Abstract

Three completely different salts, (BET-TTF)ReO₄ (1), (BET-TTF)₃(ReO₄)₂ (2), and (BET-TTF)₉(ReO₄)₄·2(THF) (3) are obtained by small variations in the electrocrystallization conditions of bis(ethylenethio)tetrathiafulvalene (BET-TTF) with the ReO₄⁻ ion. The crystal structures of the three salts show strong differences between them. In salt 1, the completely ionic BET-TTF stacks, running along the a direction, are not regular but are forming weak dimers. Two different radical cation layers configure salt 2 and the analysis of the bond-length of BET-TTF molecules forming layers 1 and 2 shows that the β-like layer 1 has mixed valence character, while layer 2 can be regarded as an anion-cation-anion [ReO₄(BET-TTF)ReO₄]⁻ sandwich layer with completely ionised BET-TTF. Only one β-like layer that consists of two different mixed valence stacks forms salt 3. Conductivity measurements show that mixed valence salts 2 and 3 are metals with a metal-semiconductor transition around 125 K and 75 K respectively whereas the completely ionised salt 1 is, as expected, a semiconductor. The effect of high quasi-hydrostatic pressure and magnetic field on the transport properties of salt 3 has also been studied. The relationship between the crystal structure and the transport properties of all three salts has been analysed by means of tight binding band structure calculations. These calculations show that layer 1 is responsible for metallic conductivity of salt 2 whereas in layer 2 the electrons are localised. EPR data of this salt indicate that there is an appreciable interaction between delocalised electrons in layer 1 and localised magnetic moments in layer 2.