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

Abstract

The influence of copper on the crystallographic phase and the microstructure of ceramics with a nominal composition of BaTiO 3+0.02 BaO+ x CuO (0⩽ x⩽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 Cu 2+ is incorporated into the BaTiO 3 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 ⩾0.3 mol% and at a sintering temperature of 1400 °C hexagonal phase occurs and the microstructure exhibits exaggerated, plate-like grains with grain sizes ⩾200 μm which is typical for hexagonal BaTiO 3 ceramics. Compared to the doping with manganese, copper is more effective to stabilize the hexagonal phase, but its solubility into the BaTiO 3 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 CuO 6 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+ (d 7), the Jahn–Teller distortion caused by the d 9 electron configuration of Cu Ti 2+ is proposed as the driving force for the phase transition cubic to hexagonal.