Electronic transitions and multiferroicity in transition metal oxides

dc.contributor.advisorGoodenough, John B.en
dc.creatorZhou, Haidongen
dc.description.abstractFour systems have been studied for the localized-itinerant electronic transition in transition-metal oxides: (i) In CaV1-xTixO3, substitution of Ti(IV) introduces Anderson-localized states below a mobility edge µ c that increases with x, crossing F ε in the range 0.2 < x< 0.4 and also transforms the strong-correlation fluctuations to localized V(IV): t1e0 configurations for x ≥ 0.1. (ii) The properties of LaTiO3+δ reveal that a hole-poor, strongly correlated electronic phase coexists with a hole-rich, itinerant-electron phase. With δ ≤ 0.03, the hole-rich phase exists as a minority phase of isolated, mobile itinerant-electron clusters embedded in the hole-poor phase. With δ ≥ 0.08, isolated hole-poor clusters are embedded in an itinerant-electron matrix. As δ > 0.08 increases, the hole-poor clusters become smaller and more isolated until they are reduced to super-paramagnetic strong-correlation fluctuations by δ = 0.12. (iii) The data of Y1-xLaxTiO3 appears to distinguish an itinerant-electron antiferromagnetic phase in the La-rich samples from a localized-electron ferromagnetic phase with a cooperative Jahn-Teller distortion in the Y-rich phase. (iv) The transition at Tt in Mg[Ti2]O4 is a semiconductor-semiconductor transition associated with Ti-Ti dimerization instabilities. The dimerization is caused by lattice instabilities resulting from a double-well Ti-Ti bond potential at a crossover from localized to itinerant electronic behavior. RMn1-xGaxO3 (R = Ho, Y) and Ho1-xYxMnO3 have been studied for the multiferroicity of RMnO3. Ga doping raises the ferrielectric Curie temperature TC and the Mn-spin reorientation temperature TSR while lowering TN of the Mn spins and the Ho magnetic ordering temperature T2. The data show an important coupling between the Mn3+-ion and Ho3+-ion spins as well as a TSR that is driven by a cooperative MnO5 site rotation and R3+-ion displacements that modify the c lattice parameter. The data also support an enhanced spin-lattice interaction in the geometrically frustrated (GF) Mn-spin system. Y doping enhances the temperature region for the P6’3cm’ magnetic phase and thereby increases TSR for Ho1- xYxMnO3. The studies of several oxygen non-stoichiometric Fe4+/Fe3+ oxoperovskite show that two mechanisms, the formation of Fe3+-O-Fe4+ pair and the disproportionation reaction 2Fe(IV)O6/2 = Fe3+ + Fe(V)O6, dominate the electronic behavior. The properties of DyBaCo2O5.5 reveal a spin-state transition from the low-spin t 6 e 0 ground state to higher spin-state at octahedral-site Co3+, which is also accounted for the metamagnetism in the sample.
dc.rightsCopyright is held by the author. Presentation of this material on the Libraries' web site by University Libraries, The University of Texas at Austin was made possible under a limited license grant from the author who has retained all copyrights in the works.en
dc.subject.lcshTransition metal oxidesen
dc.subject.lcshElectronic structureen
dc.titleElectronic transitions and multiferroicity in transition metal oxidesen
thesis.degree.grantorThe University of Texas at Austinen
thesis.degree.nameDoctor of Philosophyen

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