SUBSTITUTIONAL DISORDER AND ANION ORDERING TRANSITION IN THE (TMTSF)(2)(REO4)(1-X)(CLO4)(X) SOLID-SOLUTION

We present an x-ray diffuse scattering study of the anion ordering (AO ) transition in the (TMTSF)(2)(ReO4)(1-x)(ClO4)(x) solid solution of q uasi-one-dimensional organic conductors and superconductors. As (TMTSF )(2)ReO4 and (TMTSF)(2)ClO4 present AO instabilities at different crit ical wave vectors [q(1) = (1/2,1/2,1/2) and q(2) = (0,1/2,0), respecti vely] this solid solution allows for the study of the influence of com peting interactions on a structural transition. The order parameter of this transition can be characterized by an Ising variable representin g the two possible orientations of the ReO4- or ClO4- anion in cavitie s delimited by organic TMTSF molecules. Furthermore, the random substi tution of the anions shows that these interactions are randomly distri buted. The phase diagram of the AO transition has been determined as a function of x. At low temperatures (T) a q(1) long-range order (LRO) occurs for x less than or equal to 0.5 and a q(2) LRO occurs for 1 les s than or equal to x less than or equal to 0.97, while a short-range o rder (SRO) appears in the intermediate concentration range. For 0.7 < x < 0.94 one observes coexistence of q(1) and q(2) SRO. The basic feat ures of the (T,x) phase diagram can be accounted for by a mean-field t reatment of the Ising model with random interactions. This model, howe ver, fails to reproduce all the details of the phase diagram, such as the x dependence of the critical wave vector. Finally, we discuss the implications of this structural phase:diagram on the electronic proper ties of the solid solution.