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.
We present optimized tight-binding models with atomic orbitals to improve ab initio tight-binding models constructed by truncating full density functional theory (DFT) Hamiltonian based on localized orbitals. Retaining qualitative features of the original Hamiltonian, the optimization reduces quantitative deviations in overall band structures between the ab initio tight-binding model and the full DFT Hamiltonian. The optimization procedure and related details are demonstrated by using semiconducting and metallic Janus transition metal dichalcogenides monolayers in the 2H configuration. Varying the truncation range from partial second neighbors to third ones, we show differences in electronic structures between the truncated tight-binding model and the original full Hamiltonian, and how much the optimization can remedy the quantitative loss induced by truncation. We further elaborate the optimization process so that local electronic properties such as valence and conduction band edges and Fermi surfaces are precisely reproduced by the optimized tight-binding model. We also extend our discussions to tight-binding models including spin-orbit interactions, so we provide the optimized tight-binding model replicating spin-related properties of the original Hamiltonian such as spin textures. The optimization process described here can be readily applied to construct the fine-tuned tight-binding model based on various DFT calculations.


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