pored toga, paraelektrična faza BaTiO₃<br />ima primenu u proizvodnji mikrotalasnih tunabilnih uređaja. BaTiO₃ pokazuje<br />paralaketrično pona&scaron;anje iznad Kirijeve temperature (120&deg;C) i veliki je izazov<br />modifikovati materijal da bude paraelektričan na sobnoj temperaturi. Delimičnom<br />supstitucijom Ba²⁺ ili Ti⁴⁺ jona, jonima Sr²⁺ ili Zr⁴⁺, respektivno, snižava<br />se Kirijeva temperatura i dolazi do pomeranja fazne transformacije feroelektri<br />čno/paraelektrično na niže temperature. Monolitna keramika na bazi bazi<br />BaTiO₃ je često istraživana kako bi se ispitale promene u strukturi i svojstvima<br />uzrokovane dodatkom dopanata, međutim ne postoji praktična primena ovih<br />materijala u mikrotalasnim tunabilnim tehnologijama. S druge strane, u trendu<br />sveop&scaron;te minijaturizacije elektronskih komponenti, tanki filmovi su prepoznati<br />kao dobri kandidati za proizvodnju tunabilnih uređaja. Jedan od ciljeva ovog<br />istraživanja bio je ispitivanje strukturnih i funkcinalnih svojstava tankih filmova na<br />bazi barijum titanata, BaTiO₃, Ba_1-xSr_xTiO₃ (x=0,1, 0,2, 0,3 i 0,4) i BaTi_1-xZr_xO₃<br />(x=0,1 i 0,2), pripremljenih hemijskom depozicijom iz tečne faze. Metalne<br />soli BaCO₃, C₄H₆O₄Sr i ZrOCl₂ H₂O i Ti(OCH₂CH₂CH₂CH₃)₄ rastvorene su<br />odvojeno u kiseloj sredini i pome&scaron;ane u transparentne solove. Inkdžet &scaron;tampa<br />i spin tehnika kor&scaron;ćene su za depoziciju funkcionalnih filmova na bazi BaTiO₃.<br />Nakon deponovanja, pripremljeni filmovi su termički tretirani na različitim<br />temperaturama do 1000&deg;C. U zavisnosti od koncentracije pripremljeni su filmovi<br />debljine od 100 do 700 nm,sa veličinom zrna od nekoliko desetina nanometara. Strukturna karakterizacija potvrdila je promene u strukturi BaTiO₃ tankih filmova<br />sa dodatkom Sr²⁺ i Zr⁴⁺. Tetragonalna (feroelektrična) faza BaTiO₃ je potvrđena<br />rentgenostrukturnom analizom i Ramanovom spektroskopijom. S druge strane,<br />smanjenje tetragonalnosti je primećeno kod dopiranih uzoraka. Promene u<br />funkcionalnosti dopiranih BaTiO₃ filmova analizirane su na osnovu dielektričnih<br />i feroelektričnih merenja. Izvr&scaron;ene analize su potvrdile feroelektrično pona&scaron;anje<br />kod BaTiO₃, dok se feroelektrični odgovor u tankim filmovima smanjuje sa<br />dopiranjem. Priprema elektroda specifičnih geometrija pogodnih za tunabilna<br />merenja različitim tehnikama depozicije bio je drugi cilj istraživanja. Tehnika spaterovanja u kombinaciji sa laserskim uklanjanjem, inkdžet &scaron;tampa i fotolitografija<br />su kori&scaron;ćene za pripremu kružnih i koplanarnih elektroda na povr&scaron;ini barijum<br />titanatnih tankih filmova. Uticaj procesnih parametara svake od pomenutih<br />metoda na dimenzionalnu preciznost pripremljenih elektroda je bio predmet istraživanja<br />u okviru teze. Odabrani laser se pokazao kao neprikladan za pripremu<br />elektroda na pripremljenim barijum titanatnim tankim filmovima. Inkdžet &scaron;tampa<br />se pokazala kao korisna u pripremi elektroda mikrometarskih fimenzija, dok je za<br />pripremu sofisticiranijih geometrija fotolitografija pokazala najbolje performanse.</p> / <p>In past few decades, ferroelectrics are recognized as good candidates for wide<br />range of applications. Barium titanate is one of the most investigated perovskite<br />materials due to typical ferroelectric behavior at room temperature, with hysteresis<br />dependence of the polarization and electric field. Specified properties<br />of BaTiO₃ make this material useful in production of capacitors, memories,<br />sensors, etc. Nevertheless, paraelectric phase of BaTiO₃ may have application<br />in production of microwave tunable devices. Barium titanate shows paraelectric<br />behavior at temperatures above the Currie temperature (120 &deg;C) and it is