X-RAY SPECTROSCOPY OF STRUCTURALLY TUNABLE CHARGE DENSITY WAVE   MATERIALS

Nathan Alan Turner

This dissertation presents X-ray spectroscopic studies of a series of quasi-one-dimensional platinum-halide (PtX) linear chain complexes, [Pt(en)2][Pt(en)X2]·(ClO4)4 where X = Cl, Br, I, together with related platinum complexes. The linear chain materials exhibit a commensurate charge density wave (CDW) with an amplitude that can be systematically varied by changing the bridging halide ion, giving rise to structurally tunable fractional mixed valence states. Four studies were performed: 1. Extended X-ray absorption fine structure (EXAFS) studies were carried out on the PtX complexes. Ab initio FEFF9 modeling of the spectra revealed distinct photoelectron threshold energy values for the inequivalent platinum ions in each of the mixed-valence chains, with values that vary systematically with fractional valence state, reflecting the CDW properties. Dynamical matrix modeling was used to relate the experimentally determined mean-square relative displacement parameters for the metal-halide bond distances to the chain-axis vibrational modes that modulate the CDW structure. 2. X-ray absorption near edge structure (XANES) spectroscopy, both conventional and high energy resolution fluorescence detected (HERFD-XANES), was carried out on the PtX complexes and Pt standards. Ab initio FEFF9 modeling of the spectra and the associated angular momentum projected density of states (lDOS) were used to interpret the results, revealing systematic changes in the orbital occupancy in the linear chain materials that reflect the fractional mixed valence states of the CDW. A significant contribution to the oriented spectra of the linear chains is identified as resulting from quantum interference between the final states of the transition. 3. Resonant inelastic X-ray scattering (RIXS) measurements were carried out to probe the filled valence states, and the results are discussed in terms of the structurally tunable band structure of the linear chain materials. 4. Picosecond X-ray transient absorption (XTA) measurements were carried out on the PtCl chain complex to probe the changes in electronic distribution and local structure upon formation of the self-trapped exciton following photoexcitation, and the response is interpreted in terms of theoretical models for localized excitations.

X-RAY SPECTROSCOPY OF STRUCTURALLY TUNABLE CHARGE DENSITY WAVE MATERIALS

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