Dielectric properties of Co(CO3)(H2O)2(C3H4N2)2 and [Co(C3H3N2)2]n

We have investigated the dielectric permittivity (epsilon', epsilon '') of Co(CO3)(H2O)(2)(C3H4N2)(2) and two samples of [Co(C3H3N2)(2)](n) at low frequencies (0.1-1000 kHz) and in the temperature range 80-400 K. For the studied materials, a diffuse peak in the real part epsilon'(T) of the dielectric permittivity is observed at T-max approximate to 315 K. A detailed analysis of the data of [Co(C3H3N2)(2)](n) revealed that the temperature dependence of epsilon'(T) deviates from Curie-Weiss law below (T-max + 15 K), showing a (T - T-max)(-2) dependence. A clear dielectric dispersion occurs above T-f approximate to 287 K and it is characterized by a distribution of relaxation times according to the Kohlrausch-Williams-Watts formula. The temperature and frequency dependence of the loss tangent (tan delta)(max) can be described by the Vogel-Fulcher law, indicating thermal slowing down of non-Debye dynamics. We attribute the observed dielectric properties to a relaxor ferroelectric behaviour, which is caused by the development of a freezing state below T-f. We have analysed the electrical conductivity using the Jonscher formula sigma(omega) = sigma(dc) + A omega(s). The obtained results may be explained with the model, where the dc-conductivity is governed by thermally activated electrons and ac-conductivity due to the tunnelling of the overlapping large polarons. A large value of the dielectric permittivity epsilon' is found for[Co(C3H3N2)(2)](n) samples absorbing the moisture from the air and the phenomenon has been associated with cooperation between the H+ and HCO3- ions and the others in the relaxation process. Due to a moisture absorption property of [Co(C3H3N2)(2)](n), we suggest that this material is a potential candidate for a low cost room-temperature humidity sensor.