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

Abstract

We present oriented Pt LIII edge high energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) measurements on a series of structurally tunable quasi-one-dimensional mixed-valence platinum-halide linear chain materials, [Pt­(en)2]­[Pt­(en)2X2]­(ClO4)4 with X = Cl, Br, I, together with ab initio modeling of the spectral components and electronic structure. The materials exhibit a commensurate charge density wave (CDW) with an amplitude controlled by the bridging halide ion, giving rise to fractional mixed valence states. Ab initio FEFF9 modeling of the spectra and the associated angular momentum projected density of states (lDOS) are used to interpret the results, revealing systematic changes in the platinum orbital occupancy through the series of materials that reflect the fractional mixed valence states of the CDW. A significant contribution to a post-edge feature in the oriented spectra is identified as resulting from quantum interference between the final states of the X-ray absorption transition.

X-ray Transient Absorption Studies of Exciton Self-Trapping

“…15 This modeling also showed a small decrease in the white line intensity resulting from interference between final states d and s character, an effect observable in oriented spectra of anisotropic systems. 15,24,25 As a result of symmetry selection rules, the 5d x 2 −y 2 orbitals appearing at slightly higher energy in the lDOS calculations do not contribute to the oriented XANES spectrum for the X-ray polarization parallel to the chain axis.…”

“…There are multiple contributions to this spectral feature, as determined by our previous modeling. 15 Most of the amplitude originates from dipole transitions of p to d character that occur as a result of symmetry-allowed mixing between the a 1g 6s and 5d z 2 orbitals, with much smaller contributions from the inherently weaker 2p to 6s dipole transitions. Additional contributions result from final state interference involving the mixed 6s and 5d z 2 states.…”

“…15 Given an input structure, FEFF9 carries out self-consistent field (SCF) electronic structure calculations to determine the atomic scattering potentials and phase shifts assuming spherical muffin-tin (MT) potentials, followed by a full multiple scattering (FMS) calculation that sums scattering paths for the photoelectron emitted from the absorbing atom from neighboring atoms to infinite order using Green's function propagators and phase shifts to determine the angular momentum projected density of states (lDOS) and XANES absorption spectrum. 21,22 Ground-state (pump-off) and excited-state XANES spectra were modeled using FEFF9 parameters determined in our previous XANES study 15 as noted below, with exception of the imaginary part of the self-energy, which was set to give the 5.17 eV line width corresponding to the core-hole lifetime as appropriate for the measurements presented here, instead of the narrower line width of the measurements in the previous study, which were carried out using high energy resolution fluorescence detection (HERFD) techniques. The structure of PtCl determined by X-ray diffraction 12 was used in the Atoms program 23 (version 0.9.25) to generate the atomic positions input into FEFF9 for the ground-state spectrum.…”

“…Ab initio modeling was performed with FEFF9 (version 9.6 revision 4) following the approach used in our previous XANES studies of the PtX­(en) materials . Given an input structure, FEFF9 carries out self-consistent field (SCF) electronic structure calculations to determine the atomic scattering potentials and phase shifts assuming spherical muffin-tin (MT) potentials, followed by a full multiple scattering (FMS) calculation that sums scattering paths for the photoelectron emitted from the absorbing atom from neighboring atoms to infinite order using Green’s function propagators and phase shifts to determine the angular momentum projected density of states (lDOS) and XANES absorption spectrum. , …”

“…While femtosecond optical studies of the materials have revealed the dynamics of formation of STEs in the platinum-halide materials, − experimental characterization of the electronic properties of the resulting self-trapped states remains largely unexplored. In the work reported here, we use X-ray transient absorption to probe the electronic distribution in the STE of PtCl, which is predicted to lie in the highly localized limit. , Our approach is motivated by our X-ray absorption near-edge structure (XANES) spectroscopy study of a series of platinum-halide chain materials, which demonstrated that the Pt L III near-edge features are sensitive to variation of the fractional mixed valence charge states in the CDW ground states of the materials. In the experiment reported here, STEs are generated by optical excitation and the resulting photoinduced change in the Pt L III XANES spectrum is probed on a picosecond time scale.…”