Growth and characterisation of piezoelectric PZN-PT 91/9 and PMN-PT 66/34 single crystals for ultrasonic transducers

Today, high crystal quality in sufficient size with high crystal growth yield appears to be a key point for a wide dissemination and use in ferroelectrics applications. So, this paper is related to the growth of PZN-PT solid solution compound like (1-x)(PbZn₁3/Nb₂3/O₃)-xPbTiO₃ and PMN-PT compound like (1-y)(PbMg₁3/Nb₂3/O₃)-yPbTiO₃. For these two relaxor ferroelectric single crystals promising properties are obtained near the Morphotropic Phase Boundary (MPB) transition which occurred at values closed to x=0.09 for PZN-PT and y=0.34 for PMN-PT. The complex unit cell phase transition combined with an adequate applied electric field, oriented in the pseudo <001>_c direction; confer to these single crystals some interesting piezoelectric properties. We present some crystal growth results obtained by the Vertical Gradient Freeze-Solution Growth method (VGF-SG) for PZN-PT and by a pseudo Kyropoulos method developed for PMN-PT. The obtained single crystals, up to 120 g for PZN-PT and 160 g for PMN-PT are then analysed in term of composition and structure. Both PMN-PT and PZN-PT perovskite crystals are rhombohedral (@RT), they show a good global homogeneity. Samples demonstrate ferroelectric behaviour, and after polarisation process, dielectric constants up to 48000 near the Curie temperature (Tc ∼174°C @ 1kHz) are obtained for PZN-PT and respectively 74000 for PMN-PT (Tc ∼ 142°C @ 100 Hz). The piezoelectric coefficient d₃₃ gives values up to ∼2200 pC/N (@RT 9 kV/cm) for PZN-PT and 1700 pC/N(@RT) for PMN-PT. A value up to 94% is measured for the electromechanical coupling coefficient k₃₃.