X-ray evidence of a structural phase transition in di-tetramethyltetraselenafulvalenium perchlorate [${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$Cl${\mathrm{O}}_4$], pristine and slightly doped

Diffuse x-ray scattering studies of the quasi-one-dimensional conductor di-tetramethyltetraselenafulvalenium perchlorate [${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$Cl${\mathrm{O}}_4$], both in its pristine form and when slightly doped with the sulfur analog tetramethyltetrathiafulvalene (TMTTF), reveal the formation of a low-temperature superstructure, which may involve an ordering of the Cl${\mathrm{O}}_4$ anions. Superstructure reflections are characterized by the unexpected wave vector $\overrightarrow{\mathrm{q}}=(0,\frac{1}{2},0)$, which might also be stabilized under pressure in the superconducting salts ${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$Re${\mathrm{O}}_4$ and ${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$FS${\mathrm{O}}_3$. The occurrence of a phase transition at 24 K in pristine ${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$Cl${\mathrm{O}}_4$ provides a clue for the understanding of the dependence on the cooling rate of the electronic ground state of ${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$Cl${\mathrm{O}}_4$.