Low-Temperature Synthesis of Transition/Post-Transition Metal Intermetallics and Rare Earth Oxide Perovskites

Open Access
Author:
Henderson, Nathaniel Lee
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 28, 2009
Committee Members:
  • Raymond Edward Schaak, Dissertation Advisor
  • Raymond Edward Schaak, Committee Chair
  • Thomas E Mallouk, Committee Member
  • Christine Dolan Keating, Committee Member
  • James Hansell Adair, Committee Member
Keywords:
  • intermetallics
  • perovskites
  • second harmonic generation
  • europium titanate
  • low-temperature
  • solid state
  • solution synthesis
Abstract:
Recent years have seen the emergence of numerous low-temperature synthetic approaches toward inorganic materials, including both intermetallic and oxide compounds. Many of these processes have focused on the synthesis, modification, and conversion of nanoparticles and deposited thin-films, which can overcome energetic barriers, such as the rate-limiting solid-solid diffusion step, associated with traditional metallurgical and ceramic techniques. Through a loss of dimensionality or drastically reduced diffusion distances these approaches can access a wide range of materials at much lower temperatures than typically employed. Additionally, low-temperature methods potentially provide for the discovery of nonequilibrium or kinetically stable phases. This dissertation details our work towards the development of alternative, non-traditional synthetic methods for accessing both intermetallic and oxide compounds at low-temperatures and ambient pressures. We have used molten dispersions of low-melting metals as reactive precursors in the synthesis of various bimetallic materials, specifically M-Sn, M-Ga, M-Bi, and M-In phases (M = late transition or post-transition metal). These compounds possess a range of different chemical and physical properties, such as superconductivity, and can be rapidly obtained in solution at temperatures below 300 °C. Additionally, we find that the choice of solvent is somewhat flexible, as several of these bimetallic phases were obtainable under similar conditions in renewable plant and seed oils, such as safflower and canola oil. This provides further support for our hypothesis that the solvent acts primarily as medium for dispersion and heat-transfer and that intermetallic formation occurs through a diffusion-based approach in solution. With regards to oxide compounds, we have accessed layered perovskite-type Eu2Ti2O7, which is typically obtained through high-pressure techniques, at moderate temperatures and ambient pressure through mild oxidation of EuTiO3. This approach suppresses formation of the more thermodynamically stable pyrochlore structure and allows partial retention of the perovskite motif up to 900 °C. Our synthetic method is amenable to bulk-scaling and yields samples that possess an increased response toward second harmonic generation relative to previous reports. Finally, we have synthesized Ca-substituted EuTiO3 phases through a modified sol-gel approach. By varying the level of Ca content, we can systematically alter the length of the unit cell edge and suppress the antiferromagnetic ordering temperature, presumably through dilution of magnetic spins.