| Studies of superconductivity, magnetism and structure under pressure have made important contributions to furthering our understanding of the physical properties of materials. High pressure can change the superconducting transition temperature, create new superconductors, change the magnetic state of a material, and induce structural transitions. In Eu metal the strong local-moment magnetism in divalent Eu (4f7) suppresses superconductivity. Under sufficiently high pressure, Eu is expected to become trivalent and nonmagnetic (4f 6). Recently, superconductivity in Eu was discovered by Debessai et al. for pressures above 80 GPa. However, Eu's transition temperature lies near 2 K, nearly an order of magnitude lower than for comparable trivalent s,p,d-electron metals (Y, Sc, La, and Lu), possibly because its crystal structure is less favorable for superconductivity, or because Eu is not fully trivalent but rather mixed valent. In this thesis I present the results of extensive high pressure studies on Eu's structure, valence, and magnetism to nearly 1 Mbar pressure through a series of synchrotron techniques including x-ray diffraction, x-ray absorption near edge structure, x-ray magnetic circular dichroism, and synchrotron Mossbauer spectroscopy. In addition, the pressure-dependent superconducting and magnetic properties of selected Fe-based pnictides were also studied. |
