Dielectric and Piezoelectric Properties in the Lead-Free System Na0.5K0.5NbO3-BiScO3-LiTaO3

Phase relations, dielectric and piezoelectric properties are reported for the ternary system 98%[(1 - x) (Na0.5K0.5NbO3)-x(LiTaO3)]-2%[BiScO3] for compositions x <= 10 mol% LiTaO3. The phase content at room-temperature changed from mixed phase, monoclinic + tetragonal, for unmodified 98%(Na0.5K0.5NbO3)-2%(BiScO3), to tetragonal phase for compositions >2 mol% LiTaO3. Curie peaks at 360 to 370 degrees C were observed for all compositions, but peaks became diffuse at x >= 3 mol%, and two dielectric peaks, at 370 and 470 degrees C, were observed for 5 mol% LiTaO3. Phase segregation, and finite size affects associated with the core-shell structure, account for the occurrence of two dielectric peaks in 5 mol% LiTaO3, and diffuse dielectric behavior. The value of d(33) piezoelectric charge coefficient increased from similar to 160 pC/N for 0 mol% LiTaO3 to 205 to 214 pC/N for 1 to 2 mol% LiTaO3 solid solutions, before falling sharply at 3 mol% LiTaO3. TEM-EDX analysis revealed core-shell grain structures with segregation of Bi, Sc, and Ta in the outer similar to 100-nm shell of the 5 mol% LT sample.