INFRARED OPTICAL-PROPERTIES OF (TMTSF)2REO4 AND (TMTSF)2BF4 (WHERE TMTSF IS TETRAMETHYLTETRASELENAFULVALENE) COMPARED WITH SEVERAL MODEL-CALCULATIONS

We report high-resolution reflectivity measurements of the protonated and deuterated forms of (TMTSF)2ReO4 and (TMTSF)2BF4, where TMTSF is tetramethyltetraselenafulvalene, at 300 and 25 K, and for polarizations Ea and E b, from 30 to 8000 cm-1. Comparison of the spectra allowed assignments to be made for the many detailed vibrational features in the 25-K E a spectra, which are dominated by the electron-coupled totally symmetric (ag) modes. The data were fitted to several model calculations, including the dimer model, the tetramer model, the phase-phonon model, and a twofold-commensurate charge-density-wave model. The latter gave excellent agreement for the unsplit vibrations in both compounds, with the only variable being the semiconducting energy gaps of 1120 and 1700 cm-1 for the BF4 and ReO4 compounds, respectively. The one-dimensional density of states clearly favors the energy-band description over the isolated charge-transfer mechanism. The nature of the vibrational splittings due to the anion ordering is discussed. © 1990 The American Physical Society.