CRYSTAL STRUCTURE DETERMINATIONS AND STRUCTURAL TRENDS IN CORRELATED ELECTRON SYSTEMS

Abstract

Complex and unique crystallographic structures are interesting, even if it is only for the sense of wonder that comes from trying to comprehend how such a thing can be. But there is more to it than that, for though in the end the properties are the motivation for continued exploration--it is the properties that drive fundamental and application oriented research alike--the properties themselves stem from the structural parameters. It is the privilege and delight of solid state chemists to investigate the rich relationship between structure and properties. The richness comes from the fact that though chemistry has a rigorous basis in first principles, many of its tools have an empirical component. Thus, for every immutable principle there is another `rule of thumb'. It is the manifestation of old and established rules in completely new and unexpected ways that excites the solid state chemist.

In this dissertation I will discuss in detail the structures of four systems related to correlated electron problems. The first is Na27Ru14O48</sub, the most recent phase discovered in the Na-Ru-O system and also the most complicated. The mixed valence on the ruthenium atoms in this compound make it difficult to ascribe certain properties to specifically localized or itinerant behavior. The second is the metallic forms of two of the end members of the VnO2n-1 homologous series: V8O15 and V9O17. This series of compounds and their neighbors VO2 and V2O3 are prototypical correlated electron systems which have been studied for decades. Based on both my new findings and previous results I take a new perspective on this system and confirm some previous predictions.

In the third section I discuss two new ordered ternaries based on the CoSn structure type. CoSn is a rare type of binary that contains a kagome lattice of transition metal atoms with an atom sized void between these layers. While CoSn itself is apparently lacking in interesting electron correlations, it is proximal to FeGe and FeSn, both complex helimagnetic systems. By selective doping of Ge on one of the Sn sites in CoSn, I bring new insights into this unusual structure type.

The fourth and final system I report is the crystal structures of two closely related iron-based superconductors, Ca10(Pt3As8)((Fe1-xPtx)2As2)5 and Ca10(Pt4-&#948;As8)((Fe1-xPtx)2As2)5. Despite their nearly identical compositions, the latter has a superconducting transition more than double that of the other, and purportedly it can even exceed values triple the maximum Tc for the former compound. By making simple chemical arguments, robust and important predictions are made that serve as the basis for future confirming experiments.

This last chapter itself is characteristic of the thesis as a whole. It shows that seemingly trivial details in a crystal structure, if looked at carefully and in the right context, yield deep insights into a compound's behavior. Once explained, it seems obvious, but until then the analogy is hidden. It shows that at the heart of so many physics problems there is a chemical one.