Radicals as probes of ferroelectric phase transitions in EPR investigations of pyridinium tetrafluoroborate

This paper reports CW-EPR studies of pyridinium tetrafluoroborate undergoing solid-solid phase transitions at 240 and 205 K, of which the one occurring at the higher temperature was found to be paraelectric-ferroelectric. EPR spectra of gamma-irradiated polycrystalline samples were taken at room temperature, at the liquid nitrogen temperature, and at temperatures ranging from 163 to 273 K in the cycles of cooling and heating. The radical appearing under the effect of gamma irradiation was established to be formed by attachment of a hydrogen atom to the carbon atom C(2) in the pyridine ring. The unpaired electron was found to delocalize onto the pyridine ring. The g factor was established as 2.0021(4) and the values of the hyperfine splitting constants testify to the occurrence of an a-type interaction, A(alpha) = 0.98(5) mT, of the unpaired electron with protons at the C(3), C(5), and N(1) positions, two beta 1- and beta 2-type interactions with the hydrogen nuclei at the C(2) position, A(beta 1) = 6.68(5) mT and A(beta 2) = 4.85(5) mT, and the interaction with the nitrogen nucleus at N(1), A(N) = 0.86(5) mT. The complex character of the spectra obtained prompted us to employ computer analysis of the signals to enhance their resolution. Analysis of the temperature dependence of the linewidth parameter of individual components of the pyridine radical spectrum shows that EPR together with the computer resolution enhancement method can be a useful method for studying the nature of phase transitions in pyridinium tetrafluoroborate. (C) 1998 Academic Press.