Alkali metal TCNQ salts

B. Specific Examples Alkali Metals-TCNQ Salts... 

The spin-Peierls transition has been the object of several recent papers. Examples of the first case, with p = 1, are provided by the insulating alkali metal simple salts [47] (see below), and examples of the second case, with p = 3 or I, are given by the salt MEM(TCNQ)2 [17-19,46], by the salts of the (TMTTF)2X series [46,48], or by the salts of the (BCPTTF)2X series (BCPTTF = benzocyclopentyltetrathiafulvalene) [46,49]. 

The electrical conductivity of TTF TCNQ is of the order of 10 S m at room temperature and increases with decreasing temperature until around 80 K when the conductivity drops as the temperature is lowered. TCNQ is a good electron acceptor and, for example, accepts electrons from alkali metal atoms to form ionic salts. In TTF-TCNQ, the columns of each type of molecule interact to form delocalised orbitals. Some electrons from the highest energy filled band of TTF move across to partly fill a band of TCNQ, so that both types of columns have partially occupied bands. The number of electrons transferred corresponds to about 0.69 electrons per molecule. This partial transfer only occurs with molecules such as tetrathiafulvalene whose electron donor ability is neither too small nor too large. With poor electron donors, no charge transfer... 

What makes the TTF-TCNQ family distinct from the other salts of TCNQ with cations, such as alkali metals and tetramethylammonium, is that the charge transfer,/ in the TTF-TCNQ family is incomplete (f < 1). TTF-TCNQ members are also different from the TTF-halides in the TTF-halides, where the charge on each halide atom is unity, partial charge transfer (mixed valency) is realized by the formation of nonstoichiometric materials, while in the TTF-TCNQ family, the composition is stoichiometric (1 1), but mixed valence arises because of partial electron-transfer. 

It has long been known that alkali metals Na, K, Rb, and Cs form with the acceptor TCNQ 1 1 segregated charge-transfer salts, with complete ionization p = 1. They have regular chains at high temperature but are insulators due to Coulomb repulsions as discussed in the preceding section. They are then primarily one-dimensional magnetic materials whose properties have been studied extensively in the past [69]. 

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