acase@tulane.edu / 1 / Sijun Luo

Not available / text

Not available / text

Thesis (Ph.D.)--McMaster University (Canada), 1994. / Source: Dissertation Abstracts International, Volume: 56-08, Section: B, page: 4338. Adviser: J. E. Greedan.

Thesis (Ph. D.)--University of Texas at Austin, 2005. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2004. / Supervisor: John B. Goodenough. Vita. Includes bibliographical references.

No description available.

archives@tulane.edu / 1 / Madhu Sudan Gaire

The investigation in this thesis focuses on the synthesis of cation-ordered perovskite phases by introducing anion vacancies into the structure. Complex cation-ordered phases Ba2YMO5 and Ba3YM2O7.5 (M = Fe, Co) have been synthesized using ceramic or citrate gel methods under flowing argon. Close inspection reveals that the structures are constructed from Y2M2O102 basic units which consist of two YO6 octahedra and two MO4 tetrahedra in a rock-salt type arrangement. In the structure of Ba2YMO5 (M = Fe, Co), the neighbouring Y2M2O102 units are connected with an equivalent one in the yz-plane with YO6 octahedra sharing an apex. In the structure of Ba3YM2O7.5 (M = Fe, Co), the basic units are connected to each other by the M2O7 dimers via a chain of Y – O – M – O – M – O – Y bonds. Complex cation ordering can be achieved by carefully controlling the anion vacancies and selecting the cations with different ionic radii. The anion vacancies present in Ba2YMO5 (M = Fe, Co) (space group P21/n) allow the intercalation of anions like O2- and F- into the lattice. The fluorination of Ba2YCoO5 leads to the formation of a new orthorhombic phase Ba2YCoO5F0.42 (space group Pbnm) in which the inserted fluoride ions are distributed in a disordered manner. In contrast, the topochemical oxidation of Ba2YFeO5 leads to the formation of a new orthorhombic phase Ba2YFeO5.5 (space group Pb21m), in which Fe4+ centres are located in 4-coordinate tetrahedral sites and 5-coordinate pyramidal sites, respectively. The polar structure of Ba2YFeO5.5 is confirmed by the observation of second-harmonic generation activity and pyroelectric behaviour. Ba2YFeO5.5 also exhibits a combination of ferromagnetic and antiferromagnetic behaviours at low temperature. LaCa2Fe2GaO8 adopts a six-layer structure consisting of an OOTLOOTR stacking sequence of layers of (Fe/Ga)O6 octahedra (O) and (Fe/Ga)O4 tetrahedra (T), related to that of the four-layer brownmillerite structure (space group Pbma). The chains of tetrahedra in the structure of LaCa2Fe2GaO8 exhibit a cooperative twisting distortion in which the twisting direction of the chains of tetrahedra alternates in adjacent tetrahedral layers. LaxSr2-xCoGaO5+δ (0.5 < x < 1) adopts brownmillerite structures which consist of octahedral and tetrahedral layers with mixed valence of Co2+/Co3+. The members with x = 0.5, 0.6 and 0.7 adopt structures with I2mb space group symmetry, in which all the tetrahedra twist in the same direction. The members with x = 0.8, 0.9 and 1.0 adopt structures with Imma space group symmetry, in which the chains of the tetrahedra twist in a disordered manner. A change in the Co3+ spin state from high spin (HS) to low spin (LS) is observed as the La/Sr ratio increases. The change of the Co3+ spin state can be rationalized on the basis of internal chemical pressure.

No description available.