Vibrational spectroscopy and the structure of solids: The example of carbonate minerals

Campbell, Stephen (2023) Vibrational spectroscopy and the structure of solids: The example of carbonate minerals. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Studying material structure on the atomic scale in solids can give clues as to how that material was formed, aged, or used. Atoms in solids are constantly in motion. A material’s specific composition and crystal arrangement results in a unique profile of atomic vibrations. These vibrations, or modes, can be grouped into two main types; internal modes are higher energy vibrations related to atomic motion of single atoms or moieties within the unit cell, whereas external modes are lower in energy and correspond to the collective movement of multiple coordinated atoms or moieties. Past investigations focused on the vibrations of solids have explored the effect of crystalline ordering on internal modes both experimentally and computationally. In this thesis, I explore the viability of established experimental and computational methods to examine the influence of structural differences on the the external mode vibrations using calcium carbonate as a case study material. The experimental projects focus on infrared spectroscopy. This tool is sensitive enough to observe subtle differences in the internal modes that are linked to structural differences but has yet to be explored for the external modes. We study calcium carbonate systems using infrared spectroscopic methods to understand the impact of structure on vibrational properties. Our initial goal was to correlate novel external mode vibration data trends with the established understanding of internal mode changes and atomic structure data. We found that the broadness of the external modes makes extracting structural information using the previous analysis protocols impractical. The results highlight potential pitfalls for researchers who are new to these spectroscopic techniques. We preach caution when attempting to interpret energy shifts of external modes as structural differences amongst samples without correlating with additional experimental methods. Further experiments focused on photoacoustic infrared spectroscopy, a specialized version of the technique. Previous works have highlighted but failed to explain how this specialized setup can enhance the detection of weak internal modes. We sought to determine the mechanism for this documented phenomenon. While we could not identify the cause of the enhancement, our experiments and analysis showed that it is intrinsic to the photoacoustic method and eliminated detector saturation, often thought to be the cause, as the root mechanism. ii Finally, I used computational molecular dynamics methods to simulate calcium carbonate vibrations. Using previously published parameters, I was able to generate a vibrational density of states (VDOS) for the calcium carbonate polymorph aragonite. This work is valuable as alternative computational methods to simulate the external vibrations of calcium carbonates are computationally expensive. The computed VDOS of aragonite shows a reasonable level of qualitative agreement with experimental measurements. This work serves as a proof-of-concept and starting point to observe disorder’s effects on the calculated vibrational density of states.

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