Anion-Ordering Phase Diagram of Di(tetramethyltetraselenafulvalenium) Perrhenate,(TMTSF)2ReO4

We report high-pressure, low-temperature x-ray scattering measurements on the organic conductor ${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$Re${\mathrm{O}}_4$. The anion-ordering phase diagram has been established up to 17 kbar. In addition to the low-pressure phase characterized by the wave vector $q=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$, we have discovered a new high-pressure phase with an unexpected wave vector $q=(0,\frac{1}{2},\frac{1}{2})$. Coexistence of these two types of ordering has been observed in an intermediate pressure range.