Flux growth of high-homogeneity PMN-PT single crystals and their property characterization

The effects of B2O3 addition to the PbO flux for growing Pb(Mg1/3Nb2/3)O3-yPbTiO3 (PMN-yPT) single crystals, where y=0.28-0.34, have been investigated. The results show that the amount of B2O3 in the PbO flux is crucial in altering the chemistry of the high-temperature solution and determining the quality of the ensuing crystals. With optimum amounts of B2O3 in the PbO flux, inclusion-free high-uniformity PMN-PT single crystals have been successfully grown. The typical size of the crystals obtained is about 25-35 mm in edge length. Ferroelectric transition temperature, Tc, distribution measurements reveal that the bottom half of the grown crystals, i.e., the portion adjacent to the nucleation site, shows good compositional uniformity, with ΔTc≈±3.0°C. The PT content of this uniform composition portion is typically 2 mol% smaller than that in the initial charge, indicating that the composition of the actual flux is PbO+z(B2O3+δPT), where δ depends on the amount of B2O3 in the flux. Electrical poling studies reveal that while crystals of PMN-(31-32)%PT (Tc=140-145°C) composition are susceptible to overpoling, both PMN-28%PT (Tc=125°C) and PMN-30%PT (Tc=135°C) crystals show good resistance to overpoling. Of the latter two, PMN-30%PT exhibits better dielectric and piezoelectric properties with KT=7500-9000, k33=2200-2500 pC/N, d31=1100-1400 pC/N. The various electromechanical coupling factors are relatively insensitive to poling conditions and crystal composition, with k33=0.90-0.94, k31=0.48-0.55 and kt=0.58-0.62 for PMN-(28-32)%PT. The results show that flux-grown PMN-PT single crystals exhibit superior dielectric and piezoelectric properties in comparison to their melt-grown counterparts, possibly due to a lower concentration of defects in flux-grown crystals.