Relaxor ground state forced by ferroelastic instability in K 0.5 Bi 0.5 Ti O 3 − Na 0.5 Bi 0.5 Ti O 3

The complex interplay of local disorder with the structure and dynamics and their role in enhancing the electromechanical response makes relaxor ferroelectric materials fascinating both from the scientific and technological standpoints. It is generally believed that in chemically disordered ferroelectric solid solutions, the normal ferroelectric state gradually yields to a relaxor ground state on increasing the concentration of localized (point-defect-like) random-field centers. That a different kind of process can spontaneously stabilize a relaxor ground state is less known, especially in the family of ferroelectric perovskites. We demonstrate the occurrence of this less-known phenomenon in $(1\\ensuremath{-}x){\\mathrm{K}}_{0.5}{\\mathrm{Bi}}_{0.5}\\mathrm{Ti}{\\mathrm{O}}_{3}\\text{\\ensuremath{-}}(x){\\mathrm{Na}}_{0.5}{\\mathrm{Bi}}_{0.5}\\mathrm{Ti}{\\mathrm{O}}_{3}[(1\\ensuremath{-}x)\\mathrm{KBT}\\ensuremath{-}(x)\\mathrm{NBT}]$. Unlike the gradual evolution common to most ferroelectric solid solutions, KBT-xNBT exhibits an abrupt crossover from a normal ferroelectric ground state to a full-blown relaxor ground state for $xg0.58$. We show that this abrupt crossover to the relaxor state is caused by stabilization of an incommensurate-like/highly disordered $M$-point ferroelastic distortion in the ergodic/paraelectric temperature regime. The abrupt crossover manifests as composition driven anomalous changes in several important properties such as dielectric response, electromechanical properties, tetragonality, and coercive field, electrostrain, and mimics a scenario often encountered in composition driven interferroelectric transformations.