Magnetic and high-field EPR studies of new spin-frustrated systems.

摘要

This dissertation presents studies by magnetization, do magnetic susceptibility and EPR spectroscopy on several new spin-frustrated polyoxometalate (POM) lattices of increasing spin sizes and complexities. Measurements have been made over the wide temperature range of 1.8--95 K, frequencies of upto 400 GHz and magnetic fields upto 13 Tesla. The goal was to discover compounds with high spin ground states. The POMs studied are Na9[Na 3Cu3(H2O)9(alpha-AsW9O 33)2] (AsCu3), Na 11Cs[Cu4(H2O)2(B-alpha-GeW9 O34)2] (GeCu4), K 10[Cu5(OH)4(H2O)2(A-alpha-SiW 9O33)2) (SiCu5), Na 7Cs4[Fe6(OH)3(A-alpha-GeW9O 34(OH)3)2] (GeFe6) and Na5[Co6(H2O)30 {lcub}Co9Cl 2(OH)3(H2O)9(beta-SiW8O 31)3{rcub}] (SiCo15). Spin-frustrated lattices are of current interest due to their potential to provide large spin ground states for certain topologies. Chapter 2 describes the powder and single crystal EPR and magnetic susceptibility characterization of AsCu 3 complex, a prototypical spin-frustrated equilateral triangular system with a thermally accessible magnetic excited state. Chapter 3 is focused on the (Cu2+)4 tetramer, GeCu4, a four-spin rhombic core, found in a triplet ground state, theoretically and experimentally. Interestingly, the unpaired electrons are delocalized over only two of the four Cu2+ ions. Chapter 4 details magnetic and EPR characterization of a model five spin-coupled system, SiCu 5, which exhibits a spin doublet ground state. The spin Hamiltonian parameters have been analyzed in terms of simple crystal field---molecular orbital model in order to understand the bonding environment of the Cu 2+ ion. Chapter 5 describes magnetic susceptibility and EPR measurements of the Fe3+ hexamer, GeFe6. GeFe6 is a good example of a highly spin-frustrated system with a 111-fold degenerate diamagnetic ground state (ST = 0). Finally, Chapter 6 presents the magnetic and EPR studies of a (Co 2+)15 cluster, SiCo15. Data analysis reveals the presence of a doubly degenerate S = ½ ground state, once again a result of the spin-frustration present in the system. In conclusion, while the present experimental and theoretical studies of incorporating transition metals in POMs did not yield compounds with high-spin ground states, they are thought to be significant additions to the emerging field of spin-frustrated magnetic systems.