Colossal dielectric constants in single-crystalline and ceramic CaCu3Ti4O12 investigated by broadband dielectric spectroscopy

摘要

In the present work the authors report results of broadband dielectricspectroscopy on various samples of CaCu3Ti4O12, including so far only rarelyinvestigated single crystalline material. The measurements extend up to 1.3GHz, covering more than nine frequency decades. We address the question of theorigin of the colossal dielectric constants and of the relaxational behavior inthis material, including the second relaxation reported in several recentworks. For this purpose, the dependence of the temperature- andfrequency-dependent dielectric properties on different tempering and surfacetreatments of the samples and on ac-field amplitude are investigated. Broadbandspectra of a single crystal are analyzed by an equivalent circuit description,assuming two highly resistive layers in series to the bulk. Good fits could beachieved, including the second relaxation, which also shows up in singlecrystals. The temperature- and frequency-dependent intrinsic conductivity ofCCTO is consistent with the Variable Range Hopping model. The second relaxationis sensitive to surface treatment and, in contrast to the main relaxation, alsois strongly affected by the applied ac voltage. Concerning the origin of thetwo insulating layers, we discuss a completely surface-related mechanismassuming the formation of a metal-insulator diode and a combination of surfaceand internal barriers.