Neutron-diffraction evidence for structural phase transitions in the organic conductors bis-tetramethyltetraselenafulvalenium salts [${\left(\mathrm{TMTSF}\right)}_{2}X$, $X={\mathrm{ClO}}_4^{ -}$ and ${\mathrm{BF}}_4^{-}$]

Single-crystal neutron-diffraction experiments using the time-of-flight Laue technique establish that organic conductors, bis-tetramethyltetraselenafulvalenium salts [${\left(\mathrm{TMTSF}\right)}_{2}X$, $X={\mathrm{ClO}}_4^{ -}$ and ${\mathrm{BF}}_4^{-}$], undergo crystallographic phase transitions at low temperatures. This is the first neutron-diffraction evidence for structural phase transitions in any TMTSF salts. The phase transition in the ${\mathrm{ClO}}_4^{-}$ salt leads to the creation of two crystallographically independent TMTSF columns with the possibility of different electrontransport properties for each column which can lead to independent low-temperature phase transitions.