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This thesis investigates the structural, electrical, and possible magnetic phase transitions of ferroelectric and
multiferroic oxides under high pressures and low temperatures 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
pressure-induced 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 absence of Raman modes above 23 GPa serves as compelling
evidence for metallization, possibly due to a Mott insulator-to-metal transition caused by unit cell volume reduction
and the Jahn-Teller effect in the tetrahedral site. This transition may also involve a collapse in the local magnetic
moment.
Overall, this thesis advances the understanding of how extreme conditions influence the structural, electronic, and
magnetic properties of ferroelectric and multiferroic oxides. |