EXAFS Spectroscopy of Fractional Mixed-Valence Charge Density Wave Systems

Abstract

We present extended X-ray absorption fine structure (EXAFS) spectra and modeling of a series of structurally tunable quasi-one-dimensional mixed-valence platinum−halide linear chain materials, [Pt(en 2)][Pt(en 2)-X 2 ](ClO 4) 4 with X = Cl, Br, I. The materials exhibit a commensurate charge density wave with fractional charge states on alternating platinum ions in the chain, as well as a Peierls distortion with alternating platinum−halide bond lengths. The amplitude of the charge density wave and, correspondingly, the extent of the Peierls distortion are controlled by the identity of the bridging halide ion. We have carried out ab initio multiple scattering calculations using the FEFF9 code to relate the oriented Pt L III EXAFS spectra to the tunable electronic and structural properties. The spectral modeling reveals distinct photoelectron threshold energy values for the two inequivalent platinum ions in each of the mixed-valence chains, with values that vary systematically with fractional valence state. The difference in the photoelectron threshold energies of the two inequivalent platinum ions within each material correlates directly with the amplitude of the charge density wave, reflecting the decrease in charge density wave strength through the halide series X = Cl, Br, and I. We use dynamical matrix modeling to relate the experimentally determined meansquare relative displacement parameters for the metal−halide bond distances to the chain−axis vibrational modes that modulate the charge density wave structure. In addition, we discuss the EXAFS fitting results for the Pt−I bond lengths in the [Pt(en 2)][Pt(en 2)I 2 ](ClO 4) 4 complex in comparison to previous, mutually inconsistent structural determinations for this material.

Probing Charge Ordering in Fractional Mixed-Valence Charge Density Wave Systems with Oriented HERFD-XANES Spectroscopy

“…Structures based on previous X-ray diffraction studies , were used in the Atoms program (version 0.9.25) to generate the atomic positions input into FEFF9. Two adjustments were made to the PtI structure reported by Endres et al: the lengths of the two Pt–I bonds that form the chain axis were set to 2.702 and 3.05 Å, as determined in our EXAFS studies, and since hydrogen positions for the ethylenediamine ligands were not reported in the diffraction study, they were constructed by placing the hydrogen atoms at angles corresponding to tetrahedral bonding about the nitrogen and carbon atoms with bond lengths consistent with those of the PtBr and PtCl complexes. Since the two inequivalent platinum ions in the chain structure have different local environments, they were assigned unique potentials in the calculations.…”