Photon Factory Activity Report 2014 #32 (2015) B

BL-8A/2014S2-001

Structural Investigation on H/D Isotope Effectin a Hydrogen-bonded Purely Organic Conductor
Akira Ueda1,* Shota Yamada1,2, Takayuki Isono1, Hiromichi Kamo1, Akiko Nakao3, Reiji Kumai4,Hironori Nakao4, Youichi Murakami4, Kaoru Yamamoto5, Yutaka Nishio2, and Hatsumi Mori1
1
The Institute for Solid State Physics, The University of Tokyo, Chiba 277-8581, Japan2
Department of Physics, Faculty of Science, Toho University, Funabashi, Chiba 274-8510, Japan
3
Comprehensive Research Organization for Science and Society (CROSS), Ibaraki 319-1106, Japan
4
CMRC and Photon Factory, Institute of Materials Structure Science, High Energy Accelerator
Research Organization (KEK), Ibaraki 305-0801, Japan
5
Department of Applied Physics, Okayama University of Science, Okayama 700-0005, Japan
1 Introduction
Hydrogen bonds (H-bonds) are one of the mostfundamental and important non-covalent interactions inmaterials, and thus have attracted much attention in awide range of scientific fields. Notably, H-bonded protons(or hydrogen atoms) can be thermally transferred ordisplaced within the H-bond. This unique dynamic
behavior is known to play crucial roles in (bio)chemical
reactions/processes and some kinds of functional
solids/materials, such as dielectrics and proton conductors.
Recently, our group has explored H-bonded organic
conductors, whose H-bond dynamics and conductive electrons are cooperatively coupled in the crystal. Amongthem, -H3(Cat-EDT-TTF)2 (-H) is a new class of Hbonded conductors whose conducting layers areconnected by strong H-bonds (Fig. 1) [1], whichanticipates the emergence of interplay between the electrons and H-bond dynamics and the resulting novel
electronic properties and functionalities [2]. Therefore, inthis work, we have prepared the deuterated analogue of H, -D3(Cat-EDT-TTF)2 (-D), and investigated its
structure and properties, to gain insight into how thedifference in the H-bond dynamics by the H/Dsubstitution affects the electronic structure of this uniquemolecular system [3].

hydrogen crystal -H at room temperature. Namely, theH-bonded molecular unit, shown in the top left of Fig. 2,
is assembled (Fig. 1), to form the two-dimensional
conducting layer (Fig. 2, bottom left), in which the dimers with S = 1/2 spins are arranged two dimensionally.
On the other hand, their low-temperature structures arefundamentally different from each other: Although -Hmaintains the room temperature structure, -D undergoesa phase transition at 185 K, originating from deuteriumtransfer or displacement within the H-bond accompaniedby electron transfer between the Cat-EDT-TTF -systems(Fig. 2, top). As a result, significant charge
disproportionation occurs within the H-bonded unit,
which leads to the transformation from the dimer-Mottstate at high temperatures to a charge-ordered state with
spin-singlet pairs (Fig. 2, bottom). Reflecting thiselectronic structure change, the physical properties of -D
are drastically switched (paramagnetic semiconductor ↔
nonmagnetic insulator). Therefore, we have concluded
that the present system is a truly new type of molecularconductors, whose electronic structures and physicalproperties are coupled to the structural degree of freedomof hydrogen (deuterium) in H-bonds [3].

Fig. 2: Schematic drawing of the phase transition of -D.
Fig. 1: Crystal structures of -X3(Cat-EDT-TTF)2 (-H:X = H, -D: X = D) at room temperature.
2 Results and Discussion
The synchrotron X-ray structural analysis at BL-8A,
KEK-PF reveals that -D is isostructural to the parentReferences[1] T. Isono et al., Nat. Commun. 4, 1344 (2013).
[2] T. Isono et al., Phys. Rev. Lett. 112, 177201 (2014).
[3] A. Ueda et al., J. Am. Chem. Soc. 136, 12184 (2014).
* a-ueda@issp.u-tokyo.ac.jp