Date of Degree

9-2021

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Karl G. Sandeman

Committee Members

Timothy Benseman

Benjamin Frandsen

Nicolas Giovambattista

Sophia Suarez

Subject Categories

Condensed Matter Physics

Keywords

magnetocalorics, barocalorics, pair distribution function

Abstract

The field of calorics represents a class of materials that offer the potential for solid-state cooling and heating, and, given the global climate crisis, comprise a necessary and active area of research. A clear and thorough understanding of their internal structural interactions and their external response to the environment is necessary for overall progress in the field, as accurate theoretical modeling and efficient materials design for devices both depend on this information. Through analysis of x-ray and neutron diffraction, the atomic order and disorder that drives these interactions is revealed. This dissertation focuses on diffraction studies concerning representative samples from two classes of caloric materials, magnetocalorics and barocalorics.

The magnetocaloric effect is the change of a magnetic material's thermodynamic state when placed in a changing magnetic field. Analogously, the barocaloric effect occurs when an appropriate material is subjected to changing external pressure. Ideal functional caloric materials in both classes are sought out with attention given to a common property; an abrupt, reversible transition from one solid phase to another upon heating or cooling that occurs across a narrow temperature range. The nature of this transition can be tuned by slightly altering a compound's chemical composition and structural make-up, therefore robust analysis of the structure of a material class offers the possibility to efficiently design new materials that best suit their intended applications.

This dissertation presents two sets of diffraction studies, one with x-rays and one with neutrons, on samples from the magnetocaloric aluminum-iron-borides and barocaloric spin crossover molecules. After a brief preface in Chapter 1, Chapter 2 reviews necessary concepts relating to the fundamentals of magnetism, magnetic ordering, and the theory of phase transitions. Chapter 3 provides an historical and theoretical overview of caloric materials and introduces the AlFe2B2-based family of crystals and the Iron(II) spin crossover complexes that are the subject of the diffraction studies presented in this work. Chapter 4 presents a brief introduction to scattering and methods of refinement to analyze scattering data, in real and reciprocal space.

Chapter 5 focuses on experimental details and results from x-ray total scattering experiments performed on AlFe2B2-based crystals with data refined in real space using pair distribution function analysis, and concludes that their magnetocaloric responses are driven toward enhanced, first-order behavior upon increased iron to aluminum ratio. Chapter 6 presents results from a set of Bragg scattering experiments with neutrons performed on Iron(II) spin crossover complexes with data refined in reciprocal space using the Rietveld method, and concludes by presenting the first experimental evidence of a so-called "giant" barocaloric effect in such a material. Chapter 7 outlines a path for future research topics on both classes of materials.

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