Structural study on charge ordered states of ″-DODHT salts
Hiroyuki Nishikawa1, Yoichi Shinzaki1, Ryusuke Kondo2, Akiko Nakao3, Reiji
Kumai4, Yoshio Nogami5
1
Department of Chemistry, Ibaraki University, Mito, Ibaraki, Japan2
Department of Basic Science, The University of Tokyo, Komaba, Tokyo, Japan
3Comprehensive Research Organization for Science and Society, Tokai, Ibaraki, Japan
4
Institute of Material Structure Science, KEK, Tsukuba, Ibaraki, Japan
5
Department of Physics, Okayama University, Okayama, Japan

Normalized cell parameter

We are currently investigating the development of new donor molecules based on thenew organic donor with reducing -electron system, DODHT ((1,4-dioxane-2,3diyldithio)dihydrotetrathiafulvalene) [1]. We have already found three pressure-induced
organic superconductors, ″-(DODHT)2X (X = PF6, AsF6 and BF4∙H2O), which exhibit
insulator transition at TI = ca. 250 K at ambient pressure [2]. X-ray diffraction measurement
revealed that the satellite reflections corresponding to twofold superstructure along a-axis
were observed below the transition temperature TI for ″-(DODHT)2PF6. The magnetic
susceptibility data above 60 K was well reproduced by one-dimensional S = 1/2 alternating
Heisenberg chain model. These results indicate that the insulating state of this salt comprises acharge ordered state and the application of hydrostatic pressure above 13.2 kbar brings thesuperconducting phase by suppression of the charge ordered insulating phase [3]. In thiscontext, the nature of the insulating state under pressure of this salt is important issue forinvestigation of the relationship between charge ordering and superconductivity. Among
″-DODHT salts with octahedral anions, only the PF6 and AsF6 salts showed transition tosuperconducting states under pressures of above ca. 1.3 GPa. Their magnetic behaviors,
however, are rather different; temperature dependence of magnetic susceptibility of PF 6 salt
followed 1-dimensional alternating Heisenberg model, while that of AsF6 salt did not and anabrupt drop of susceptibility was observed at around 90 K. On the other hand, ″-DODHT
salts with larger octahedral anions such as SbF6 and TaF6 did not show the superconducting
transition under the pressure. Nevertheless the magnetic susceptibility was well reproduced byalternating Heisenberg model for both the salts. In order to clarify the difference of physicalbehaviors among ″-DODHT salts, we carried outb
X-ray structural experiments using synchrotron beam
1.01
line at Photon Factory in KEK (BL-8A).
150K
For the AsF6 salt, satellite reflections
corresponding to (a*/2, b*/2, c*) appears with
1.00
cbroadness of the spots below 95 K, where themagnetic susceptibility decreased to non-magnetic
0.99
state abruptly. This result was in contrast with the PF6250Ksalt, indicating a different CO structure. The precise
0.98 astructural analysis including superlattice reflections
was difficult due to the streak-like spots, we collected
V"-(DODHT)2AsF6
the data at 150 K and 250 K; just below and above the0.97
0100
200300temperature where the length of the b-axis wereTemperature / K
significantly changed (Figure 1). We will compare theFig 1. Temperature dependence of cellstructure at these two temperatures and discuss theparameter of ″- (DODHT)2AsF6.
insulating state of this salt.
[1] H. Nishikawa et al., Chem. Commun., 494-495 (2003). [2] H. Nishikawa et al., J. Am.
Chem. Sco., 730-731 124 (2002). [3] H. Nishikawa et. al., Phys. Rev. B, 72, 052510 (2005).