Exploring Electronic Transitions in Model Ferroelectric and Multiferroic Oxides under Extreme Conditions of Pressure and Temperature

Sahu, Mrinmay (2024) Exploring Electronic Transitions in Model Ferroelectric and Multiferroic Oxides under Extreme Conditions of Pressure and Temperature. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Mrinmay Sahu (18RS031)) 18RS031.pdf - Submitted Version Restricted to Repository staff only Download (10MB)

Abstract

This thesis investigates the structural, electrical, and possible magnetic phase transitions of ferroelectric and multiferroic oxides under high pressure and low temperature using synchrotron X-ray diffraction, Raman spectroscopy, and dielectric constant measurements. The primary goal is to understand how these transitions affect the materials’ ferroelectric and multiferroic behaviour. Detailed descriptions of experimental tools used to study the structural, vibrational, and electronic properties of these materials are provided. Ferroelectric polycrystalline Eu-doped BaTiO₃ powder shows a pressure-induced structural transition from a tetragonal to a mixed cubic and tetragonal phase at 1.4 GPa. The internal deformation of the TiO₆ octahedra, caused by the charge difference from Eu doping, preserves the tetragonality of the sample even under pressure. The pressureinduced increase in the intensity of selected Raman modes indicates local structural inhomogeneity responsible for spontaneous polarization. Low-frequency electronic scattering responses suggest pressure-induced carrier delocalization, while the high-pressure dielectric constant data indicate the presence of microscopic ferroelectric ordering in the system. The material exhibits characteristics of both ferroelectricity and semi-metallicity due to pressure-driven localized clusters of ferroelectric ordering and carrier delocalization. Eu doping plays a crucial role in the structural and vibrational properties of BaTiO₃ at low temperatures. It undergoes a rhombohedral to orthorhombic phase transition around 190 K, followed by a transition to a tetragonal crystal structure around 280 K. Below 100 K, intrinsic anharmonicity, seems insufficient to explain the observed frequency shift. In the low-temperature rhombohedral phase, most of the frequency shift is attributed to a three-phonon decay process, and the four-phonon decay process becomes significant in the orthorhombic and tetragonal phases. Our findings suggest that Eu incorporation within BaTiO3 facilitates electron delocalization at low temperatures, potentially enhancing electron-phonon coupling and forming distorted octahedral polarons. Multiferroic Fe₄Nb₂O₉ polycrystalline powder exhibits a pressure-induced structural transition from a trigonal to a monoclinic phase at 8.8 GPa, accompanied by a possible electronic transition and enhanced ferroelectric polarization, suggesting re-entrant multiferroicity. This transition is attributed to a significant distortion of NbO₆ octahedra. Multiferroic inverse spinel Co₂TiO₄ undergoes two pressure-induced structural phase transitions: from cubic to tetragonal at approximately 7.2 GPa with a minimal alternation in unit cell volume, and from tetragonal to orthorhombic phase with a mixed space group at around 17.3 GPa. The second transition corresponds to a first-order phase transition involving a significant reduction in unit cell volume of approximately 17.5%. The bulk compressibility of the inverse spinel Co₂TiO₄ and its high-pressure post-spinel phases is estimated as the average polyhedral compressibility within each phase. The significant reductions in unit cell and octahedral volume, coupled with the high-spin to low-spin transition, result in a pressure-induced metallization, leading to the collapse of local magnetic moments in the Cmcm-orthorhombic phase. The absence of Raman modes beyond 23 GPa serves as compelling evidence for metallization. Additionally, the sample’s color transformation from brownish to black under high pressure indicates a narrowing of the band gap. The study highlights a Mott insulator-to-metal transition within a narrow pressure range, likely facilitated by the reduction in unit cell and octahedral volume at the phase boundary, along with the influence of the Jahn-Teller effect in the tetrahedral site. Overall, this thesis advances the understanding of how extreme conditions influence the structural, electronic, and magnetic properties of ferroelectric and multiferroic oxides.

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