Doctoral Dissertations
Date of Award
12-2025
Degree Type
Dissertation
Degree Name
Doctor of Philosophy
Major
Materials Science and Engineering
Major Professor
Veerle Keppens
Committee Members
William Weber, David Mandrus, Maik Lang, Kathrine Page
Abstract
The deliberate introduction of configurational entropy has emerged as a powerful strategy for stabilizing complex oxides and enabling novel functionalities. In this dissertation, the synthesis and investigation of multifunctional high-entropy oxides (HEOs) are undertaken with a focus on perovskite and garnet structures. A family of rare-earth perovskites, REBO3 (RE = La, Pr, Nd, Sm, Eu, Gd; B = Sc, Al, Cr, Ni, Fe), was synthesized with equimolar B-site occupancy. All compositions crystallize in the orthorhombic Pnma structure and exhibit thermally activated transport. Magnetic ordering was observed at low temperatures in LaBO3, PrBO3, and SmBO3, with the transition temperatures shifting in relation to the rare-earth ionic moment.
Detailed investigation of the high-entropy perovskite Eu(ScCrFeNiAl)O3 revealed a subtle interplay between local bonding, charge ordering, and polaron transport. SQS-based PDF analysis identified anomalously short Eu–Ni and long Ni–O distances, indicating electrostatic coupling between Eu and Ni, while bond-valence results confirmed the coexistence of Ni2+, Cr3+ and Cr4+ at the ground state. Temperature-dependent XRD analysis showed a structural anomaly near 140 K, correlating with a crossover in conduction from Ni-mediated small-polaron hopping to Cr- or oxygen-mediated hopping. Magnetization measurements confirmed an antiferromagnetic transition at ~17 K and additional features near ~130 K. Complementary photoluminescence spectroscopy revealed intensity quenching below 150 K possibly due to reduced electron–phonon coupling.
Extending the high-entropy design to the garnet system, Y3Fe4(Sc/Cr/Ga/Al)1O12 and Y3Fe3(Sc/Cr/Ga/Al)2O12 were synthesized via Fe3+ - sublattice substitution in YIG. Both retain the cubic Ia̅ 3d structure, yet PDF and DOS analysis exposed distinct site preferences for Sc3+/Cr3+ toward octahedral coordination and, in Y3Fe3(Sc/Cr/Ga/Al)2O12, nanoscale Al3+ short-range ordering invisible to average probes. This emergent short-range order disrupts long-range spin connectivity, producing cluster-glass-like magnetism, while Cr3+ substitution enhances photoluminescence although the short-range order delocalizes the local crystal-field environment.
Together, these studies demonstrate that entropy-driven compositional complexity not only stabilizes unconventional oxide phases but also engenders tunable couplings among structural, electronic, magnetic, and optical degrees of freedom – offering new design principles for multifunctional materials.
Recommended Citation
Tanveer, Rubayet, "Synthesis and Investigation of Multifunctional High Entropy Oxides. " PhD diss., University of Tennessee, 2025.
https://trace.tennessee.edu/utk_graddiss/13643
Included in
Ceramic Materials Commons, Semiconductor and Optical Materials Commons, Structural Materials Commons
Comments
Following comments were addressed as suggested:
- Notes: - Throughout dissertation: Line spacing of the text, spacing between headings/subheadings and text, and spacing between sections of text must consistently be single-spacing (since that is what you are using). Please revise where you are not using single-spacing or you’ve added extra spaces between headings/subheadings and sections of text. - Title page: Move your name and graduation date closer to the bottom of the page. - Copyright page: Move the text up to the top of the page. - Table of Contents: All period leader lines and page numbers for the entries must be in the same format (i.e. all bolded or none bolded). The page numbers for the entries must be aligned in a straight line down the right side of the page. - List of Figures: Each entry must have a period leader line that connect the figure title to the page number.