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

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• 作者：Minoru Mitsumi ; Tomoko Murase ; Hideyuki Kishida ; Tetsuya Yoshinari ; Yoshiki Ozawa ; Koshiro Toriumi ; Takuya Sonoyama ; Hiroshi Kitagawa ; and Tadaoki Mitani
• 刊名：Journal of the American Chemical Society
• 出版年：2001
• 出版时间：November 14, 2001
• 年：2001
• 卷：123
• 期：45
• 页码：11179 - 11192
• 全文大小：300K
• 年卷期：v.123,no.45(November 14, 2001)
• ISSN：1520-5126

文摘

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 threecompounds have been examined using UV-visible-near-IR, IR, polarized Raman spectroscopy, X-rayphotoelectron spectroscopy (XPS), and X-ray crystal structure analyses (except for 1). Compound 3 was furthercharacterized through electrical transport measurements, determination of the temperature dependence of latticeparameters, X-ray diffuse scattering, and SQUID magnetometry. 3 crystallizes in the monoclinic space groupC2/c and exhibits a crystal structure consisting of neutral 1-D chains with a repeating -Pt-Pt-I- unit lyingon the crystallographic 2-fold axis parallel to the b axis. The Pt-Pt distance at 293 K is 2.684 (1) Å in thedinuclear unit, while the Pt-I distances are essentially equal (2.982 (1) and 2.978 (1) Å). 3 shows relativelyhigh electrical conductivity (5-30 S cm^-1) at room temperature and undergoes a metal-semiconductor transitionat T_M-S = 205 K. The XPS spectrum in the metallic state reveals a Pt²⁺ and Pt³⁺ mixed-valence state on thetime scale of XPS spectroscopy (~10^-17 s). In accordance with the metal-semiconductor transition, anomaliesare 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 drasticincrease in the Pt-I distance near the transition temperature is observed. Furthermore, a steep increase in U₂₂of iodine atoms in the 1-D chain direction has been observed. The lattice parameters exhibit significanttemperature dependence with drastic change in slope at about 205-240 K. This was especially evident in theunit 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 staterevealed 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 orderingcorresponding to -Pt²⁺-Pt²⁺-I-Pt³⁺-Pt³⁺-I- or -Pt²⁺-Pt³⁺-I-Pt³⁺-Pt²⁺-I- in an extremely short timescale. Diffuse lines corresponding to 2-D ordering progressively decrease in intensity below 252 K and areconverted to the diffuse planes corresponding to 1-D ordering near T_M-S. Furthermore, diffuse planes condensedinto superlattice reflections below T_M-S. Polarized Raman spectra show temperature dependence through adrastic low-energy shift of the Pt-I stretching mode and also through broadening of bands above T_M-S.