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 quasi-one-dimensional organic conductors and superconductors. As (TMTSF)₂ReO₄ and (TMTSF)₂ClO₄ present AO instabilities at different critical wave vectors [q₁=(1/2,1/2,1/2) and q₂=(0,1/2,0), respectively] this solid solution allows for the study of the influence of competing interactions on a structural transition. The order parameter of this transition can be characterized by an Ising variable representing the two possible orientations of the ReO₄^- or ClO₄^- anion in cavities delimited by organic TMTSF molecules. Furthermore, the random substitution of the anions shows that these interactions are randomly distributed. The phase diagram of the AO transition has been determined as a function of x. At low temperatures (T) a q₁ long-range order (LRO) occurs for x<~0.5 and a q₂ LRO occurs for 1<~x<~0.97, while a short-range order (SRO) appears in the intermediate concentration range. For 0.7<x<0.94 one observes coexistence of q₁ and q₂ SRO. The basic features of the (T,x) phase diagram can be accounted for by a mean-field treatment of the Ising model with random interactions. This model, however, 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 properties of the solid solution.