Rare-earth transition-metal intermetallics: structure-bonding-property relationships
The purpose of this thesis is to explore novel rare-earth, transition metal compounds and to investigate their structure and bonding and how these influence their properties. We have focused on the synthesis, characterization and electronic structure investigations, as well as physical properties of ternary rare-earth Fe-rich intermetallics, e.g., LaFe13-xSi x, and RE2-xFe4Si14-y, etc;The LaFe13-xSix, (1.0 ≤ x ≤ 5.0) compounds with structures related to the cubic NaZn13type are an important series of compounds to study for possible efficient magnetic refrigeration. A systematic structural study of the compositional variation in LaFe 13-xSix exhibits a structural transformation from the cubic NaZn13-structure type to a tetragonal derivative due to preferential ordering of Fe and Si atoms. Temperature-dependent single crystal X-ray diffraction at various temperatures and electronic structure calculations (both Extended Huckel and TB-LMTO) was performed on the cubic phases to examine the origin of the large magnetic entropy change. The giant magnetocaloric effect of cubic LaFe13-xSix alloys results from coupling between magnetic ordering and structural transformation;In light of the potential superstructures that could be observed for RE2-xFe4Si14-y as well as possible modifications of semiconducting beta-FeSi2, we report herein a thorough examination of the chemical composition and structure of RE2-xFe 4Si14-y (RE = Y, Gd-Lu) using a combination of electron microscopy and X-ray diffraction, and demonstrate a new superstructure for this class of compound. The structures are built up of rare-earth silicide planes with approximate compositions "RE1.2Si1.9" alternating with beta-FeSi2-derived (hkl)-type slabs. Investigation by transition electron microscopy (TEM) reveals a superstructure in the crystallographic ab-plane. Fe Mossbauer spectra confirm two different iron environments in superstructure. Magnetic susceptibilities suggest weak magnetic coupling between rare-earth elements, and resistance measurements indicate poor metallic behavior with a large residual resistivity at low temperatures, which is consistent with disorder. Electronic structure calculations on model structures identify a pseudogap in the densities of states for specific valence electron counts that helps to provide a useful electron counting scheme for this class of rare-earth/transition metal/main group compound.