Electrical and magnetic characterisation of a series of TCNQ salts.
The electrical and magnetic properties of a series of 12 isostructural TCNQ salts of bis-pyridinium cations are reported and these properties are related to the structural characteristics. The role of the cation lattice in stabilising a regular TCNQ stack in these salts is discussed. For comparison with this series, the electrical and magnetic properties of two TCNQ salts of bis-pyridinium cations with ordered structures and two salts of dialkyldiphenylphosphonium cations are reported.Within the isostructural series of salts, the stoichiometry takes values of between 1:3 and 1:5, and the cation length determines the stoichiometry. In all these salts, the hydrated cation lattice is disordered and the TCNQs stack in regular columns. The room temperature conductivities, measured along the stacking axis, are in the range 0.05 to 500 S/cm with the highest conductivities observed in dehydrated salts having the approximate stoichiometry, 1:4.5. The 1:5 salts possess the lowest conductivity of the series and in these salts the cation lattice is partially ordered. The 1:3 and 1:4 salts are small band gap semi-conductors whereas the conductivity of the 1:5 salts is not simply activated within the experimental range of temperatures. In these salts the conductivity obeys the empirical power law: [mathematical equation]. The temperature dependence of the magnetic susceptibility of all but the 1:3 salts of the isostructural series is characteristic of a system of partially localised triplet excitons. The magnitude of the singlet-triplet exchange energy exhibits a stoichiometric dependence and is lower in the 1:5 salts where the spin concentration is lower. The magnetic susceptibility of the 1:3 salts varies with temperature according to the Curie-Weiss law. The apparent low spin concentration supports the evidence found for alloy formation in these salts where the cation lattice is partially occupied by neutral or monoquaternised base.
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Sheffield Hallam University (United Kingdom).