Photon Factory Activity Report 2016 #34 (2017)

BL-8B/2014G089

Charge ordering in δo-(BEDT-TTF)2TaF6
Tadashi Kawamoto1,*, Kohei Kurata2, Takehiko Mori1, and Reiji Kumai3
Department of Materials Science and Engineering, Tokyo Institute of Technology, Tokyo 152-8552,
Japan
2
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Tokyo 152-8552,
Japan
3
ERLCondensed Matter Research Center and Photon Factory, Institute of Materials Structure Science,High Energy Accelerator Research Organization (KEK), Tsukuba, 305-0801, Japan
1

1 Introduction
The δ-type organic conductor (BEDT-TTF)2MF6 (M =P, As, and Sb) shows a phase transition at approximately
room temperature (260 – 300 K) [1-4]. The PF6 salt has
been investigated using x-ray diffraction, and the twofold superlattice reflections corresponding to 2kF havebeen observed [4]. However, the low temperature crystal
structure shows that the phase transition is not a 2kF
charge-density-wave (CDW) but a charge order [5].
Recently, two δ-type polymorphs, monoclinic (δm) andorthorhombic (δo) phases have been found in (BEDTTTF)2TaF6 [6]. Both phases show a semiconductorinsulator phase transition at 276 K and 300 K for the δmand δo phases, respectively. The two-fold superlatticereflections are observed at around the phase transition
temperatures for both phases. However, the wave vectorof the superlattice reflections, q = (b* + c*)/2, of the δoTaF6 salt differs from that of the δo-PF6 salt (q = c*/2)
despite the same crystal structure [6]. The present paper
reports the low-temperature crystal structure of δo(BEDT-TTF)2TaF6.
2 Experiment
X-ray oscillaiton photographs were taken using animaging plate at BL-8B of PF, KEK. The sample wascooled down to 39 K by a helium gas stream coolingmethod. The structure was solved by the direct method(SHELXT) and refined by the full-matrix least-squares
procedure (SHELXL) [7,8].
3 Results and Discussion
Although some reflections with l ≠ 2n, where n is aninteger, are observed in h0l reflections, we have chosen
P2/c as suggested by SHELXT in order to clarify theaverage strucure in the low-temperature insulating phase[9]. This space group is a sub-group of the hightemperature space group Pnna, where the superlattice isgiven by a’ = 2c, b’ = a, and c’ = -c-b based on thesuperlattice wave vector. Therefore, the crystallographic
independent molecules are four donors and two anions.
Although two donors A and C are flat, B and D molecules
are bent (Fig.1); the bent TTF skeleton is characteristic of
the neutral BEDT-TTF. This indicates that the lowtempertaure phase is a charge ordered state. Moreover,the charges of the donors estimated from the bond lengths
are QA = +0.8(2), QB = +0.5(2), QC = +0.7(2), and QD =+0.2(2). These are in agreement with the molecularshape; the flat molecules are hole rich. The chargeordered pattern obtained shows that the hole rich donors
are dimerized by the largest transfer integral; the spin
singlet state observed in the electron spin resonancespectra can be realized.
In summary, the phase transition of the present
compound is not a 2kF CDW but a charge orderingtransition originated from the intersite Coulomb repulsion.

Fig. 1: Molecular shape projected along the molecular
short axis (a) and the donor layer projected onto the abplane (b) of δo-(BEDT-TTF)2TaF6 at 39 K. In (b), blackrounded rectangles denote hole rich molecules A and C,and dashed ellipses represent dimers.

References[1] H. Kobayashi et al., Chem. Lett. 12, 581 (1983).
[2] R. Laversanne et al., Solid State Commun. 52, 171(1984).
[3] P. C. Leung et al., Acta Crystallogr. C 40, 1331
(1984).
[4] G. K. R. Senadeera et al., J. Phys. Soc. Jpn. 67, 4193
(1998).
[5] Y. Nogami et al., J. Phys. IV Fr. 12, 233 (2002).
[6] T. Kawamoto et al., Magnetochem. 3, 14 (2017).
[7] G. M. Sheldrick, Acta Crystallogr. A 71, 3 (2015).
[8] G. M. Sheldrick, Acta Crystallogr. C 71, 3 (2015).
[9] Crystal data of δo-(BEDT-TTF)2TaF6 at 39 K:
monoclinic, space group P2/c, a = 13.2594(5) Å, b =14.5036(4) Å, c = 33.8724(7) Å, β = 101.206(2)º, V =6389.8(3)Å3, and Z = 8. The final R1 value is 0.1387 forobserved 20909 reflections (F2 > 2σ(F2)).
* kawamoto@o.cc.titech.ac.jp