Electron microscopic studies of crystal growth and defects in inorganic solids

Abstract

This project focuses on the growth mechanisms and microstructures of several inorganic solid based on the electron microscopic investigation. By controlling the reaction time to record the growth process of crystals, or observing any traces remained in the final crystals, possible explanations about the overall developments and growth mechanisms are presented for the studied objects. For (Mn,Co)₃O₄ spinel solid solution synthesised by a traditional high-temperature calcination, the relationship between a cubic and a tetragonal spinel was confirmed. High-resolution transmission electron microscopy (HRTEM) images further show specific planar defects caused by a cubic to tetragonal unit cell distortion during cooling. This type of defect acts as a twin boundary or a co-sharing plane between adjacent domains in a single-like crystal. (Mn,Co)₃O₄ spinel solid solution with specific morphologies was then synthesised by a solvothermal method. By recording the morphological change of particles based on different reaction times, two different types of precursors were found to form in a solution through different mechanisms. The multi-reacted mechanisms were then found to be an important factor leading to the development of core-shell or shell-free spherical particles composed of nanocrystals. A further trial to synthesise Mn₃O₄ spinel with specific morphologies was done by the assistance of a heteropolyacid. The particles turned out to be the Mn₃O₄ @ MnWO₄ core-shell spheres composed of nanoparticles. In this reaction, the heteropolyacid helped to aggregate the amorphous precursors into a spherical shape, followed by a decomposition for the nucleation of MnWO₄. Crystallisation of Mn₃O₄ nanoparticles and MnWO₄ nanorods occurred at differently reacted stages, caused by an environmental change in the spherical precursors, was the main factor that separated the Mn₃O₄-rich core and the MnWO₄-rich shell. A spinel solid solution, (Mn,Co,Al)₃O₄, with a specific morphology was found to form in a mimetic Tianme glaze. In this glass system, a liquid phase separation occurred at the high temperature which enriched the crystalline source on the surface of the glass. Nucleation of (Mn,Co,Al)₃O₄ then occurred followed by a rapid crystal growth along specific directions to form the two-dimensional dendrites. The dendrites were later found to appear the rainbow-like colour on the glaze causing mainly by the thin film interference of light.