Drye, Tyler BrunsonChemical substitution into iron-pnictide parent compounds (e.g. AFe<sub>2<\sub>As<sub>2<\sub> where A=Ba, Sr, or Ca) has proven to be an effective means to induce bulk high-temperature superconductivity in these systems. By doping CaFe<sub>2<\sub>As<sub>2<\sub> with rare-earth lanthanides (La, Ce, Pr, and Nd), we have observed a 47 K superconducting phase coexisting with a lattice distorting “collapse” transition. Both of these effects have important ramifications: the collapse transition occurs when interlayer As atoms form a bond, shrinking the <italic>c-axis<\italic> lattice constant and simultaneously quenching the iron magnetic moment. This transition is further explored in context of a similar system, Sr-doped BaNi<sub>2<\sub>As<sub>2<\sub>. The superconducting phase, given the right combination of conditions, appears with a critical temperature as high as 49 K, but always in a very small volume of the sample (as determined by shielding effects). This has led to interesting theories about the nature of this superconductivity. A recently posited idea of “interfacial superconductivity” has been ruled out by our tests. Additionally, increasing the concentration of rare-earth atoms does not increase the superconducting volume fraction, but, in fact lowers the transition temperature, excluding the hypothesis that rare-earth defects are responsible for the minority superconducting phase. New pressure measurements have shown that the superconducting phase is stabilized when antiferromagnetic order is fully suppressed.enStructural Changes and the Nature of Superconductivity in Rare-earth Doped CaFe2As2DissertationCondensed matter physicsInorganic chemistryMaterials ScienceCaFe2As2Iron Pnictidesiron superconductorstructural collapse