Weiss Group and Molecular Devices InterGroup Meetings

Group Meetings are weekly in 339 Davey at 1³⁰ PM on Thursdays. (note change in day of the week)
InterGroup and MRSEC meetings are held in 339 Davey. They include and follow MRSEC seminars, and include lunch.
All are welcome to these series.

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Quasi-one-dimensional (1D) halogen-bridged complexes have been attractive much attention because of not only many marked physical properties but also controllability of the electronic states by varying those components. It has been established that Pd and Pt complexes form MII-MIV charge density wave (CDW) states, whereas Ni complexes form M^III-M^III Mott-Hubbard states. Recently synthesized Ni-Pd mixed metal complexes [Ni_1-xPd_x(chxn)₂Br]Br₂ (0=x=1) have been interested because competition of electronic structure between CDW and Mott-Hubbard states is expected. In the presentation, I report the local valence structures of [Ni_1-xPd_x(chxn)₂Br]Br₂ (x=0, 0.40, 0.76, 1) observed by scanning tunnel microscopy (STM) measurements.

Fig. 1 shows an STM image of [Ni(chxn)2Br]Br2 in the range of 200 Å × 200 Å. Bright spots in the image are observed every 5 Å by 7 Å. Because Ni-Ni distances along b (1-D chain) and c axes are 5.16 and 7.12 Å, these spots reflect the periodicity of Ni(chxn)2 units on bc plane. The observed periodicity in the image of [Ni(chxn)₂Br]Br₂ show a Mott-Hubbard Ni^III–Ni^III state, which is consistent with the result obtained by single-crystal X-ray analysis.

An STM image of [Pd(chxn)₂Br]Br₂ in the range of 200 Å × 200 Å is shown in Fig. 2. Bright spots are observed every 10 Å by 7 Å. Because Pd-Pd distances along b (1-D chain) and c axes are 5.29 and 7.07 Å, respectively, these spots along the chains reflect the two-fold periodicity of the lattice owing to the CDW state of [Pd(chxn)₂Br]Br₂. The phase of CDW is almost aligned in bc plane, consistent with the X-ray diffuse scattering study which shows this complex is so called 2D CDW.

Fig. 3 shows an STM image of [Ni_1-xPd_x(chxn)₂Br]Br₂ (x= 0.76). In this complex, two parts in which bright spots are observed every 5 and 10 Å along the chain coexist. This result shows Mott-Hubbard and CDW states coexist in this complex, indicating ca. x=0.8 is the boundary between Mott-Hubbard and CDW states. Detailed results will be discussed.

Dr. Hiroyuki Nishikawa, Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachi-Oji, Tokyo 192-0397, Japan
NEW ORGANIC SUPERCONDUCTORS: A MOLECULAR DESIGN TO SUPERCONDUCTIVITY

Organic (super)conductors have developed mainly based on p-electron system donors, tetrathiafulvalene (TTF) derivatives. In the course of the studies, the extension of a -system was revealed to be a very useful way to obtain a stable metallic salt which retains metallic nature without an insulator transition. In order to develop an organic superconductor, however, it is necessary to destabilize such stable metallic states, which leads to highly correlated electron systems. The reduction of p-electron system of the donors is one of the hopeful strategies for this purpose and we have already prepared new organic conductors composed of p-reduced donors.

This talk will present our new concept for a molecular design towards organic superconductors and preparation, structures and physical properties of new organic superconductors from ‘post-TTF’ donors designed by this concept.

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