Copied from MDF News Flash (No.6) May 22, 2010
Selective annihilation of a multi-instability component in (EDO-TTF)2PF6
The title complex is known to show the peculiar metal-insulator (MI) transition associated with the distinct deformation of donor molecule at just below room temperature (ca. 280 K). The transition mechanism was attributed to the cooperation of Peierls transition, order-disorder (OD) transition of the counter anion and charge-ordering (CO) transition. Due to the multi-instability of this quasi-one-dimensional molecular conductor, the susceptible nature to external stimuli was anticipated. In fact, the ultra-fast and highly efficient photo-induced phase transition to a highly conducting state was observed for the low-temperature insulator phase, in which one photon converted 50 - 500 donor molecules within 0.1 ps followed by the relaxation to the metastabe state within ca. 1.5 ps.
Recently, the researchers in Kyoto and Okazaki inspected the cooperativity of the mechanisms by examining the alloy system. Although the methyl substituted donor molecule, MeEDO-TTF, also produced the 2:1 PF6 complex, its crystal and electronic structures were different from those of (EDO-TTF)2PF6. Despite the differences, the electrooxidation of a mixture of these donor molecules yielded the single crystalline alloys of [(EDO-TTF)1-x(MeEDO-TTF)x]2PF6 in the range of 0 < x ≤ 0.55 and 0.91 ≤ x < 1, the room-temperature crystal structures of which were isostructural to those of x = 0 and x = 1 pristine complexes, respectively.
The detailed comparison of EDO-TTF rich alloys proved that the multi-instability same to that observed for x = 0 complex caused the MI transition of the alloy with x = 0.05 at 242 K, while x = 0.22 - 0.55 alloys showed no phase transition. The molar fraction dependence of transition feature was visualized by comparing the crystal structures at 150 K (Fig. 1b - 1d). The periodicity of the donor arrangement was doubled and the anion was ordered at 150 K from that at 300 K (Peierls and OD Transitions; cf. Fig. 1a). The flat (F) and bent (B) shapes of EDO-TTF in x = 0.05 alloy at 150 K corresponded to the rich and poor charges on the molecules, respectively (CO transition; Fig. 1b). The most striking result was obtained for the MI transition of x = 0.13 alloy at 188 K. In this case, only the unit cell doubling and partially suppressed OD transition feature were detected for the crystal structure at 150 K (Fig. 1c), both of which were no more observed for x = 0.48 alloy (Fig. 1d). These results indicate that the alloying suppressed all the instabilities. The difference of suppression efficiencies resulted in the selective annihilation of CO feature among the components of the multi-instability for x = 0.13 alloy.
For details, see an article, "Tuning of Multi-instabilities in Organic Alloy, [(EDO-TTF)1-x(MeEDO-TTF)x]2PF6", Chem. Mater., 22(10), 3121–3132 (2010) by T. Murata, X.F. Shao, Y. Nakano (A05(a)), H. Yamochi (A05(a)), M. Uruichi, K. Yakushi, G. Saito, K. Tanaka.