ORCID Identifier(s)

0000-0001-8895-1887

Graduation Semester and Year

2022

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Chemistry

Department

Chemistry and Biochemistry

First Advisor

Robin Macaluso

Abstract

The structural and electronic effects of the lone pairs play an important role in magnetic, photocatalytic, and semiconducting behaviors of materials. Investigating the role of stereochemically active ns2 lone pairs derived from p-block cations in solid-state materials is important to phenomena associated with polarizable bonding, e.g., ferroelectricity. The effect of lone pairs in ferroelectric polarization has been studied on the lead metaniobate in which the covalency between Pb and O stabilizes the in-plane polarization which leads to orthorhombic symmetry. PbTiO3 has been widely studied for ferroelectric polarization associated with the off-centering of Ti in TiO6 due to the characteristic 6s2 lone pair. The structural distortion associated with lone pairs can be structured with long-range periodicity. However, lone-pair induced structural distortions can also be short-range in nature; investigation of these requires large X-ray and/or neutron fluxes available at synchrotron and neutron sources. The general theme of this dissertation focuses on structural characterization of Pb2+ containing solid-state materials using total elastic scattering where both Bragg and diffuse scattering of diffraction patterns are analyzed by Rietveld and pair distribution function techniques, respectively. Chapter 1 presents the two chemical models that explain lone pairs of electrons, their interactions, and their influence on structural distortions. In the classical model, lone pair distortion is explained by the hybridization of s and p atomic nonbonding. The revised lone pair model, on the other hand, state that lone pair is formed from the interaction of the cation s and p atomic orbitals with the oxide anion p orbital. Chapter 2 presents the fundamentals of structural characterization of materials including powder X-ray diffraction, Rietveld refinement and atomic pair distribution function. Rietveld refinement is used to determine the crystallographic information and phase purity of our samples. Since we are interested in the effect of the lone pairs on the structure and properties of the Pb2+ containing solids, atomic pair distribution function is employed to find the local structure of the materials and ultimately characterize the lone pairs. In Chapter 3, I will discuss how the Pb 6s2 lone pair driven structural distortions resulting in hybridization between the Pb 6s2 lone pair and O 2p. This hybridization causes a change in the electronic structure of PbVO3Cl resulting in a thermochromic transition from yellow to red at 200 °C. This thermochromic phenomenon was not observed in BaVO3Cl, which does not contain a stereochemically active lone pair. Chapter 4 presents the structural, dielectric, and heat capacity behavior of the cation- and anion-deficient pyrochlore Pb1.5Nb2O6.5 upon cooling. We find that both the vacancies and the lone pair driven distortions of the Pb cations are globally disordered in the cubic Fd3 ̅m structure, and local distortions are present at all temperatures that can be described by cristobalite-type cation ordering. Finally, Chapter 5 reports the Rietveld refinement results of neutron diffraction data on the cubic pyrochlore Fd3 ̅m Pb1.5Nb2O6.5 against different models including cubic Fd3 ̅m 16d, cubic Fd3 ̅m 96g, F4 ̅3m, cristobalite P212121, and cristobalite P41212 at 100K, 200K, and 300K. The data were refined using the Rietveld methods in the GSAS-II software.

Keywords

Lone pairs, Pair distribution function

Disciplines

Chemistry | Physical Sciences and Mathematics

Comments

Degree granted by The University of Texas at Arlington

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