Morphotropic phase boundary, segregation effect and crystal growth in the NBT–KBT system

Lead-free piezoelectric materials of sodium–potassium bismuth titanate, (1−x)NaBi(TiO3)2–xKBi(TiO3)2, ceramics and single crystals were prepared and their crystallographic and dielectric properties were measured. Single crystals with dimensions of several centimeters were grown by using the melt growth technique. The morphotropic phase boundary between rhombohedral and tetragonal symmetry determined by X-ray diffraction and Raman spectra was found at x=0.2. No coexistence of rhombohedral/tetragonal phases was found in the NKBT system. Compositions analyses showed that severe phase segregation occurred during the crystallization of the solid solution. From the segregation point, a homogeneous NKBT crystal near the MPB composition is difficult to grow.