X-Ray Absorption Near-Edge Spectroscopy of Structurally Tunable Charge Density Wave Systems*

Mixed-valence halide-bridged transition metal linear chain (MX) complexes have proven to be excellent model systems for investigating a range of phenomena inherent to correlated electron systems owing to their structural tunability. In these materials, the nature of the ground electronic state is determined by competing electron-electron and electron-phonon interactions, which can be systematically tuned by varying the metal ion (M) and the halide ion (X) that comprise the linear chain structure. Platinum-halide materials exhibit a charge density wave ground state with fractional charge states on alternating Pt ions in the chain, together with a Peierls distortion with alternating Pt-halide bond lengths. We have carried out oriented Pt LIII XANES measurements on a series of PtX materials with X = Cl, Br, and I, in which the halide ion controls the amplitude of the charge density wave. FEFF9 modeling of the XANES spectra and LDOS shows systematic changes in orbital occupancy, reflecting variation of the amplitude of the charge density wave with coupling strength.