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Tuning of Multi-Instabilities in Organic Alloy, [(EDO-TTF)1−x(MeEDO-TTF)x]2PF6

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journal contribution
posted on 2010-05-25, 00:00 authored by Tsuyoshi Murata, Xiangfeng Shao, Yoshiaki Nakano, Hideki Yamochi, Mikio Uruichi, Kyuya Yakushi, Gunzi Saito, Koichiro Tanaka
Electrocrystallization of a mixture of 4,5-ethylenedioxytetrathiafulvalene (EDO-TTF) and 4,5-ethylenedioxy-4′-methyltetrathiafulvalene (MeEDO-TTF) yielded alloyed salts, [(EDO-TTF)1−x(MeEDO-TTF)x]2PF6 (x = mole fraction of MeEDO-TTF). EDO-TTF rich alloys (x ≤ 0.5) were isostructural to (EDO-TTF)2PF6, the metal−insulator transition of which is caused by the cooperation of Peierls, anion-ordering, and charge-ordering mechanisms. Incorporation of MeEDO-TTF into (EDO-TTF)2PF6 extended the donor−anion distance to weaken donor−anion interactions, while donor−donor interactions within the conduction layer remained the same magnitudes to those of (EDO-TTF)2PF6 having a quasi-one-dimensional electronic structure. These alloys showed a metallic behavior or nearly constant resistivity around room temperature, and their transport properties strongly depended on x value. x ≤ 0.13 alloys showed a sharp metal−insulator transition due to Peierls instability and anion-ordering, and transition temperature decreased as x increased. Although charge-ordering in the donor column was also elucidated for the insulator phase of x ≤ 0.05 alloys, uniform +0.5 charge of donor molecules was preserved in the low-temperature phase of x = 0.13 alloy. In the case of the x ∼0.5 alloy, any abrupt anomalies were not detected down to low temperature, while broad resistivity minimum was observed at 200−270 K. x ≥ 0.9 alloys were isostructural to (MeEDO-TTF)2PF6, which forms a two-dimensional donor layer. Similar to (MeEDO-TTF)2PF6, MeEDO-TTF rich alloys exhibited a semiconductor-to-semiconductor phase transition. The transition became gradual, and the transition temperature was suppressed with decreasing x value. Raman spectra indicated that nearly localized and distinct charge-disproportionated states, which are observed in the high- and low-temperature phases of pristine (MeEDO-TTF)2PF6, respectively, coexist in the high-temperature phase.

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