Far-infrared powder absorption measurements of some tetramethyltetraselenafulvalene salts [(TMTSF)2X]

We have measured the far-infrared [(10–100)-${\mathrm{cm}}^{\mathrm{-}1}$] powder spectra of six compounds of the form (TMTSF${)}_2$X, where X=${\mathrm{PF}}_6$, ${\mathrm{AsF}}_6$, and ${\mathrm{SbF}}_6$ (octahedral anions) and ${\mathrm{ClO}}_4$, ${\mathrm{BF}}_4$, and ${\mathrm{ReO}}_4$ (tetrahedral anions), and TMTSF is tetramethyltetraselenafulvalene. The octahedral-anion spectra are all very similar, having four sharp resonances which have been attributed to three lattice modes and one internal mode. These assignments were made on the basis of isotope shifts upon deuteration and wave-number temperature dependence. The intensity of the lines is proportional to the dc conductivity and shows the spin-density-wave transition around 12 K. The spectra of the tetrahedral-anion compounds, on the other hand, show extra structure below the anion-ordering temperature $T_{\mathrm{AO}}$; this is attributed to the activation of zone-boundary phonons due to zone folding which accompanies the formation of the ordered anion superlattice. In particular, a feature appears in each compound around 30 ${\mathrm{cm}}^{\mathrm{-}1}$ which actually exists above $T_{\mathrm{AO}}$ but is very broad, and is interpreted as a zone-boundary acoustic phonon. This is believed to be responsible for the E∥a 30-${\mathrm{cm}}^{\mathrm{-}1}$ feature in the ${\mathrm{ClO}}_4$ compound, previously observed by Ng, Timusk, and Bechgaard to be sensitive to magnetic field and possibly related to the superconductivity.