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Abstract The subject of this thesis is the preparation and characterization of transition metal oxides RE

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Abstract

The subject of this thesis is the preparation and characterization of transition metal oxides RE

2−x

A

x

MO

4

(RE = La; A = Sr, Ca, Ba; M = Co, Mn, Ni) with the layered K

2

NiF

4

structure as well as the analysis of electrical tranport properties and magnetic susceptibility measurements in these systems. A detailed description of the preparation procedure of these compounds can be found in this work. The study of the electrical transport properties of the cobaltate system revealed a bahaviour which is strongly influenced by localization of charge carriers. Finally, it turned out that the Efros-Schklovskii model was very well suited in order to describe the measured data. Besides this, in cooperation with D. Meier, N.

Hollmann and E. Rose it was found, that the heat conductivity of this strongly anisotropic system is largest in c -direction wheras the electrical conductivity and the magnetic moment are strongest within the ab -planes. Cluster calculations gave evidence that the Co

3+

-ions is in the LS state for higher doping levels ( x ≥ 0.5 ) and in the HS state for lower doping levels ( x =0.3). Coming to the stripe ordered nickelates, soft resonant X-ray measurements of J. Schlappa revealed a localization of the holes on the oxygen sites and not on the Ni- sites. Finally, neutron measurements of D. Sennf succeeded in determining the magnetic exchange parameter of the complex CE-type order in the manganite system.

Kurzzusammenfassung

Das Thema dieser Dissertation ist zum einen die Preparation und Charakterisierung von Übergangsmetallsoxiden RE

2−x

A

x

MO

4

(RE = La; A = Sr, Ca, Ba; M = Co, Mn, Ni) mit K

2

NiF

4

Schichtstruktur und zum anderen die Untersuchung elektrischer Transporteigen- schaften sowie magnetischer Susceptibilitätsmessungen an diesen Systemen. Eine detail- lierte Beschreibung des Kristallpräparationsverfahrens dieser Verbindungen kann in dieser Arbeit gefunden werden. Die Analyse der elektrischen Transport-Eigenschaften an dem Kobaltat-System zeigte ein Verhalten welches stark druch Lokalisierung der Ladungsträger bestimmt ist. Letztlich zeigte sich, daß das Efros-Schklovskii Model am besten geeignet war, um die gemessenen Werte zu beschreiben. Daneben wurde in Zusammenarbeit mit D.

Meier, N. Hollmann und E. Rose herausgefunden, daß die Wärmeleitfähigkeit dieses stark

anisotropen systems am stärksten in c -Richtung ist, während die electrische Leitfähigkeit

und auch das magnetische Moment am stärksten innerhalb der ab -Ebenen sind. Cluster-

Rechnungen deuten darauf hin, daß sich die Co

3+

-Ionen für höhere Dotierungen ( x ≥ 0.5 )

im LS Zustand befinden, während sie für niedrigere Dotierungen ( x =0.3) im HS Zustand

sind. Bei den Streifen-Ladungsgeordneten Nickelaten zeigten weiche resonante X-ray Mes-

sungen von J. Schlappa, daß die Löcher auf den Sauerstoffen und nicht auf den Nickelionen

sitzen. Schließlich konnten anhand von Neutronenmessungen von D. Senff die magnetischen

Austauschparameter der komplexen CE-Typ Ordnung in dem Manganat System bestimmt

werden.

Referenzen

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