Date of Award

12-2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemistry

Advisor

Hwu, Shiou J

Committee Member

Pennington , William T

Committee Member

Daw , Murray S

Committee Member

Brumaghim , Julia L

Abstract

Inline with the focus of our research, my goal for this dissertation study is to further explore new classes of magnetic insulators exhibiting novel properties. The scope of this work is twofold: 1) the exploratory synthesis of polyanion-based mixed-framework compounds containing transition metal oxide magnetic nanostructures, and 2) the investigation of the structure/property correlation with respect to the spin couplings in confined lattices. These studies are of fundamental and technological importance regarding the long-term development of low-dimensional magnetic solids for device applications in especially information storage and quantum computing. Several interesting and significant transition metal based silicate, phosphate arsenate and vanadate compounds with structural frameworks featuring low-dimensional layers, chains, and oligomers were synthesized. Their synthesis and characterizations are outlined in this dissertation in the order of the size of the magnetic nanostructures following the introduction (Chapter 1) and experimental (Chapter 2) sections. The first project, as illustrated in Chapter 3, was undertaken for the attempted synthesis of the solid-solution series Cs2-xRbxCu3P4O14 (x = 0, 0.33, 0.50, 0.80). The primary focus of this study is on the size effect of monovalent cation to the bulk magnetic properties. The second project was the exploratory synthesis of 'nanostructured' manganese oxide materials. In this project, new manganese-based phosphates and arsenate systems have been explored. We were able to isolate several new and interesting compounds displaying interesting structures and properties. (Cs3Cl)Mn4O4(AsO4)3 (Chapter 4) contains structurally isolated Mn4O4 cubanes residing in each of the anti-ReO3 type (Cs3Cl) cages. The field dependent magnetic studies show superparamagnetic type behavior similarly found in magnetic nanoparticals. A new manganese phosphate compound, BaRb3KMn6(PO4)6, adopting a new structure type reveals a novel 'bangle'-like magnetic nanostructure unit (Chapter 5). Also, a new manganese(III) phosphate compound, Ba2Mn3O2(PO4)3, exhibits structurally isolated nanowire (Chapter 6). This compound presents once again the utilities of closed-shell, nonmagnetic oxyanions for synthesizing structurally isolated and electronically insulated magnetic nanostructures. This study also allows us to further understand the issues concerning low-dimensional magnetic structures in extended solids, magnetic anisotropy, coherent length for magnetic coupling, and their relationships with quantum tunneling of magnetization, a well-recognized phenomenon in single molecule magnets (SMMs). The mixed-valance manganese(II,III) systems, such as Na3Mn7(AsO4)6, KNaMn3(AsO4)3 (Chapter 7), μ3-oxo Ba2CsRbMn7O2(AsO4)6 and μ4-oxo CsMn3O(AsO4)2 (Chapter 8), presents the complexity of spin interactions in compounds containing two-dimensional Mn-O lattices thus magnetic anomalies. The third project (Chapter 9) deals with a new series of fresnoite-type solids; Ba2Mn(Mn1.4Si0.6O7)Cl, Rb2MnV2O7Cl and Cs2MnV2O7Cl, where noncentrosymmetric lattices and ferromagnetic properties are evident. This discovery offers opportunities for the search of new 'multiferroic' materials for device miniaturization. In summary, the man body of this dissertation reports the exploratory synthesis, structural characterization, and physical properties studies of selected chemical systems that contain magnetic nanostructures. The Appendix summarizes remaining new compounds discovered in this study that exhibit equally interesting features.

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