129I Moessbauer spectroscopic studies of tetrathiafulvalene-iodine charge-transfer salts.

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The Mössbauer effect of 129I has been applied to investigate tetrathiafulvalene–iodine (TTF–In) charge-transfer salts at 16 K. The electrically conductive TTF–I0.71 salt shows a broad single-line with the isomer shift of −0.36 mm s−1, suggesting that the iodine atoms on the average are not completely ionic (I−1.0) but are negatively charged by about 0.9 (I−0.9) in the low temperature phase. This is due to the weak interactions of the iodine atoms with the sulfur atoms of the TTF molecules. This fact is consistent with the crystallographic data and the electron spin resonance spectra of the salt. This salt is recognized as a partially oxidized mixed-valence compound. The spectrum of the TTF–I3 salt consists of two sets of quadrupole octets, corresponding to the central and terminal iodine atoms of the slightly distorted I3− ion. The charge density localized on each iodine atom of the I3− ion is close to that of the isolated I3− ion. That is, the salt is regarded as a simple ionic compound, for which a high electrical conductivity can not be expected. It is confirmed that a small amount of the TTF–I2 salt coexists in the TTF–I0.71 phase obtained from the solution of excess TTF and iodine. In the salt the iodine atoms exist as polyiodides consisting predominantly of I3− units.
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