Evidence of Cu Deficiency: A Key Point for the Understanding of the Mystery of the Giant Dielectric Constant in CaCu₃Ti₄O₁₂

Abstract

The compositions of the grains and the grain boundaries in CaCu 3 Ti 4 O 12 (CCTO) have been characterized by a high-resolution field emission transmission electron microscope equipped with a high-angle annular-dark-field detector and energy-dispersive X-ray spectroscopy (EDS). The excess ions and their clusters at grain boundaries led to the observed strain contrast. These ions are further clarified as Cu ions by Z-contrast (Z: atomic number) image and EDS. The quantitative EDS analyses further indicate that the grain boundaries of CCTO are Cu rich and the grains are Cu deficient. The Ca-or Ti-rich amorphous phases were observed at corners or edges of the grains of CCTO.

Giant dielectric response in Dy2/3Cu3Ti4O12 ceramics

“…However, much work has inclined to the extrinsic explanation of internal barrier layer capacitor (IBLC) effects originating from semiconducting grain surrounded by insulating grain boundaries in CaCu 3 Ti 4 O 12 ceramics, and intragrain insulating barrier or electro polarization effects for single crystal samples [9][10][11][12]. The presence of oxygen vacancies, aliovalences of Ti and Cu ions and segregation of Cu might offer a plausible explanation for the IBLC mechanism in CaCu 3 Ti 4 O 12 ceramics [13][14][15][16][17].…”

Preparation of high dielectric constant thin films of CaCu3Ti4O12 by sol-gel

“…Zhang and Tang (2004) found evidence for the existence of Ti 3+ as well as Ti 4+ ions in CCTO and attributed the high grain conductivity to polaron transport along Ti 3+ -O-Ti 4+ linked path. The observed segregation of Cu ions to grain boundaries led Fang et al (2006) and Fang and Mei (2007) to suggest that Cu ions act as acceptors and increase the grain boundary resistivity while the electron hopping between Cu 3+ and Cu 2+ is thought to be the origin of the high conductivity of grains.…”

“…Several models have been proposed to explain the difference in the conductivity between the grains and the grain boundaries (Li et al 2004Zhang and Tang 2004;Fang et al 2006;Fang and Mei 2007). Zhang and Tang (2004) found evidence for the existence of Ti 3+ as well as Ti 4+ ions in CCTO and attributed the high grain conductivity to polaron transport along Ti 3+ -O-Ti 4+ linked path.…”

Effects of Cu stoichiometry on the microstructure, electrical conduction, and dielectric responses of Y2/3Cu3Ti4O12

“…For CCTO ceramics, the insulating grain boundary and semiconducting grain have been determined to be Cu-rich and Cu-deficient phase, respectively [9][10][11][12][13][14]. It is known that the lattice distortion derived from the substitution ions with larger radius may improve the formation of Cu-rich grain-boundary layer which could enhance the resistance of the grain boundaries [8,[15][16][17][18][19][20][21].…”