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

Abstract

We have investigated the dielectric permittivity (ε′, ε′′) 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 ε′(T) of the dielectric permittivity is observed at T_max ≈ 315 K. A detailed analysis of the data of [Co(C₃H₃N₂)₂]_n revealed that the temperature dependence of ε′(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 ≈ 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 δ)_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 σ(ω) = σ_dc + Aω^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 ε′ 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.