ORCID Identifier(s)

ORCID 0000-0002-3375-4998

Graduation Semester and Year

Summer 2025

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Chemistry

Department

Chemistry and Biochemistry

First Advisor

Dr. Robin Macaluso

Second Advisor

Dr. Muhammad Huda

Abstract

Structure–property relationships in some oxychalcogenide materials are strongly governed by the interplay between stereochemically active lone pairs and anion vacancy formation. Through a combination of density functional theory (DFT), synchrotron and neutron total scattering and optical characterization, this work elucidates how local symmetry breaking, chemical substitution, and vacancy dynamics influence electronic and optical behavior across several lone pair-containing systems. In crystalline TeO2, all three polymorphs exhibit wide band gaps with Te4+ 5s2 lone pairs contributing significantly near the valence band edge. Oxygen vacancy formation is thermodynamically favorable and induces asymmetric charge redistribution and enhanced polarizability, supporting its potential as a fast-responding nonlinear optical (NLO) material. Comparative analysis of the dielectric tensor and refractive index of oxygen vacancy containing TeO2 models demonstrates strong alignment between theory and experiment, confirming lone pairs and vacancy coupling as a key mechanism for optical enhancement. In La2O2Te, we explore solid-state and hydrogen-assisted synthesis to probe how oxygen content influences optical properties. Structural and morphological differences are observed via X-ray diffraction (XRD) and scanning electron microscope (SEM), while UV-vis and photoemission spectroscopy reveal additional low-energy transitions linked to oxygen vacancy-induced defect states. DFT calculations confirm the emergence of occupied La 5d states just below the CBM, consistent with low energy electron transition (< 1.0 eV). These results underscore oxygen vacancies’ dual role in shaping both structure and defect-mediated optical behavior in oxytellurides. While in sulfur-doped tin niobate, Sn2Nb2O7-xSx, sulfur substitution enables systematic tuning of the lone pair activity and local structure. Optical band gap narrowing is observed with increasing sulfur concentration. Total scattering analysis reveals that the P212121 space group, rather than the high-symmetry Fd-3m model, accurately captures the local distortions and site-specific displacements driven by S/O ordering and Sn2+ lone pairs. As sulfur content increases, long-range lone pair-induced distortion is suppressed, but local asymmetry is enhanced near sulfur-rich regions, indicating a delicate balance between covalency and coordination flexibility. The results establish Sn-based heteroanionic pyrochlores as a tunable platform for electronic and optical property design via targeted anion substitution and lone pairs’ stereochemical activity. Collectively, this study provides a unified framework for understanding how local structural features, particularly lone pair asymmetry and anion vacancy formation, drive macroscopic functionality in oxychalcogenides. These findings lay the groundwork for the rational design of semiconducting and optoelectronic materials based on lone pair active cations and mixed-anion chemistry.

Keywords

Heteroanionic, Stereochemically Active Lone Pairs, Oxygen Vacancy, Optical Properties, Semiconductor, Oxysulfide, Density Functional Theory, Neutron Diffraction, Synchrotron Diffraction, Pair Distribution Function

Disciplines

Computational Chemistry | Inorganic Chemistry | Materials Chemistry

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