Metallic behavior and periodical valence ordering in a MMX chain compound, Pt2(EtCS2)4I

A new one-dimensional (I-D) halogen-bridged mixed-valence diplatinum(II,III) compound, Pt-2(EtCS2)(4)I (3). has been successfully synthesized from [Pt-2(EtCS2)(4)] (1) and [Pt-2(EtCS2)(4)I-2] (2). These three compounds have been examined using UV-visible-near-IR, IR, polarized Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray crystal structure analyses (except for 1). Compound 3 was further characterized through electrical transport measurements, determination of the temperature dependence of lattice parameters, X-ray diffuse scattering, and SQUID magnetometry. 3 crystallizes in the monoclinic space group C2/c and exhibits a crystal structure consisting of neutral 1-D chains with a repeating -Pt-Pt-I- unit lying on the crystallographic 2-fold axis parallel to the b axis. The Pt-Pt distance at 293 K is 2.684 (1) Angstrom in the dinuclear unit, while the Pt-I distances are essentially equal (2.982 (1) and 2.978 (1) Angstrom). 3 shows relatively high electrical conductivity (5 -30 S cm(-1)) at room temperature and undergoes a metal-semiconductor transition at TM-S = 205 K. The XPS spectrum in the metallic state reveals a Pt2+ and Pt3+ mixed-valence state on the time scale of XPS spectroscopy (similar to 10(-17) S). In accordance with the metal-semiconductor transition, anomalies are observed in the temperature dependence of the crystal structure, lattice parameters, X-ray diffuse scattering, and polarized Raman spectra near TM-S. In variable-temperature crystal structure analyses, a sudden and drastic increase in the Pt-I distance near the transition temperature is observed. Furthermore, a steep increase in U-22 of iodine atoms in the I-D chain direction has been observed. The lattice parameters exhibit significant temperature dependence with drastic change in slope at about 205-240 K. This was especially evident in the unit cell parameter b (I-D chain direction) as it was found to lengthen rapidly with increasing temperature. X-ray diffraction photographs taken utilizing the fixed-film and fixed-crystal method for the metallic state revealed the presence of diffuse scattering with line shapes parallel to the a* axis indexed as (-, n + 0.5, 1) (n; integer). Diffuse scattering with k = n + 0.5 is considered to originate from the 2-fold periodical ordering corresponding to -Pt2+-Pt2+-I-Pt3+-Pt3+-I-or-Pt2+-Pt3+-I-Pt3+-Pt2+-I- in an extremely short time scale. Diffuse lines corresponding to 2-D ordering progressively decrease in intensity below 252 K and are converted to the diffuse planes corresponding to I-D ordering near TM-S. Furthermore, diffuse planes condensed into superlattice reflections below TM-S. Polarized Raman spectra show temperature dependence through a drastic low-energy shift of the Pt-I stretching mode and also through broadening of bands above TM-S.