A rhombohedral structure-properties relation in Pb-free Bi 0.5 (Na 1− x K x ) 0.5 TiO 3 ceramics

A structure-properties relation in Bi 0.5 (Na 1− x K x ) 0.5 TiO 3 ceramics was investigated for K contents ranging from 0 to 20 mol% substituting for Na in (Bi 0.5 Na 0.5 )TiO 3 ceramics. A rhombohedral perovskite solid solution was confirmed up to x = 18 mol%. An inverse relation between depolarization temperature ( T d ) and piezoelectric coefficient ( d 33 ) was observed in the rhombohedral phase region. As x increased toward the morphotropic phase boundary (MPB), T d was lowered while d 33 was enhanced. Structurally, the rhombohedral lattice distortion (90− α ) decreased while the lattice constant ( a R ) increased with increasing x . 90− α appeared to be coupled with the compositiondependent variations in T d and d 33 ; the smaller 90 − α , the lower T d , and the higher d 33 .