Abstract
A combined powder X-ray lattice parameter and ceramic impedance spectroscopy study is presented on materials within the CaO–CuO–TiO2 ternary phase diagram. Several compositions containing CaCu3Ti4O12 (CCTO) and small amounts of secondary phases such as TiO2, CaTiO3 and CuO are analysed and two different defect mechanisms are identified as the cause of the non-stoichiometry in CCTO. The first mechanism involves a variation in the Cu content, which explains the large differences in the intrinsic bulk and extrinsic grain boundary (GB) resistance, and the formation of the ceramic internal barrier layer capacitor (IBLC) structure. The second mechanism is associated with Ca–Cu anti-site disorder causing an unusually high intrinsic bulk permittivity above that predicted from Clausius–Mossotti calculations.
Highlights
The ternary oxide compound CaCu3Ti4O12 (CCTO) is a 1 : 3 A-site ordered perovskite (A9A993B4O12) where the oxygen octahedra are strongly tilted and the A99 site Cu cations adapt a fourfold square–planar coordination
The difference in electrical conductivity between the grain boundary (GB) and bulk areas in CCTO is large: the GB charge transport activation energy EA is about 0.5–0.8 eV, while the bulk EA is about 10– 100 meV,[15,20] the nominal resistance of the GB and bulk areas vary by a factor of up to # 105
The presence of excess CuO led to an increased rb and indications of a second defect mechanism were found, which may explain the large difference between the GB and bulk resistivity in terms of the difference in Cu content
Summary
The ternary oxide compound CaCu3Ti4O12 (CCTO) is a 1 : 3 A-site ordered perovskite (A9A993B4O12) where the oxygen octahedra are strongly tilted and the A99 site Cu cations adapt a fourfold square–planar coordination. The difference in electrical conductivity between the GB and bulk areas in CCTO is large: the GB charge transport activation energy EA is about 0.5–0.8 eV, while the bulk EA is about 10– 100 meV,[15,20] the nominal resistance of the GB and bulk areas vary by a factor of up to # 105. It was shown previously that this distinct ceramic IBLC structure becomes more pronounced during the ceramic sintering process and is associated with segregation of Cu towards the GB areas.[21] More recently it was shown that the interior conducting grain phase of CCTO grows out of an insulating matrix during sintering, most likely by segregation of Cu out of the conducting areas.[22] Here, in this work, such Cu segregation is explicitly associated with a defect mechanism, which offers a plausible explanation for the large differences in conductivity between the bulk and GB regions. A second defect mechanism is associated with the Ca–Cu antisite defects and the increased eb bulk permittivity
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
