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Local Structure of Metastable Defect Complexes and Phase Transitions

  • Author(s): MacKeen, Cameron
  • Advisor(s): Bridges, Frank
  • et al.
Creative Commons Attribution-NonCommercial 4.0 International Public License
Abstract

To understand and predict phenomena in solid materials, we often look at how the lattice structure brings about such an effect. Our lab analyzes the extended x-ray absorption fine structure (EXAFS) to closely study the physical configuration of atoms in a lattice. I will talk about the interesting local structure of the following compounds: Organic lead-halide perovskites MAPbI3 andMAPbBr3, ytterbium doped calcium fluorite (CaF2:Yb), and lithium niobate doped with Zn,Er,In, and Hf.

Organic perovskites have been a material of high interest for harvesting light. In order to improve crystallization of solution based methylammonium lead iodide (MAPbI3), we aim to maximize lead-iodide coordination via an excess of iodide precursors: methylammonium iodide (MAI) and hydrodic acid (HI). We find that a surplus of HI results in higher order and coordination for the nearest Pb–I atomic shell (six iodine neighbors). In another study, we look into the disorder and vibrational properties of MAPbBr3 in different structural phases. This dissertation includes a temperature dependent study on the local structure of MAPbBr3, and a rigorous quantifying of the anharmonicity parameter, C3, that represents dynamic disorder. C3 increases nonlinearly with temperature and results in an asymmetric pair distribution for the nearest Pb–Br shell.

Calcium fluorite doped with ytterbium is a doped simple FCC crystal with a down-shifted anomalous luminescence. A long lived excited state decays with a yellow-green emission and can be induced with UV exposure. With two consistent methods quantifying amount of Yb in the 2+ and 3+ state, we present compelling evidence that the impurity trapped exciton model does not explain this system. Further, we can elucidate the various metastable states of this system and their relation to strange observations.

Finally, lithium niobate has piqued interest in frequency conversion device engineering as a non-linear optical material. There lacks a general model for how the photorefraction is affected by different species of dopants, but there are proposed charge compensating substitution schema. We judge models of substitution schema based on solution energy calculations using our EXAFS results of lithium niobate doped with zinc, indium, erbium, and hafnium.

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