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|Inhomogeneous conductivity behavior of poly- and single-crystalline CaCu3Ti4O12 samples: a strong evidence for the existence of polaronic stacking fault defects|
|Keywords: Dielectrics, Scanning Probe Microscopy , Impedance Spectroscopy|
|High-k dielectrics with colossal dielectric constant (CDC) (ε’ > 103) are a matter of great interest since their using as passive components could provide an improvement in the efficiency of electronic devices and ensure the miniaturization of capacitive electronic elements . Calcium copper titanate (CaCu3Ti4O12), a non-ferroelectric material, whose exceptional dielectric properties have been reported in 2000 , attracts the attention of scientific community because of its CDC (ε’ >105), observed for ceramics and single crystals. Moreover, the absence of ferroelectric transition makes CCTO properties quite stable, with a wide range of temperature-independent dielectric behavior. Despite the efforts of the scientific community, a conclusive explanation about the polarization mechanisms that justify the exhibition of CDC is still missing. Discarding the existence of any ferroelectric transition, the experimental data, collected during the last decade, point to an extrinsic barrier mechanism(s) as the origin of the main dielectric polarization, where the following approaches are dominated: 1) multiple micro-capacitors (accumulation of the electric charge at the grain-to-grain boundary interface) ; 2) electrode process polarization state (the depletion layers of Schottky contact) [4, 5]; 3) plane defect model (the twinning parallel to the (100), (010), and (001) planes) [6, 7]. In Ref. , the third approach was significantly developed and established as a polaronic stacking fault defect model (the stacking faults at the nanoscale level work as a large association of internal barrier layer capacitors), thus resulting in the huge dielectric constant observed in CCTO based materials. This phenomenon was named as nanoscale barrier layer capacitance (NBLC) mechanism and a huge enhancement of polaronic defects was predicted. Meanwhile, as far as we know, any direct evidence of the existence of these polaronic defects have been shown, both for poly- and single-crystalline CCTO samples until now. In this work we show the efficiency of Scanning Probe Microscopy method, implemented in electrophysical modes, and impedance spectroscopy analysis to disclose the origins of the unusually high dielectric constant characteristic of CaCu3Ti4O12 (CCTO) for poly and single-crystalline samples. In the case of polycrystalline samples two main mechanisms responsible for the CDC have been identified. There is a microscale barrier layer capacitance (MBLC) mechanism, attributed to the potential grain-to-grain barriers, and a nanoscale barrier layer capacitance (NBLC) mechanism, attributed to the potential barriers created by the structural defects such as twinning or slip planes. The NBLC mechanism was found to be consistent of terrace- ledge and bump domains structure which simultaneously contribute to the CCTO conductivity within the dimension of one grain. The same NBLC mechanism was identified as the origin of CDC in the case of the single crystal samples. Both in polycrystalline and single crystal structures, the alternation of nano-barriers and gaps observed in the topography image scan correspond to the conductive and insulating stripes in current distribution image scans. These results are in line with the macroscopic behavior of polycrystalline and single crystalline CCTO samples, considering that the observed CDC behavior in both CCTO materials require the coexistence of conductive barriers and insulating gaps caused by the twin boundaries, planar defects and slip planes within the crystallographic structure of the sample. This approach has been initially proposed in Ref. 6 and significantly developed in Refs. 7 and 8 to yield the structural model for planar defects. Following this model, the ideal arrangement of a perfect crystal of CCTO, where the CuO4 square planes containing Cu2+ ions share corners with the TiO6 octahedra containing Ti4+ ions, could be disrupted as a result of twinning in a CCTO crystal [6-8]. This disturbance may create the plane defects and make the Cu and Ti environments conductive by accommodating Cu+ and Ti3+ ions. Simultaneously, it creates the insulating barrier because of the associated charge balance which requires the existence of oxygen vacancies in the defect plane . These sources have been associated with the different stacking faults predicted for CCTO. The present work highly contributes for a general understanding of the anomalous colossal dielectric constant behavior in CCTO poly and single crystalline structures at the macro- and nanoscale levels.|
Polytechnic Institute of Coimbra/University of Aveiro/I3N