Publication Date

2026

Document Type

Dissertation/Thesis

First Advisor

Chmaissem, Omar

Degree Name

Ph.D. (Doctor of Philosophy)

Legacy Department

Department of Physics

Abstract

Ionic conductors are increasingly central to electrochemical energy technologies because they enable charge transfer through the motion of mobile ions. Among them, superionic conductors are crystalline solids that exhibit exceptionally high ionic conductivities, making them promising candidates for safer and more sustainable next‑generation energy storage and conversion devices. Despite their technological importance, key gaps remain in our ability to connect macroscopic transport to microscopic structural disorder and correlated dynamics that enable fast‑ion motion. Neutron scattering provides a powerful set of tools for establishing these connections.

In total scattering experiments, Bragg diffraction measures the average crystallographic structure, while diffuse scattering probes short‑range correlations arising from local disorder and collective motion. This dissertation develops and applies diffuse scattering techniques that separate static and dynamic contributions and probe correlated local structure through three‑dimensional differential pair distribution functions (3D-ΔPDF). Complementarily, quasi‑elastic neutron scattering (QENS) probes ionic motion through near‑elastic energy exchange, accessing both single‑particle diffusion (predominantly incoherent scattering) and correlated motion (coherent scattering). Building on modern instrumentation and computation, this dissertation develops analysis approaches for single‑crystal, large‑volume reciprocal‑space measurements in a “4D‑QENS” framework, enabling characterization of QENS line shapes that are difficult to isolate in powder-averaged measurements or along limited high‑symmetry directions.

These methods are first applied to superionic SrCl₂, whose high-symmetry, fluorite-type average structure enables efficient reciprocal‑space sampling. Combining 4D-QENS and 3D-ΔPDF, this dissertation probes local structural defects and dynamics within the ionic conducting phase. The data support a conduction mechanism dominated by metastable configurations involving doubly occupied chlorine sites that generate and propagate vacancies, enabling long‑range hopping diffusion.

These techniques are then applied to Cu₂₋ₓSe, a material of interest for its ultralow thermal conductivity and a highly conducting phase near room temperature. Although Cu₂₋ₓSe adopts an average anti-fluorite structure, the copper sublattice sites are constrained to tetrahedrally shaped volumes in the structure. In 4D-QENS, the correlated dynamics display a reciprocal‑space dependence that indicates especially rapid motion within the tetrahedral volumes which copper occupies. Consistent with QENS analysis, 3D-ΔPDF identifies short‑range correlations consistent with in‑phase motions along ⟨110⟩.

This dissertation establishes a transferable, joint diffuse-scattering and 4D‑QENS methodology for linking local structural correlations to correlated ionic dynamics in superionic conductors. These methods enable mechanistic interpretation beyond average-structure models and provide new constraints for theories of coherent QENS in crystalline fast‑ion systems.

Extent

130 pages

Language

en

Publisher

Northern Illinois University

Rights Statement

In Copyright

Rights Statement 2

NIU theses are protected by copyright. They may be viewed from Huskie Commons for any purpose, but reproduction or distribution in any format is prohibited without the written permission of the authors.

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