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Structural and Electronic Properties of (TMTTF)2X Salts with Tetrahedral Anions

Comprehensive measurements of the pressure- and temperature-dependent dc-transport are combined with dielectric spectroscopy and structural considerations in order to elucidate the charge and anion orderings in the quasi-one-dimensional charge-transfer salts (TMTTF)2X with non-centrosymmetric anions X = BF4, ClO4 and ReO4. Upon applying hydrostatic pressure, the charge-order transition is suppressed in all three compounds, whereas the influence on the anion order clearly depends on the particular compound. A review of the structural properties paves the way for understanding the effect of the anions in their methyl cavities on the ordering. By determining the complex dielectric constant ê(ω, T) in different directions we obtain valuable information on the contribution of the anions to the dielectric properties. For (TMTTF)2ClO4 and (TMTTF)2ReO4, eb' exhibits an activated behavior of the relaxation time with activation energies similar to the gap measured in transport, indicating that the relaxation dynamics are determined by free charge carriers.

Fabre salts ; one-dimensional conductors ; charge order ; anion order ; electronic transport ; pressure dependence ; dielectric spectroscopy

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