Neutron diffraction studies of disorder in R(2)T(17) (R = neodymium, dysprosium, samarium, terbium and T = iron, silicon, aluminum) and (R)iron oxide perovskite systems

The properties of materials are strongly dependent on their crystalline structure. Even a slight disorder from the ideal structure may have an impact on physical properties. Study of the structural dependence of properties can provide physical insight into the material, and serve as guidelines for material scientists to seek better materials. In this dissertation, the disorder in two types of structures has been studied.; In the La_0.6Sr_0.4FeO_3−δ compounds, the oxygen vacancies significantly change the degree of positional disorder; consequently, the significant change of magnetic properties occurs. The oxygen vacancies were created by different treatments, such as annealing samples under various gases including N₂, air and mixtures of CO/CO ₂. The air- or oxygen-annealed samples have almost no oxygen vacancies while those made in reducing atmospheres show 7–11% oxygen vacancies. The rhombohedral distortion decreases in the reduced samples to such an extent that they appear almost cubic. All the samples exhibit antiferromagnetic ordering at room temperature. The samples with little or no oxygen vacancies show a magnetic moment of ∼1.4μ_B at the Fe site while those having >7% oxygen vacancies show a moment of ∼4.0μ_B.; The novel disordered rhombohedral (DR) compounds [1] is derived from the CaCu₅ structure, with the same unit cell as the well known ordered rhombohedral 2:17 structure, but with excess Fe dumbbells replacing some of the rare-earth atoms. The presence of excess Fe dumbbells due to the disorder makes their magnetization higher than that of their ordered parent compounds. In addition, experiments showed that the Curie temperature increases as the degree of disorder increases, which makes this compound more interesting for the permanent magnetic material. The neutron studies have been carried out on samples of (NdDy)_xFe_17−ySi_y with 1.5 ≤ x ≤ 2 and 1 ≤ y ≤ 3, Tb_2−xSm _xFe_17−ySi_y with 0.5 ≤ x ≤ 1 and 1 ≤ y ≤ 3, Nd_2−xTb_xFe_17−ySi _y with 0.5 ≤ x ≤ 1 and 1 ≤ y ≤ 3, Nd_2−xDy _xFe_17−y(Al_0.5Si_0.5)_y to determine the stability range of the disordered rhombohedral 2:17 phase and to examine the effects of Fe excess, and Tb and Sm substitution on the Curie temperature.