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This dissertation presents EPR spectroscopic, dc magnetic susceptibility, and thermo-magnetic studies of three spin S = 1 systems. This work is primarily focused on the complexes [(CH3CN)5VOV(CH3CN)][BF4]4, Cr(C4H13N3)((O2)2.H2O and Cr(NH3)3(O2)2. Chapter 2 discusses the experimental techniques used in this work. Chapter 3 details the powder and single crystal magnetic susceptibility, magnetization and high field/frequency EPR characterization of [(CH3CN)5VOV(CH3CN)5][BF4]4 complex. Analysis of the magnetic susceptibility and high frequency EPR data in the realm of Heisenberg exchange interactions and quantum mechanical spin interactions, reveal that the ferromagnetic spin state ST = 2 is the ground state. The systematic study of this dinuclear V3+ complex provides a model system for understanding molecular magnetism. Chapter 4 presents magnetic susceptibility and heat capacity experiments as a function of temperature over 1.8-300 K and magnetic field 0-9 T on the 3-dimensional antiferromagnet Cr(NH3)3(O2)2. The compound undergoes an antiferromagnetic phase transition at 8.46 K. The analysis of magnetic field vs transition temperature showed that the compound shows standard antiferromagmentic behavior. The ligand is shown to play a key role in the ordering processes of such systems. Replacing NH3 by C4H13N3 gives the 2-d compound Cr(C4H13N3)(O2)2.H2O. Chapter 5 presents detailed study of magnetic susceptibility, torque magnetometry, heat capacity and magnetocaloric effect measurements on Cr(C4H13N3)(O2)2.H2O. Crystal structure analysis of Cr(dien) indicated the availability of low dimensional spin exchange pathways. Magnetic susceptibility and zero field heat capacity measurements showed that Cr(dien) undergoes an antiferromagnetic ordering transition at TN = 2.55 K. The specific heat and magnetocaloric effect measurements have been performed for magnetic fields from 0-18 T and from temperatures of 0.2 K to 2 K to define previously undetermined phase boundary in the field-temperature phase space. Analysis of the magnetocaloric effect data revealed the compound undergoes a quantum phase transition at a zero temperature magnetic field value of 12.3 T. Therefore the results presented here should provide a significant contribution to the understanding of quantum phase transitions in Cr(IV) compounds.
Quantum Phase Transitions, Electron Paramagnetic Resonance
Date of Defense
April 14, 2010.
A Dissertation submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Includes bibliographical references.
Naresh S. Dalal, Professor Directing Dissertation; James S. Brooks, University Representative; Harold W. Kroto, Committee Member; Geoffrey F. Strouse, Committee Member.
Florida State University
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