The effect of KNN on dielectric, ferroelectric and piezoelectric
properties of BNKT ceramics was investigated. XRD
revealed that KNN addition had no remarkable effect on the crystal structure of BNKT ceramics in the studied composition
range. em decreased and Td shifted toward lower temperatures
with increasing KNN concentration. Deformed hysteresis loops
at small KNN concentrations were observed, suggesting the
coexistence of polar and NP phases for modified BNKT
ceramics. At 3 mol% KNN, a pinched-type hysteresis loop with
low remnant polarization (11 mC/cm2) and a small piezoelectric
constant (27 pC/N) was observed. Alternatively, the
unipolar field-induced strain was enhanced from 0.11% for pure
BNKT to 0.22% for 3 mol% of KNN under an applied electric
field of 50 kV/cm, corresponding to a normalized strain (Smax/
Emax) of 434 pm/V. This significant strain enhancement is a
result of the reversible phase transition between a NP phase in a
zero field and a field-induced ferroelectric phase.