Electric-field-induced non-ergodic relaxor to ferroelectric transition in BiFeO3- x SrTiO3 ceramics

Abstract

While BiFeO₃-based solid solutions show great promise for applications in energy conversion and storage, realizing this promise necessitates understanding the structure-property relationship in particular pertaining to the relaxor-like characteristics often exhibited by solid solutions with polar-to-non-polar morphotropic phase boundaries. To this end, we investigated the role of the compositionally-driven relaxor state in (100 - x)BiFeO₃-xSrTiO₃ [BFO-xSTO], via in situ synchrotron X-ray diffraction under bipolar electric-field cycling. The electric-field induced changes to the crystal structure, phase fraction and domain textures were monitored via the {111}_pc, {200}_pc, and 1/2{311}_pc Bragg peaks. The dynamics of the intensities and positions of the (111) and (111̄) reflections reveal an initial non-ergodic regime followed by long-range ferroelectric ordering after extended poling cycles. The increased degree of random multi-site occupation in BFO-42STO compared to BFO-35STO is correlated with an increase of the critical electric field needed to induce the non-ergodic-to-ferroelectric transition, and a decrease in the degree of domain reorientation. Although both compositions show an irreversible transition to a long-range ferroelectric state, our results suggest that the weaker ferroelectric response in BFO-42STO is related to an increase in ergodicity. This, in turn, serves to guide the development of BFO-based systems into promising platform for further property engineering towards specific capacitor applications.

Fig. 1. Azimuthal angle-dependency of the XRD patterns for (a) BFO–35STO, and (b) BFO–42STO. The peaks marked with the star symbol (*) belong to a minor secondary phase, most likely Sillenite (Bi₂₅FeO₄₀). The panels (c and d) highlights the {111}_pc and the 1/2{311}_pc reflections for BFO–35STO and BFO–42STO, respectively.

Fig. 2. Bipolar E-field cycling along the polarization direction for BFO–35STO. (a) Contour map of the {111}_pc reflections at ψ = 0°. (b) Deconvoluted peak fitting evolution for selected in situ diffracted intensity distributions. The insert in (a) represents the E(t) evolution, where the marked yellow symbols correspond to the fitted intensity distributions in (b).

Fig. 3. Evolution of the fitted I_int and 2θ over E(t) cycling for BFO–35STO. The right panel is an enlarged view of the first E-field cycle (from 0 to 2.5 min), while the right panel covers the entire time scale (from 0 to 27 min). The green area marks the region where signatures of ferroelectricity are absent, i.e. without {111}_pc intensity interchange, while the orange and blue area denote regions where the onset of an electric-field-induced phase transformation and ferroelastic domain interchange take place, respectively.

Fig. 4. Bipolar E-field cycling along the polarization direction for BFO–42STO. (a) Contour map of the {111}_pc reflections at ψ = 0°. (b) Deconvoluted peak fitting evolution for selected in situ diffracted intensity distributions. The insert in (a) represents to the E(t) evolution, being the marked yellow symbols correspondent to the fitted intensity distributions in (b). In contrast to BFO–35STO, between 0 and 14 kV mm⁻¹ the {111}_pc peak is not visibly split before the first E_max.

Fig. 5. Evolution of the fitted I_int and 2θ over E(t) cycling for BFO–42STO. The right panel is an enlarged view of the first E-field cycling (from 0 to 2.5 min), while the right panel covers the entire time scale (from 0 to 27 min). The green area denotes a region where the ferroelectric behavior is absent, implying the I_int related to an unsplit (111)_pc peak. The blue area denotes a region where (111)_pc splits into two peaks, denoting the appearance of the rhombohedral distortion, and ferroelastic domain interchange by the periodic I_int oscillation.

Fig. 6. Macroscopic P–E (a and b), J–E (c and d), and S–E (e and f) hysteresis loops, f₁₁₁ texture indexes (g and h), and lattice strain for (111) (i and j), and (111̄) domains (k and l), for BFO–35STO and BFO–42STO, respectively. The cycles shown in (g–l) corresponds to the first two bipolar periods and is representative of the entire dataset.

Fig. 7. Contour map of the {200}_pc reflections at ψ = 0° under bipolar E-field cycling for (a) BFO–35STO, and (b) BFO–42STO. Lattice strain hysteresis loop for (c) BFO–35STO, and (d) BFO–42STO. In both compositions the {200}_pc peak remains unsplit during the entire field cycling.