Résumé
We present an x-ray diffuse scattering study of the one-dimensional substituted perylene radical-cation (M)2X-S salts, where M is the 1,2,7,8-tetrahydrodicyclopenta[cd,lm]-perylene or the 3,4,9,10-tetra- methylperylene radical cation, X the PF6- or AsF6- anion, and S a solvent molecule similar to CH2Cl2. These isostructural salts consist of stacks of radical cations and of anion and/or solvent chains (X/S) located in the channels between the organic stacks. Despite the similarity of the TMP and CPP radical cations, the magnetic and electronic properties of both types of salts differ. We show that the structural difference between the TMP and the CPP salts resides in the transverse correlations between the positions of the X/S chains: the X/S chains are much more disordered in the TMP salts than in the CPP ones. We have studied the X/S chains disorder by using an Ising model on a triangular lattice with nearest and next-nearest neighbor interactions. This study shows that the disorder in the TMP salts is due to a competition between the direct interactions between the chains X/S and those which are mediated by the organic stacks. In the more ordered CPP salts, the later interactions are screened by the conducting electrons of the organic stacks. Accordingly, it is suggested that the TMP stacks undergo a site charge localization (4kF-site charge localization). This interpretation is consistent with C13 cross polarization magic angle spinning NMR, conductivity and electrochemical measurements. The consequences on the Peierls instabilities of both salts are briefly discussed.