Large dielectric constant and Maxwell-Wagner relaxation in Bi _2/3Cu₃Ti₄O₁₂

We studied frequency and temperature dependences of impedance, electric modulus, and dielectric permittivity of Bi_2/3Cu₃Ti ₄O₁₂ in the ranges of 10^-1-10⁶ Hz and -150-200°C, respectively. We first observed two electrical responses in the impedance and modulus formalisms. Then we detected a Debye-like relaxation in the permittivity formalism. Most interestingly, we found that the large dielectric constant of Bi_2/3Cu₃Ti₄O ₁₂ is independent of the temperature and frequency below 150°C. The results are interpreted in terms of a two-layer model with conducting grains partitioned from each other by poorly conducting grain boundaries. Using this model, we attributed the two electrical responses in impedance and modulus formalisms to the grain and grain-boundary effects, respectively, while the detected Debye-like relaxation and large dielectric constant were well explained in terms of Maxwell-Wagner relaxation.