Investigations of the Effect of Transition Metal Substitution on the Pre-edge of X-ray Absorption Near-Edge Spectra

Transition-metal K-edge X-ray absorption spectroscopy (XAS) describes a group of techniques that are element specific and provide information about the chemical environments and structures in a material. X-ray Absorption Near-Edge spectroscopy (XANES) is a particularly useful XAS technique as it provides information regarding the chemical environment of a transition-metal centre and the electronic structure of the material being studied. As such, XANES has found use in a variety of scientific fields including biochemistry, geochemistry, and fundamental physics. One particularly information rich region of a transition-metal K-edge XANES spectrum is the pre-edge region, which contains information about the oxidation state and coordination environment of the transition-metal. Due to the wealth of information contained in the pre-edge region, it is important to fully understand all the features contained in this region and the factors that may affect them. To this end, a series of studies have been performed, with the goal of increasing the understanding of the pre-edge region of transition-metal K-edge spectra and how it may be used to characterize a material. A series of Sr2Fe2-xMoxO6 (0.25 \leq x \leq 1.0) double perovskites have been studied in order to investigate how changes in the oxidation state and coordination environment may be studied when both change simultaneously. These materials were also studied due to their technological relevance, as the Sr2FeMoO6 double perovskite has been widely studied due to its interesting and technologically relevant physical properties. However, Sr2FeMoO6 is just a single composition in the Sr2Fe2-xMoxO6 solid-solution, and it is important to understand how the composition impacts the transition metal valence states. Fe K- and Mo K-edge X ray absorption near-edge spectra have been collected to investigate how the oxidation state and coordination environment change with composition. When the Mo content is low, Fe adopts a 3+ oxidation state and Mo adopts a 6+ oxidation state. As the Mo content is increased, the Fe and Mo cations are both partially reduced, resulting in a mixture of Fe3+ and Fe2+ and Mo5+ and Mo6+. The reduction of the metal centers apparently drives a change in unit cell from a cubic Fm\bar{3}m structure to a tetragonal I4/m structure. The results reported here show that by careful analysis of the pre-edge, simultaneous changes in the oxidation state and coordination number may be discerned and analyzed separately. In order to study the effects on the pre-edge from changes in the spin-state of the transition-metal centre and variations in the metal-ligand bond covalency, a series of Cu2FeSn3-xTixS8 thiospinels have been studied. The Cu2FeSn3-xTixS8 thiospinels exhibit an interesting magnetic phenomenon known as a spin-crossover transition (SCO), in which the Fe2+ transitions from a low spin state to a high-spin state (or vice versa). The effects of such a transition on the XANES spectrum has been studied by collecting a series of Fe K-edge XANES spectra at varying temperatures. Further, XANES has been used to investigate the changes in the electronic structure of these materials as Ti is substituted for Sn. The room-temperature Fe K-edge XANES spectra showed that the pre-edge intensity increased with increasing Ti content as a result of the Fe–S bond becoming more covalent. (Ti K- and S K-edge XANES spectra, supported by electronic structure calculations, confirmed this analysis.) Temperature-dependent Fe K-edge XANES spectra were also collected to study the SCO transition and showed that the main-edge features decreased in intensity with decreasing temperature, corresponding to variations in the average Fe2+ spin-state. The rare-earth orthoferrites (REFeO3, RE = rare-earth, Y) were studied in order to understand the relationship between the Fe–O–Fe bond angle and an ill-studied feature known as an intersite-hybrid peak contained in the Fe K-edge spectra of some materials. This feature is attributed to non-local transitions of Fe 1s electrons to Fe 3d states on the next-nearest-neighbor atom that are hybridized with Fe 4p states on the absorbing atom through O 2p states. In this study, it is shown that the intensity of this feature is strongly dependent on the Fe–O–Fe bond angle; the lower the Fe–O–Fe bond angle, the less intense the intersite-hybrid peak is. Fe L- and O K-edge XANES spectra were also collected in order to investigate the electronic structure of these technologically relevant materials