Geometric magnetic frustration, frustrated ferroelectricity, and superconductivity in transition metal compounds

Understanding the relationships between the structures and properties of materials has been a long-standing goal in solid state chemistry. Systems that display correlated electron behavior -- that is, when the interactions between electrons within a single atomic site or between adjacent sites are non-negligible -- are particularly rich in properties but challenging to understand. The first part describes a study of the properties of two layered triangular lattice materials. NaVO2 contains triangular layers of V3+ (d2) ions. The octahedral coordination of the vanadium ions splits the five d-orbital-derived bands into a triply degenerate t2g and a doubly degenerate eg set. A Jahn-Teller distortion, along with a form of geometric magnetic frustration, give rise to two successive orbital ordering transitions. NaFeO2 contains triangular layers of Fe3+ (d5) with the electrons spread out across the d-orbital-derived bands. In this case, no structural distortion is observed down to 1 K. Instead, geometric magnetic frustration prevents magnetic ordering, giving rise to a series of intricate magnetic states. The second part describes preliminary results on a series of reduced niobium pyrochlores in which electric dipoles do not adopt long range order. This is analogous to geometric magnetic frustration, but little is known about these systems displaying frustrated ferroelectricity. The third part describes a series of detailed chemical characterizations of members of the new "iron-based" superconductors. Fe1+deltaSe is shown to be extremely sensitive to defects and composition, with the best superconductors found when it is nearly stoichiometric, with formula Fe 1.01Se. 77Se NMR provides evidence of the importance of spin fluctuations in the observed superconductivity. LaNiPO is shown to be similarly sensitive to composition. The low temperature specific heat of LaNiPO is consistent with the presence of spin fluctuations. Taken together, these results suggest that spin fluctuations may be important for superconductivity in the entire family of materials.