| This dissertation details the synthesis and characterization of a variety of B-site substituted transition-metal oxides. The goal of this work was to develop new materials which may exhibit large magnetoresistance as well as to gain an understanding of the role the B site cation plays in the observed magnetoresistance properties. During the course of this research the following systems were investigated.; A series of metastable Mn substituted n = 2 Ruddlesden-Popper (RP) strontium ferrates, Sr3Fe2−xMnxO7−δ , where x = 2/3, 1 and 4/3, were studied. These compounds are oxygen deficient and contain some Fe3+. They are insulators and exhibit spin-glass like behavior at low temperatures. Mössbauer and XANES spectra show that the B cations are not fully oxidized and that the Fe4+ in these compounds are charge-disproportionated into Fe3+ and Fe5+ at low temperature.; Two oxygen deficient n = 2 RP type compounds, Sr3FeMoO 6.5 and Sr3FeNbO6.5, have been prepared. Sr 3FeMoO6.5 is semi-conducting, exhibits ferromagnetic cluster-glass type magnetism and a maximum MR of −15% at 5 K in an applied field of 5 T. Sr3FeNbO6.5 exhibits spin-glass type magnetism and is an insulator. Mössbauer data is consistent with the iron being Fe3+ in Sr3FeMoO6.5, while the oxidation state of Fe in Sr3FeNbO6.5 is both 3+ and 2+.; The first molybdenum oxynitride pyrochlore, Sm2Mo2O 3.83N3.17, was synthesized by heating Sm2Mo 2O7 in flowing ammonia. Sm2Mo2O 3.83N3.17 forms with the cubic pyrochlore structure, space group Fd3m (a = 10.4975 Å). The sample is semiconducting and the temperature-dependent magnetic susceptibility follows Curie-Weiss behavior. XANES measurements indicate the molybdenum has a formal oxidation state significantly larger than 4+.; The compounds Ln2FeMoO7 (Ln = Y, Dy, Ho) have been synthesized by solid-state reaction. These materials adopt an anion deficient super-structure of fluorite with the space group C2/ c. XANES data confirms that Fe is trivalent while Mo is pentavalent. Magnetic data indicate that Y2FeMoO7 undergoes a field dependent AFM transition around 115K and a spin-glass like transition at T g ∼ 20 K. Dy2FeMoO7 and Ho2FeMoO 7 exhibit nearly ideal Curie-Weiss paramagnetism.; The last chapter details the synthesis and characterization of the monoclinic double-perovskite Sr2CrNbO5.53. This compound is an insulating spin-glass. Neutron diffraction data indicates that the oxygens preferentially occupy the equatorial positions around the metal-octahedra. Low-temperature neutron diffraction reveals that below 150 K the Cr 3+ t2g bands split due to the non-symmetric distribution of oxygen atoms around the Cr octahedra. |
