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

TL;DR

This study investigates the origin of colossal dielectric constants in CaCu3Ti4O12 through broadband dielectric spectroscopy on single crystals and ceramics, revealing surface-related effects and relaxational behaviors up to 1.3 GHz.

Contribution

It provides a comprehensive broadband analysis of CaCu3Ti4O12, including single crystals, and proposes a surface-related mechanism involving metal-insulator diodes for the colossal dielectric response.

Findings

Colossal dielectric constants are influenced by surface treatments and ac-field amplitude.

The second relaxation is sensitive to surface modifications and voltage.

Intrinsic conductivity follows the Variable Range Hopping model.

Abstract

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