A family of oxygen-deficient perovskite phases with compositions, Ca₂Mn_2-xNb_xO_γ has been synthesised and characterised using X-ray Powder Diffraction and Thermogravimetry. Property characterisation has included the study of electrical properties by Impedance Spectroscopy and the investigation of magnetic properties for one composition, x = 1.0. Some compositions were tested for possible application as electrode materials in gas sensors. The Ca₂Mn_2-xNb_xO_γ system 0 ≤ x ≤ 1.2, with variable oxygen content, γ, can accommodate up to 20% vacancies at oxygen sites and Mn occurs in valence states ranging from +2 to +4. Depending on the B-cation ratio and oxygen content, two solid solution form: an extensive GdFeO₃-type solid solution over the entire range of cation content, 0 ≤ x ≤ 1.2, with zero or small oxygen deficiencies and a closely-related, grossly oxygen-deficient solid solution over the range 0.3 ≤ x ≤ 0.8 with a simple cubic perovskite structure. Unit cell volume and orthorhombic GdFeO₃-type distortion vary greatly with Mn valency, oxygen content and B-cation content. The orthorhombic GdFeO₃-type structure of composition x = 1.0 was refined by the Rietveld method. Mn and Nb are disordered over the B-sites. Jahn-Teller activity of Mn³⁺ does not result in cooperative distortions of (Mn,Nb)O₆ octahedra, but it was observed to effect the oxygen stoichiometry. The Nb-rich composition, x = 1.0, can accommodate large amounts of Ca-vacancies (up to 10%). Electrical conductivity varies greatly with the composition. Results suggest that t_2g electrons are responsible for a variation of conductivity and activation energy rather than e_g electrons. Conductivities are believed to depend mainly on the degree of π-orbital overlap between Mn and oxygen and hence, on interatomic distances. At low temperatures, the material exhibits spin glass-like behaviour.

Manganese (Mn) in excess of crop requirements is a serious problem when manganiferous soils become acid, waterlogged or amended with organic materials. We investigated the dynamics of manganese phytotoxicity and tested management options for growing crops in acid soils with excess Mn. We hypothesized that Mn phytotoxicity is governed by water use and expressed as continuous negative interaction between current plant/leaf growth rate (RGR PLant/Leaf) and future Mn accumulation rate (RAR Mn); and that under growth conditions where RGRPlant/Leaf exceeds RAR Mn, excess Mn can be managed by maximizing RGR Plant/Leaf and minimizing RAR Mn. The parameters RGR Plant/Leaf and RAR Mn were calculated using the conventional growth analysis techniques. The dynamics of Mn phytotoxicity was investigated by growing Mn-tolerant Lee and Mn-sensitive Forrest soybeans in the greenhouse using Wahiawa series, a manganiferous Oxisol in Hawaii. The soybeans were grown at soil pHs 4.78, 5.5, and 6.00 and five growth conditions (control, 80-90% field capacity, 40% shading, green manure and phosphorus at 150 mg kg-1). RAR Mn consistently exceeded RGR Leaf in most treatments. Over the range of soil pH and growth conditions, we found strong positive correlation between RGR Leaf and RAR Mn, this correlation mediated by a more fundamental correlation of both rate processes to plant water use. The dynamics of Mn phytotoxicity, referred to as the 'dual feedback effect' model described a continuous negative interaction between current RGR Leaf and future RAR Mn and between current RAR Mn and future RGR Leaf. Manganese accumulation rate exceeded plant growth rate, leaf Mn increased with time and growth treatments did not affect growth rate unless soil pH was increased to eliminate excessive Mn in the soil. Field experiments were conducted in Rugao series, an acid Alfisol in Northern Philippines. The soil is acid (pH 4.40) with abundant Fe-Mn concretions within the surface 20-cm. Preliminary field experiment showed Mn phytotoxicity in local soybeans cv. PSB Sy2 and PSB Sy6 as leaf symptoms in addition to low plant growth rates and grain yields associated with leaf Mn exceeding a critical value of 500 mg kg-1. Results of a second field experiment showed that cultivar, liming, and the management of phosphorus (P), manure and mulching modified plant growth rate and enhanced tolerance to excess soil Mn. Lime control (2 t ha-1) neutralized half of the exchangeable AI while keeping saturated paste Mn in excess. Mulching did not affect saturated paste-and increased RAR Mn without affecting RGR Leaf. Increases in RGR Leaf due to P and manure were accompanied by increases in RAR Mn. This increase in RGR Leaf translated to increased yields even when RAR Mn and soil solution Mn were increased as in the case of manure addition. Increases in grain yield due to manure exceeded the increases due to lime or P. Chicken manure was more effective than green manure in increasing grain yield. Plant growth rate exceeded Mn accumulation rate, leaf Mn decreased with time and manure treatments alleviated Mn phytotoxicity despite an increase in soil Mn.

Photocopy of typescript. / Thesis (Ph. D.)--University of Hawaii at Manoa, 1978. / Bibliography: leaves 219-235. / Microfiche. / xiii, 311 leaves ill., maps

Thesis (M.S.)--Auburn University, 2007. / Abstract. Vita. Includes bibliographic references.

Thesis (M.S.)--University of Texas at El Paso, 2007. / Title from title screen. Vita. CD-ROM. Includes bibliographical references. Also available online.

Dissertation (Ph.D.) -- Worcester Polytechnic Institute. / Keywords: rat brain; osmotic shock; blood brain barrier; manganese-enhanced MRI. Includes bibliographical references.

Thesis (Ph.D. - Hydrology and Water Resources)--University of Arizona. / Includes bibliographical references (leaves 46-66).

Thesis (Ph.D.)--University of Wollongong, 2004. / Typescript. Includes bibliographical references.

Thesis (Ph. D.)--University of Hong Kong, 1996. / Includes bibliographical references (leaf 184-194).

Thesis (Ph. D.)--University of Wisconsin--Madison, 1963. / Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 69-71).