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UBC Theses and Dissertations

Study of the temperature-dependence of the far-infrared spectrum of (TMA) (I) (TCNQ) Fortier, Normand


The temperature-dependence (20-300K) of the far-infrared spectrum (20-1000cm⁻¹) of Trimethylammonium Iodide Tetracyanoquinodimethane (TMA)(I)(TCNQ) in its ordered phase, has been measured for the first time. Our measurments confirmed the existence of two phase transitions near 150 and 100K. At least three absorption bands are activated below the 150K phase transition. We have assigned two of them to the totally symmetric mode Ag⁹ and Ag¹⁰ of the TCNQ molecule. We have also observed evidence of Brillouin zone splitting, where one band above 150K, split into three, below 150K. Since the conduction band of (TMA)(I)(TCNQ) is 1/3 filled, we have interpreted this result as suggesting that the 150K transition is a Peierls distortion. We also observe a sharp increased in infrared absorption at 365cm⁻¹ and a weaker one at 100cm⁻¹. Since the D.C. conductivity measurments indicates that a 1000cm⁻¹ band gap is present at all temperatures, we interpret these absorption "edges" in terms of transitions between localised states in the band gap. These localised states would be introduced by either chemical impurities or by some randomness in the potential at each TCNQ site.

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