<p>Ispitivani su dvostruki slojeviti hidroksidi, LDH i me&scaron;oviti oksidi nastali njihovom<br />razgradnjom, sa različitim sadržajem Mg-Al-Fe i pro&scaron;irenom supstitucijom aluminijuma.<br />Koprecipitacione metode male i velike prezasićenosti odabrane su za sintezu LDH op&scaron;te<br />formule [Mg1_-xM(III)_x(OH)₂](CO₃)_x/2 &sdot; mH₂O, gde M(III) pretstavlja Al i/ili Fe, a x udeo<br />trovalentnih anjona, x = M(III) / [M(II) + M(III)], variran u intervalu 0,15 &lt; x &lt; 0,7.<br />Katalizatori na bazi me&scaron;ovitih oksida sa gvožđem pogodni za upotrebu u redoks<br />reakcijama, formirani su nakon termičke aktivacije - razlaganja LDH. Izvr&scaron;ena je<br />karakterizacija me&scaron;ovitih oksida nastalih iz LDH, kao i netretiranih LDH, ispitivanjem<br />kristalne strukture (XRD), termičke stabilnosti (TG-DTA), teksture (adsorpcija N₂), redoks<br />(H₂ TPR) i kiselo-baznih karakteristika (NH₃ TPD), a utvrđena je i priroda prisutnih vrsta<br />gvožđa (Moessbauer spektroskopija). Pokazano je kako metoda pripreme i obim<br />supstitucije M(III) jona utiču na strukturne, povr&scaron;inske i redoks karakteristike LDH i<br />njihovih me&scaron;ovitih oksida u korelaciji sa katalitičkim svojstvima u odabranim test<br />reakcijama (razgradnja N₂O i redukcija sa NH₃). Utvrđeno je da na katalitičku aktivnost<br />dominantno utiču redoks karakteristike i priroda vrste gvožđa u me&scaron;ovitom oksidu, ali<br />takođe i strukturne karakteristike polaznog LDH, pri čemu je povr&scaron;inska struktura aktivnih<br />centara intervalentnog Fe²⁺-Fe³⁺gvožđa ključna za redoks procese ispitivanih reakcija.<br />Kod svih me&scaron;ovitih oksida dobijenih iz LDH aktivni centri imaju relativno jednake jačine,<br />dok razlike u katalitičkom pona&scaron;anju potiču od razlika u dostupnosti aktivnih centara.<br />Najbolje performanse u katalitičkim reakcijama pokazali su uzorci sa najmanjom<br />stabilno&scaron;ću inicijalnih LDH kod kojih je količina supstituisanih M(III) jona bila blizu<br />granice za inkorporaciju u LDH matricu (x = 0,5), a naročito uzorak sintetisan metodom<br />velike prezasićenosti sa malom količinom dodatne Al(OH)₃ faze.</p> / <p>Layered double hydroxides (LDHs) and derived mixed oxides with different Mg-Al-Fe content and extended aluminum substitution were investigated. High and low supersaturation precipitation methods were used for the synthesis of LDHs with general formula [Mg1_-x M(III)_x (OH)₂](CO₃)_x/2 &sdot; mH₂O where M(III) presents Al and/or Fe and x the content of trivalent ions, x = M(III) / [M(II) + M(III)], varied between 0.15 &lt; x &lt; 0.7. Iron-containing mixed oxides, suitable as catalysts for redox reactions, were formed after thermal activation - decomposition of LDH. Both, LDHs and derived mixed oxides were characterized with respect to crystalline structure (XRD), thermal stability (TG-DTA), textural (N₂ adsorption), redox (H₂ TPR) and acid-base properties (NH₃ TPD) as well as the nature of the iron species (Moessbauer spectroscopy). It was demonstrated how preparation method and the extent of M(III) substitution influence the structure, surface and redox properties of LDHs and derived mixed oxides in correlation to catalytic properties in chosen test reactions (N₂O decomposition and reduction with NH3). The catalytic behavior is mainly determined by the redox properties and the nature of the iron species in mixed oxides, but also by the structural properties of initial LDHs. It was confirmed that surface structure of intervalent Fe²⁺-Fe³⁺ iron active sites is crucial for the redox processes in chosen reactions. The strength of active sites is similar for all mixed oxides derived from LDH, but the differences in catalytic behavior arise from the differences in active sites accessibility. The best catalytic results were obtained when the stability of the initial LDH matrix was lowest e.g. when the amount of substituted M(III) was near the limit for the incorporation into LDH matrix (x = 0.5), especially for the sample synthesized with high superasaturation method containing a small amount of additional Al(OH)₃ phase.</p>

In this thesis, I study the electronic and structural phase diagrams of the superconducting 122 iron pnictides systems Ba1-xKx(Fe1-yCoy)2As2 and EuFe2(As1-xPx)2 by means of the local probe techniques 57Fe Mössbauer spectroscopy (MS) and muon spin relaxation (muSR). For both isovalent substitution strategies - Co/K for Fe/Ba and P for As, respectively - the antiferromagnetic Fe ordering and orthorhombic distortion of the parent compounds BaFe2As2 and EuFe2As2 are subsequently suppressed with increasing chemical substitution and superconductivity arises, once long-range and coherent Fe magnetic order is sufficiently but not entirely suppressed. For Ba1-xKx(Fe1-yCoy)2As2 in the charge compensated state (x/2=y), a remarkably similar suppression of both, the orthorhombic distortion and Fe magnetic ordering, as a function of increasing substitution is observed and a linear relationship between the structural and the magnetic order parameter is found. Superconductivity is evidenced at intermediate substitution with a maximum Tsc of 15 K coexisting with static magnetic order on a microscopic length scale. The appearance of superconductivity within the antiferromagnetic state can by explained by the introduction of disorder due to nonmagnetic impurities to a system with a constant charge carrier density. Within this model, the experimental findings are compatible with the predicted s± pairing symmetry. For EuFe2(As1-xPx)2, the results from 57Fe MS and ZF-muSR reveal an intriguing interplay of the local Eu 2+ magnetic moments and the itinerant magnetic Fe moments due to the competing structures of the iron and europium magnetic subsystems. For the investigated single crystals with x=0.19 and 0.28, 57Fe MS evidences the interplay of Fe and Eu magnetism by the observation of a transferred hyperfine field below Tafm at which the Eu subsystem orders into a canted A-type AFM magnetic structure. Furthermore, an additional temperature dependent out-of-plane tilting of the static Fe hyperfine field is observed below the onset of static Eu ordering. ZF-muSR shows a strong increase of the local field at the muon site below Tafm=20 K and a crossover from isotropic to anisotropic Eu spin-dynamics between 30 and 10 K. The temperature dependence of the spin dynamics, as derived from the muSR dynamic relaxation rates, are related to a critical slowing down of Eu-spin fluctuations which extends to even much higher temperatures (~100 K). They also effect the experimental linewidth observed in the 57Fe MS experiments. The strong influence of the Eu magnetic order onto the primary observables in both methods prevents conclusive interpretation of the experimental data with respect to a putative interplay of Fe magnetism and superconductivity.

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