Second harmonic generation in the paraelectric phase in powders and ceramics of BaTiO3

A second harmonic signal has been clearly observed in powders and ceramics of BaTiO₃ in the high-temperature phase. The effect manifests itself at temperatures 500 K above the temperature of the phase transition from the tetragonal phase to the cubic phase, which is also significantly higher than the Burns temperature of BaTiO₃. The temperature dependence of the second harmonic generation signal I _2?? in the paraelectric phase becomes a straight line when plotted in the Arrhenius coordinates lnI _2??(1/T). The experimental results obtained indicate the presence of noncentrosymmetric local regions in the paraelectric phase over a wide range of temperatures. The contribution from these polar regions to the second harmonic generation increases as the phase transition temperature is approached. The temperature dependence of the second harmonic generation in barium titanate ceramics is significantly affected by the technique used for pressing and annealing of the samples.