Synthesis and Investigation of Multifunctional High Entropy Oxides

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.

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