Investigation of Thermoelectric Properties of Ruthante Pyrochlore and A new Si-based High Entropy Pyrochlore

Abstract

The pyrochlore system has a general formula of A2B2O7, where A is typically a rare earth and B is a transition metal ion. It crystallizes in a face-centered cubic structure with space group Fd-3m. The structure consists of interpenetrating corner-shared tetrahedron networks made up of A 3+ and B 4+ ions. If either A or B happens to be magnetic, the system generally shows strong geometrical frustration. The Geometrical frustration arises when the magnetic interactions are incompatible with the lattice symmetry. For example, antiferromagnetically coupled spins on a triangular or tetrahedral motiff. Recently, the iridates family of pyrochlore has garnered a lot of attention due to its novel ground state, which can be adjusted through chemical doping at the A site. Other interesting features under discussion include the metal-to-insulator transition, magnetic transition at low temperatures, and anomalous lattice contraction. The competition between three energy scales, namely, spin-orbit interaction, onsite Coulomb repulsion, and crystal field splitting energy, plays a crucial role in determining the Weyl semimetallic ground state, making these systems both interesting and challenging to investigate. This study aims to explore the Ruthenates series with Ru4+ at the B site. Ruthenate Pyrochlores with 4d transition metal ions on the B site are of huge interest because of their electronic properties, which can be tuned by altering the A site element. We synthesized four compounds of the (Eu1-xBix)2Ru2O7 series with 0%, 2%, 3.5%, and 100% Bi doping at the A site using conventional solid-state synthesis. After structural analysis, we conducted some thermoelectric measurements to evaluate their performance for a thermoelectric application, which are discussed in a separate chapter. As the second part of my thesis, I worked on High entropy pyrochlores(HEP). In recent years, HEP has gained significant interest due to its ultra-low thermal conductivity, high stability at high temperatures, and high coefficient of thermal expansion. Therefore, it is considered a promising material for thermal barrier coatings (TBC), particularly in new high-temperature gas turbine engines. The main objective in this part of the thesis is to synthesize some high entropy pyrochlores and prove the concept of entropy-stabilized phase using the thermal reversibility method. We attempted to synthesize A2(Ti0.2Zr0.2Hf0.2Si0.2Ce0.2)2O7 , where A = Nd, Dy and ; Dy2(Ti0.2Zr0.2Hf0.2Si0.2Sn0.2)2O7, and Dy2(Ti0.25Zr0.25Hf0.25Si0.25)2O7 using traditional solid-state synthesis. We carried out a detailed structural characterization of the synthesized sample and measure their thermal conductivity up to high temperatures (873K).