Photon Factory Activity Report 2005 #23 Part B (2006)
Electronic Structure of Condensed Matter

1B, 4C/2004G226

Incommensurate Structure of the Organic Superconductor (MDT-TS)I0.85Br0.41
Tadashi KAWAMOTO1*, Yoshimasa BANDO1, Takehiko MORI1, Toru KAKIUCHI2, Hiroshi SAWA2,3,Kazuo TAKIMIYA4, Tetsuo OTSUBO4
1
Department of Organic and Polymeric Materials, Graduate School of Science and Engineering,Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan2
Department of Materials Structure Science, The Graduate University of Advanced Studies, Tsukuba,Ibaraki 305-0801, Japan3Institute of Materials Structure Science, High Energy of Accelerator Research Organization,
Tsukuba, Ibaraki 305-0801, Japan4
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University,Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan
Introduction
In organic superconductors, the ratios of the donor
molecules to anions are represented by an integer(typically 2:1) [1].
By contrast, the MDT-TSF(methylenedithio-tetraselenafulvalene) series salts areincommensurate
ambient
pressureorganic
superconductors and the charge transfer degrees deviate
from 0.5 [2]. Moreover, (MDT-TSF)(AuI2)0.436 has shown
the characteristic Fermi surface reconstruction by anincommensurate anion potential [3]. (MDT-TS)(AuI2)0.441,
where MDT-TS is 5H-2-(1,3-diselenol-2-ylidene)-1,3,4,6tetrathiapentalene, shows a metal-insulator transition at
TMI = 50 K in spite of the basically same crystal structure
as those of the MDT-TSF superconductors [4]. The
ground state of this salt changes from an“incommensurate antiferromagnetic insulating state” with
TN = 50 K to a superconducting phase at 3.2 K under 10.5
kbar [5]. We have discovered new organic conductor
based on MDT-TS and IxBry. The new conductor shows
superconducting phase transition under high pressure.
The present paper reports the incommensurate lattice
structure of (MDT-TS)I0.85Br0.41.
Results and Discussion
Figure 1 shows the synchrotron radiation x-ray
oscillation photograph at room temperature.
Thisphotograph clearly displays incommensurate layer lines.
We distinguish the donor lattice and the anion lattice byindices h and h’, respectively. In Fig. 1, there are several
spots around h = 0 line. These spots are originated in theconvolution between h = 1 and h’ = 3 lines. Although
there is no clear spot at h’=1 line which comes from thelength of I3 or I2Br , the Raman spectra show that theanion has a unit of I3 . Therefore, the anions exist as trihalide anion chains composed by I and Br. The ratio
between the length of the a-axes (a/a’) is 1.2612(2) fromFig. 1, and the composition is estimated as (MDTTS)I0.85Br0.41 from the x-ray oscillation photograph and theenergy dispersion spectroscopy. This composition meansthat the MDT-TS molecule does not make a simple I2Br or
IBr2 salt. We can speculate that the charge transfer degree

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is 0.420 under assuming tri-halide anions. This value lies
in between (MDT-TS)(AuI2)0.441 and (MDT-TS)(I3)0.407.
This result indicates that we can control the chargetransfer degree (band-filling) in organic conductors basedon MDT-TS with linear anions by changing the length ofthe anion.

Figure 1: X-ray oscillation photograph. The verticaldirection is along the donor stacking direction.

References[1] T. Ishiguro, K. Yamaji, and G. Saito, OrganicndSuperconductors, 2 Ed. (Springer, Berlin, 1998).
[2] K. Takimiya et al., Angew. Chem. Int. Ed. Engl. 40,1122 (2001); Chem. Mater. 15, 3250 (2003); ibid 15,1225 (2003); T. Kawamoto et al., Phys. Rev. B 65,140508(R) (2002); ibid 71, 172503 (2005); J. Phys. Soc.
Jpn. 74, 1529 (2005).
[3] T. Kawamoto et al., Phys. Rev. B 67, 020508(R)(2003); Eur. Phys. J. B 36, 161 (2003).
[4] K. Takimiya et al., Chem. Mater. 16, 5120 (2004)[5] T. Kawamoto et al., Phys. Rev. B 71, 052501 (2005).
* kawamoto@o.cc.titech.ac.jp