Transformation from insulating p-type to semiconducting n-type conduction in CaCu3Ti4O12-related Na(Cu5/2Ti1/2)Ti4O12 ceramics

Abstract

A double doping mechanism of Na+ + ½ Ti4+ → Ca2+ + ½ Cu2+ on the general formula Ca1−xNax(Cu3−x/2Tix/2)Ti4O12 has been used to prepare a series of isostructural CaCu3Ti4O12 (CCTO)-type perovskites. A complete solid solution exists for 0 ≤ x ≤ 1 and all compositions exhibit incipient ferroelectric behaviour with higher than expected intrinsic relative permittivity. Although CCTO ceramics typically exhibit n-type semiconductivity (room temperature, RT, resistivity of ∼10–100 Ω cm), Na(Cu5/2Ti1/2)Ti4O12 (NCTO) ceramics sintered at 950 °C consist of two insulating bulk phases (RT resistivity > 1 GΩ cm), one p-type and the other n-type. With increasing sintering temperature/period, the p-type phase transforms into the n-type phase. During the transformation, the resistivity and activation energy for electrical conduction (Ea ∼ 1.0 eV) of the p-type phase remain unchanged, whereas the n-type phase becomes increasingly conductive with Ea decreasing from ∼ 0.71 to 0.11 eV with increasing sintering temperature. These changes are attributed to small variations in stoichiometry that occur during high temperature ceramic processing with oxygen-loss playing a crucial role.