Our primary goal, to explore the ground state behavior of low-dimensional antiferromagnets to inform the understanding of the AFM phase of the high-temperature copper(II) oxide superconductors, requires systems with weakly superexchange interacting copper(II) ions such that the magnetic spectrum can be explored up to saturation. The two-halide, bi-halide, and pyrazine bridges typically confer weak interactions with variation based on geometric and electronic effects, so compounds with these features are explored here. In this dissertation the design, synthesis, and characterization of a variety of copper(II) coordination compounds were addressed and the compounds were evaluated in terms of their proximity to low-dimensional antiferromagnetic lattices. With a large family of compounds of varying class, including copper(II) halide salts with bulky organic cations, copper(II) halide neutral compounds with N or O donor organic ligands, and copper(II) pyrazine coordination polymers, our group has developed magneto-structural correlations and design routes which have nearly enabled the synthesis of designer magnetic materials.
Thus, our group has developed scaffolding techniques to design layers, chains, ladders and dimers of copper(II) ions. With the X-ray crystal structure for reproducible compounds, we explore the hydrogen bonding, crystal packing, and coordination geometry which confer the desired superexchange strength, sign and dimensionality, which we evaluate further via SQUID magnetometry. The magnetization up to 50 kOe in the following projects inform on the dimensionality and approximate strength of interactions as well as the ground state behavior of singlet state systems. The magnetic susceptibility is explored to quantify the exchange based on analytical models for the various dimensionalities. The structure and magnetism of the compounds are evaluated for consistency or correlations and for the degree of isolation between the layers, chains or ladders.
Recently, our group has expressed interest in the characterization of our compounds via single crystal EPR spectroscopy to supplement powder studies to further evaluate the dominant magnetic interactions and isolation between features. The process developed in the dissertation for the evaluation of our compounds with EPR spectroscopy is outlined in chapters 2 and 3 and pursued in a several projects, including single-crystal experiments in chapter 10 and 15 used to evaluate the major contributors to the angular dependence of the linewidth.
In this dissertation the synthesis and characterization of twenty-eight compounds are outlined in detail. Included are dimeric and monomeric copper(II) bromide compounds with the bulky non-coordinating cation, triethylbenzylammonium, (compounds 5-7), including a special ladder compound with ferromagnetic rungs and antiferromagnetic rails which exists in an understudied regime of the Haldane chain. Several salts and neutral copper(II) halide compounds with the intrinsically cationic 1-(4’-pyridinium)-4-hydroxypyridine and 1-(4’-pyridinium)-pyridinium or closely related moieties (compounds 8-18), including a unique honeycomb-like compound evaluated by detailed calculations. A large family of copper(II) pyrazine bridged compounds with halide substituted 4-pyridone ligands including chains, ladders and layers (compounds 21-28) achieved well-isolated lattices with the potential to extract magneto-structural guidelines for the pyrazine-bridged scaffold. The attempted deuteration of, and dilution with zinc of, (QuinH)2CuBr4⋅2H2O, are detailed in chapters 4 and 5 to observe ordering-phenomena. Two halide-bridged copper(II) pyrazine compounds (19 and 20) with interesting implications are evaluated in detail in chapter 11 via unique polycrystalline temperature dependent magnetization studies. Several additional compounds are outlined briefly in chapter 16. Throughout the dissertation design elements which inform future studies are emphasized, and magneto-structural correlations are evaluated for compounds with exceptionally weak exchange (∼1 K) to very strong exchange (∼100 K).
|Advisor:||Turnbull, Mark M., Landee, Christopher P.|
|Commitee:||Smith, Luis, Greenaway, Frederick T.|
|School Location:||United States -- Massachusetts|
|Source:||DAI-B 81/5(E), Dissertation Abstracts International|
|Subjects:||Chemistry, Inorganic chemistry, Physics|
|Keywords:||Antiferromagnetism, Crystal structure, Electron paramagnetic resonance (EPR), Magnetic susceptibility, Magnetization, Synthesis|
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