Metallic Behavior and Periodical Valence Ordering in a MMX Chain Compound, Pt2(EtCS2)4I

• Published in: Journal of the American Chemical Society Vol. 123; no. 45; pp. 11179 - 11192
• Main Authors: Mitsumi, Minoru, Murase, Tomoko, Kishida, Hideyuki, Yoshinari, Tetsuya, Ozawa, Yoshiki, Toriumi, Koshiro, Sonoyama, Takuya, Kitagawa, Hiroshi, Mitani, Tadaoki
• Format: Journal Article
• Language: English
• Published: American Chemical Society 14.11.2001
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja010900v

A new one-dimensional (1-D) halogen-bridged mixed-valence diplatinum(II,III) compound, Pt2(EtCS2)4I (3), has been successfully synthesized from [Pt2(EtCS2)4] (1) and [Pt2(EtCS2)4I2] (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) Å in the dinuclear unit, while the Pt−I distances are essentially equal (2.982 (1) and 2.978 (1) Å). 3 shows relatively high electrical conductivity (5−30 S cm-1) at room temperature and undergoes a metal−semiconductor transition at T M - 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 (∼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 T M - 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 1-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 (1-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, l) (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 1-D ordering near T M - S. Furthermore, diffuse planes condensed into superlattice reflections below T M - 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 T M - S.