Crystal structure and related properties of copper-doped barium titanate ceramics

The influence of copper on the crystallographic phase and the microstructure of ceramics with a nominal composition of BaTiO3 + 0.02 BaO + x CuO (0 less than or equal to x less than or equal to 0.02) was investigated. Systematic studies of X-ray diffraction and electron paramagnetic resonance (EPR) with varying doping level and sintering temperature were performed. The EPR data show that Cu2+ is incorporated into the BaTiO3 lattice at Ti-sites. Like in the case of other 3d transition elements as, e.g., Mn, Fe or Ni, also Cu decreases the phase transition temperature cubic-hexagonal and stabilizes the hexagonal phase at room temperature. At Cu concentrations greater than or equal to0.3 mol% and at a sintering temperature of 1400 degreesC hexagonal phase occurs and the microstructure exhibits exaggerated, plate-like grains with grain sizes greater than or equal to200 mum which is typical for hexagonal BaTiO3 ceramics. Compared to the doping with manganese, copper is more effective to stabilize the hexagonal phase, but its solubility into the BaTiO3 lattice is significantly lower. Depending on doping level and sintering temperature the EPR data exhibit in the whole temperature range between 40 and 450 K a superposition of two spectra of Cu-Ti(2+) which are attributed to tetragonally distorted CuO6 octahedra in a tetragonal and hexagonal crystal surrounding, respectively. Analogous to the case of Ti-Ti(3+) (d(1)), Mn-Ti(3+) (d(4)), Fe-Ti(2+) (d(6)) or Ni-Ti(3+) the Jahn-Teller distortion caused by the d9 electron configuration of Cu2+ is proposed as the driving force for the phase transition cubic Ti to hexagonal. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.