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UBC Theses and Dissertations

The far-infrared powder absorption spectra of bis-tetramethyltetraselenafulvalene salts [(TMTSF)₂X, X=(PF₆, AsF₆, SbF₆, BF₄, ClO₄ and ReO₄)] Homes, Christopher C.


The far-infrared powder absorption spectra from 20-250 cm⁻¹ have been examined in a series of six protonated and deuterated (TMTSF)₂X compounds, with X=PF₆, AsF₆ and SbF₆ (octahedral anions) and X=BF₄, ClO₄ and ReO₄ (tetrahedral anions) as a function of temperature from 10K to 290K . The octahedral-anion spectra are all very similar, having 4 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 roughly proportional to the d.c. conductivity, and indicate a phase transition around 12K. The spectra of the tetrahedral-anion compounds are also similar. They show, however, extra structure below the anion-ordering temperature, TAOI including a strong feature at 30 cm⁻¹. The 30 cm⁻¹ feature shows a magnetic field depedence and may be related to the superconductivity. Simple one- and two-dimensional models are presented that indicate that this feature may be due to the activation of a transverse acoustic zone-boundary phonon, due to zone-folding, which accompanies the formation of a superlattice created by the anion-ordering transition. Extended measurements from 100-300 cm⁻¹ have been performed on protonated AsF₆ and SbF₆ compounds, allowing the identification of two internal modes. Extended measurements from 100-400 cm⁻¹ have been performed on protonated and deuterated BF₄ and ReO₄ compounds, which show internal modes of both the TMTSF and anion molecules, and a torsional mode of the TMTSF methyl groups. These assignments were made on the basis of isotope shifts upon deuteration an extensive published vibrational analysis of TMTSF.

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