Electrical and Dielectric Properties of ACu$_3$Ti$_4$O$_{12}$ Compounds

We studied frequency and temperature dependences of impedance and permittivity of several compounds in the system ACu$_3$Ti$_4$O$_{12}$ (A=Ca, Bi$_{2/3}$, Y$_{2/3}$, La$_{2/3}$) in the ranges of 10$^{-1}$$\sim$10$^{6}$ Hz and -150$\sim$200 $^\circ$C, respectively. We found all compounds investigated displayed similar electrical and dielectric properties, namely, they all have two electrical responses in the impedance formalism and a Debye-like peak in the permittivity formalism, besides, their dielectric constants are independent of frequency and temperature in a wide range. We explained the experimental results in terms of a two-layer model with conducting grains partitioned from each other by poorly conducting grain boundaries. We attributed the two electrical responses in the impedance formalism to the grain and grain boundary effects, respectively, while the Debye-like peak in the permittivity formalism to a Maxwell-Wagner relaxation.