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Synthesis of NdBaFe2O5.000 metastable

The present study focuses on the NdBaFe2O5 + δ oxide. It crystallizes for a broad range of values of δ in the tetragonal system, ideally with the iron ions with a square base pyramidal coordination for the stoichiometrically precise NdBaFe2O5.000 compound.
In this thesis methods for obtaining oxide NdBaFe2O5.000 have been developed, most suitable reagents and most suitable experimental conditions were planned and discussed. The final product is intended to characterize the behavior of the mixed-valence oxide through pair-distribution-function and neutron diffraction.

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1 Introduction Perovskites constitute a large family of compounds that includes oxides. The prototype structure is derived from the simple formula ABO 3 , where A and B are cations of different sizes as for example in CaTiO 3 . This kind of structure is based on cubic close packing of oxygen, with one-quarter of the oxygen replaced by the large A cation. The large cation positions are in coordination twelve with respect to the surrounding oxygen. The B cations occur instead in octahedrons that share only apices. The total valence of both ions (A+B) must be equal to 6. The idealized structure is primitive cubic, but differences in size between the A and B cations can deform the structure to a number of different distortions, of which tilting is the most common one. Other modifications of this basic structure can result in more complicated perovskite-type compounds. Among these are the so called double, or ordered, perovskites. This term actually covers two different types of perovskite structures, which have little in common. One of them has usually doubled edges (8 times the volume) of the simple perovskite unit cell and the B cations are ordered. The other has a double volume unit cell (with respect to the simple perovskite prototype) and ordered A cations. We will consider here the double-cell perovskite type with the A-cation ordered. Ordering at the A site is typically achieved with divalent Ba (A cation) and trivalent rare earth element, RE (A’ cation). Oxygen deficiency around the smaller A’ cation is common. The structural prototype, YBaCuFeO 5+δ , was synthesized 1 in 1988, opening interesting prospectives for substitutions of Cu with Fe in order to understand magnetic properties in ordered perovskites. he present study is focused on the oxide NdBaFe 2 O 5+δ (Figure 1). It crystallizes for a wide range of δ is a transition in which a Ba NdFeO Figure 1 Structure of NdBaFe 2 O 5+δ . T in tetragonal system as shown in Figure 1; ideally with iron atoms in square pyramidal coordinations for the precise stoichiometry NdBaFe 2 O 5.000 as described in Ref.2. The latter composition has iron in average valence state +2.5. Such phases are called mixed-valence compounds. While at room temperature only one valence state is distinguishable, +2.5, below certain temperature a transformation called Verwey transition occurs into two separate valence states +2 and +3. Verwey transition was observed 3 for the first time in magnetite (Fe 3 O 4 ). It valence-mixed compound transforms into a valence-ordered compound. The process is also called charge ordering. Magnetite is a common iron oxide mineral. It shows a strong magnetism and it is probably the oldest recognized natural magnetic material. It crystallizes in the cubic system and 1

Laurea liv.I

Facoltà: Scienze Matematiche, Fisiche e Naturali

Autore: Davide Ciceri Contatta »

Composta da 49 pagine.

 

Questa tesi ha raggiunto 135 click dal 30/05/2008.

Disponibile in PDF, la consultazione è esclusivamente in formato digitale.