Structure-Property Relationships in Select Rare Earth Materials and their Hydrides

Abstract

Rare-earth elements and compounds find numerous applications ranging from clean energy generation and high-end permanent magnets to materials for magnetocaloric cooling and quantum information science. For this reason, it is of the essence to have well-understood structure-property relations as they are the first step in transforming the science of rare-earth materials into engineering applications. Among the tens of thousands of known three-dimensional atomic arrangements of atoms, that is, crystal structures, those that are distinctly layered provide a unique opportunity to examine specific chemical and magnetic interactions in a controlled fashion.

Part of this thesis examines the interactions between heavy and light lanthanides in select representatives of the rare-earth intermetallic family that adopt the layered CeScSi-type structure. I show how crystallography controls the fundamental inter-lanthanide interactions leading to near-perfect magnetic compensation at predictable chemistries. Further, I demonstrate how said compensation leads to unusual magnetic memory effects, and, when coupled with random and minor perturbations in a conventionally-assumed uniform lanthanide distribution, large spontaneous and conventional exchange biases.

Additionally, I examine the effects a non-magnetic LaFeSi, that crystallizes in a layered CeFeSi-type structure, has on the responsive physical behaviors of LaFe13 xSix when naturally combined in a metal-metal composite. I show how dynamic stress fields, exerted by the magnetically inert LaFeSi matrix as temperature and/or magnetic field vary, alter the progression of magnetoelastic transformation in the ferromagnetic LaFe13-xSix. I show how controlling the constituent ratios effects the dynamic and static stress fields allowing for manipulation of magnetic properties and magnetocaloric effect.

Lastly, I also examine how inserting hydrogen into the aforementioned materials effects both inter- and intralayer interactions and, consequently, macroscopic physical properties. Utilizing Density Functional Theory (DFT) computations, performed collaboratively, we predict that Pr0.75Gd0.25ScGeH exhibits competing Kondo and indirect 4f exchange interactions which I experimentally explore. Furthermore, I note how hydrogen insertion affects the fundamental interactions seen in the La-Fe-Si composites and produces non-conventional phenomena such as superconductivity in non-magnetic LaFeSi, co-existing with robust ferromagnetism of LaFe13 xSix grains, as well as the development of a nearly anhysteretic giant magnetocaloric effect near and above room temperature.