Effect of K0.5Na0.5NbO3 on Properties at and off the Morphotropic Phase Boundary in Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3 Ceramics

Lead-free (1-y)(Bi1/2Na1/2TiO3–xBi1/2K1/2TiO3)–yK0.5Na0.5NbO3 [(1-y)(BNT–xBKT)–yKNN] (0.1≤x≤0.6 and 0≤y≤0.05) ceramics were prepared with compositions both at as well as off the morphotropic phase boundary (MPB) of x=0.2 and y=0. It was found that KNN addition destabilizes a ferroelectric order in BNT–BKT at zero electric-field, resulting in low remanent polarization, remanent strain and effective small signal d33eff [23 pC/N for (x,y)=(0.2,0.05), 60 pC/N for (0.2, 0.02)], while it does not affect the maximum polarisation and strain at high fields and thus enhances large signal piezoelectric coefficient, d33* [200 pm/V for (0.2,0.05), 217 pm/V for (0.2,0.02)]. This effect of KNN was found to be independent from the MPB and thus equally high d33* values were obtained for compositions at and far off the MPB. Thus, adding KNN to BNT–BKT removes the restriction to remain at exactly MPB position and provides more flexibility while offering equal or even improved strain.