UC Santa Barbara

Understanding the impact of structure on observable magnetic and electronic properties is an important aspect in materials chemistry. These insights can be used to develop theoretical models and predict new compounds that may be of interest in a wide range of applications. To study these structure–property relationships, many techniques are typically used, including X–ray diffraction, energy–dispersive X–ray spectroscopy, microscopy, magnetization and transport measurements, and density functional theory calculations. A combination of these measurements and computations can give a clearer picture of the underlying mechanisms that link structure and property so closely.Some materials exhibit a structural change concurrent with its magnetic ordering, and this magnetostructural coupling is proposed to enhance certain magnetic effects. This phenomenon has been explored as a potential route to discovering new materials that exhibit desirable effects, such as in magnetocaloric materials with applications in solid state refrigeration. One such material is AlFe₂B₂, which exhibits a large magnetocaloric effect that was previously not linked to its structure. Chapter 2 of this dissertation reports an in–depth variable temperature synchrotron X–ray diffraction study that establishes the importance of magnetostructural coupling in the magnetocaloric effect observed in AlFe₂B₂. In other systems, substituting an element with an isovalent, similar sized element can have pronounced changes on the magnetic and electronic properties. Chapter 3 explores the newly established magnetocaloric MnPdGa and the differences between MnPdGa and MnPtGa. While MnPtGa is also a magnetocaloric that adopts the same crystal structure and is isovalent to MnPdGa, DFT calculations elucidate electronic differences in the two compounds, which may cause their different magnetocaloric performances.

Lastly, chapter 4 and appendix B contains a hole–doping study of the AV₃Sb₅ (A = K, Rb, and Cs) kagome superconductors which have a competing charge density wave order. With very careful and systematic Sn substitution on the Sb site, the superconducting and charge ordering critical temperatures are tuned, and the effects of hole–doping on A = K, Rb vs. A = Cs suggest that the different A site kagomes have subtle but important differences in their electronic structures. DFT calculations support this idea, and while the exact interaction mechanisms between the superconducting and charge density wave ordering phases in AV₃Sb₅ require further study, the results presented here show a strong structure–property dependence in AV₃Sb₅.