Undoped Forms Research Articles

Polyindole and poly(5-cyanoindole) : electrochemical and FT-IR spectroscopic comparative studies

Indole (C 8NH 7) and 5-cyanoindole (C 9N 2H 6) can be electropolymerized in the presence of ClO 4 − anions. The electrochemical behaviour of both polymers is discussed. FT-IR studies of their doped and undoped forms agree with a 2,3 type structure of regular, statistical or random conformation

Oxygen partial pressure dependence of optical absorption in lithium niobate

Optical absorption of undoped and Fe-doped LiNbO 3 crystals grown under various oxygen partial pressures, P O 2 , has been investigated. The 2.9 eV band found in undoped crystal forms only during the growth process and is proportional to P 1 4 O 2 . This band consists of the superposition of three broad absorption bands A 1, A 2, and A 3, and is assigned to a hole localized on an oxygen ion. The 2.5 eV band found in a crystal grown in a reduced atmosphere is identified as an oxygen vacancy rather than an anti-site Nb as determined by the measurement of density and by performing optical bleaching experiments. The concentrations of the oxygen vacancies are very sensitive to the presence of Fe impurities in the as-grown crystals. Based on these results, it is proposed that lithium niobate is best formulated as [ Li (1 − x) Nb 2 5 ][ Nb] O (3 + δ) where x and δ represent the deviation from stoichiometry.

Effect of lithium doping on the conductivity of poly( p-phenylene vinylene)

The effect of lithium doping on the conductivity (σ) of poly( p-phenylene vinylene) (ppv) has been studied via a.c. impedance measurements. Ppv has been synthesized via a water-soluble polymer electrolyte precursor. Its conductivity in both undoped and doped forms has been measured as a function of temperature. While doping with LiCl has not resulted in reliable data, doping with lithium triflate (LiCF 3SO 3) has improved the conductivity of ppv by more than two orders of magnitude. Plots of log(σ) versus 1/ T show that the conductivities of undoped and doped ppv follow Arrhenius behavior with equal activation energies ( E a ≈ 0.37 eV). The increase in conductivity without a change in activation energy is attributed to the increase in the electronic density of ppv.