Field-induced phase transition in Bi_1/2Na_1/2TiO₃-based lead-free piezoelectric ceramics

Abstract

The origin of the electric field-induced strain in the polycrystalline ceramic 0.92Bi 1/2 Na 1/2 TiO 3 -0.06BaTiO 3 -0.02K 1/2 Na 1/2 NbO 3 was investigated using in situ high-resolution X-ray and neutron diffraction techniques. The initially existing tetragonal phase with pseudocubic lattice undergoes a reversible phase transition to a significantly distorted rhombohedral phase under electric field, accompanied by a change in the oxygen octahedral tilting from a 0 a 0 c + to a À a À a À and in the tilting angle. The polarization values for the tetragonal and rhombohedral phases were calculated based on the structural information from Rietveld refinements. The large recoverable electric field-induced strain is a consequence of a reversible electric field-induced phase transition from an almost nonpolar tetragonal phase to a ferroelectrically active rhombohedral phase.

Strain Mechanisms in Lead-Free Ferroelectrics for Actuators

“…The origin of the large electromechanical response of incipient piezoelectrics is generally attributed to an electric field induced phase transition. [140][141][142][143][144][145][146][147] Figure 2.14 at point displays an ergodic relaxor state, as previously discussed. For BNT-based lead-free incipient piezoelectrics, it was demonstrated that the electric field induced phase transition coincides with the inflection point of the strain output 148 and the transformation is followed by switching of the newly created domains.…”

“…In contrast, other BNT-based materials feature passive phase transformations as a consequence of compressive stresses. 141,142 Since stresses seem to play a secondary role, the non-zero intensity of the ½(311) pc SSR indicates that the electric field is a conjugate thermodynamic variable of the octahedral tilt angle ∅. This entails that the correlation length of ( − − − ) tilting increases considerably upon application of electric field.…”

“…This strain mechanism is qualitatively similar as proposed for other BNT-based materials. 140,141 However, Krauss et al 258 proposed that the electric field induced phase transition is from an antiferroelectric state to a ferroelectric state.…”

“…It should be noted that small FD distortions are a common feature of several BNT-based lead-free materials. 141 In general, this is due to competing FD and AFD instabilities. 367 In order to corroborate the existence of AFD instabilities leading to octahedral tilting, neutron diffraction was performed because this technique is sensitive to the position of all O 2-isotopes due to their relatively large neutron scattering length.…”

“…Therefore, the interdependent relationship between grain orientation and constraints, switching events, and intergranular stresses should determine the relative content of electric field induced phases. 144,452,453 BNT-based ceramics have been shown to feature either an electric field induced transformation into a single phase 142 or a phase mixture 141,143,145,146,434,454 depending on chemical composition. 430 Moreover, the electric field required to induce a phase transition and its completeness depends on crystallographic direction.…”