Kirshenbaum, Kevin CharlesI report on superconductivity in undoped SrFe₂As₂ and find that it is caused by lattice strain in the as-grown crystals that can be removed or returned with annealing or pressure, respectively. To study the magnetic/structural transition I measure the evolution of these transitions in solid solutions of the [Ca, Sr, Ba]Fe₂As₂ series and determine that the Neel temperature is independent of the size of the antiferromagnetically ordered moment. I present the first reported phase diagrams for Ni- and Pt-substitution in SrFe₂As₂, showing that the simple charge-counting picture of chemical substitution cannot completely describe the onset and offset of the superconducting phase. Finally, I use the transport scattering rate to explain the variation in T_c seen in transition metal substituted 122s. I will show that pair breaking can explain the variation in the optimum transition temperature, and that the rate of suppression of T_c with scattering will show that the pairing symmetry of the iron-based superconductors is a sign-changing, multiband s-wave order parameter that must include both inter- and intraband scattering.DissertationCondensed matter physicsPhysicscondensed matter physicsIron-based superconductorslow temperaturematerial synthesisSuperconductivitytransport measurements