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) (Na_0.5K_0.5NbO₃)-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%(Na_0.5K_0.5NbO₃)-2%(BiScO₃), to tetragonal phase for compositions >;2 mol% LiTaO₃. Curie peaks at 360 to 370°C were observed for all compositions, but peaks became diffuse at x-3 mol%, and two dielectric peaks, at 370 and 470°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 d₃₃ piezoelectric charge coefficient increased from ~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 ~100-nm shell of the 5 mol% LT sample.