great<br />challenge to make material paraelectric at room temperature. Partial substitution<br />of Ba²⁺ or Ti⁴⁺ ions, by Sr²⁺ or Zr⁴⁺, respectively, decreases the Currie temperature<br />of barium titanate and moves phase transition ferroelectric/paraelectric<br />to lower temperatures. Bulk BaTiO₃was often investigated in order to improve<br />structure and functionality by addition of dopant, but do not have practical<br />application in microwave tunable technologies. On the other hand, with the<br />trend of overall miniaturization of electronic devices, thin films are recognized<br />as good candidates for production of tunable devices. One of the aims in this<br />research was investigation of structural and functional properties of barium<br />titanate based thin films, BaTiO₃, Ba_1-xSrxTiO₃ (x=0,1, 0,2, 0,3 i 0,4) and<br />BaTi_1-xZrxO₃ (x=0,1 i 0,2), prepared by chemical solution deposition. Metal<br />salts of BaCO₃, C₄H₆O₄Sr and ZrOCl₂ H₂O and Ti(OCH₂CH₂CH₂CH₃)₄were<br />separetly disolved in acetic environment and mixed in clear transparent sols.<br />Inkjet printing and spin coating were used for deposition of functional BaTiO3<br />based fims. After deposition prepared films were thermally treated at different<br />temperatures up to 1000 &deg;C. In dependence of sol concentration thickness of<br />obtained films is from 100 to 700 nm and grain size is few tens of nanometers. Structural characterization confirmed changes in structure of barium titanate thin<br />films by addition of Sr^₂₊ i Zr⁴⁺. Tetragonal (ferroelectric) phase of BaTiO₃ is<br />confirmed by X-ray diffraction and Raman spectroscopy. On the other hand, decreasing<br />of tetragonality was noticed in doped samples. Changes in functionality<br />of doped BaTiO₃ thin films were analyzed by dielectric and ferroelectric measurements.<br />Performed analysis confirmed ferroelectric behavior of barium titanate<br />thin films, and decrease in ferroelectric answer of doped films. Investigation of<br />possibility of complex shaped electrodes preparation,suitable for tunability measurements, by different deposition techniques was the second goal of this research.<br />Sputtering technique in combination with laser removal, inkjet printing and photolithography were used for preparation of complex circular and coplanar electrodes<br />on the surface of barium titanate based thin films. Influence of processing<br />parameters for the each of mentioned technique on dimensional precision of prepared<br />electrodes was investigated. Selected laser was not appropriate for production<br />of electrodes on prepared barium titanate based thin films. Inkjet printing<br />was useful for production of electrodes in micrometer range, but for more sophisticated<br />geometries photolithography shows the best performance.</p>

Systematic measurements of the magnetic moment in dependence on temperature and magnetic field of hexagonal 6H-BaTiO3 + 0.04 BaO + x/2 Fe 2 O 3 (0.005 x 0.05) ceramics were performed to study the influence of Fe ions on the magnetic properties. While the samples show Curie–Weiss paramagnetism for Fe concentrations 1.0 mol%, antiferromagnetic interactions become manifest for 2.0 and 5.0 mol% iron. With increasing Fe content the antiferromagnetic interaction, which is assumed to be caused by a superexchange mechanism Fe 3+ Ti(1) − O 2− O(2) − Fe3+ Ti(2) , becomes stronger. At external magnetic fields smaller than 1 T a further, ferromagnetic interaction between Fe 3+ ions is detected below 200 K. The interactions between Fe 3+ ions in the samples with 2.0 and 5.0 mol% iron are also manifest in the EPR spectra by numerous lines with low intensity. Q-band EPR investigations of 5.0 mol% Fe doped single crystals confirm the existence of only one type of Fe 3+ –V O associates in the samples.

No description available.