Grain Size Effect in Lead-Free Bi0.5Na0.5TiO3-based Materials: Exploring the Ferroelectric Behaviour

Abstract

High density ceramics of lead-free Bi0.5Na0.5TiO3 (BNT) and 94%Bi0.5Na0.5TiO3 - 6%BaTiO3 (BNBT-6), ranging in grain size from 80 nm to 10μm, were successfully sintered from nanometre and micrometre grain size powders by spark plasma sintering and conventional sintering techniques. High temperature X-ray diffraction (XRD) was used to determine the crystal structure of the crushed ceramics while domain imaging techniques including back-scattered scanning electron microscopy and piezoresponse force microscopy were used to examine the microstructure. The influence of grain size on the intrinsic and extrinsic properties was investigated by measuring the temperature and frequency dependence of the dielectric response as well as the ferroelectric and piezoelectric properties in the unpoled and electrically poled states. Grain size was shown to influence the room temperature defect structure in BNT, transforming the average crystal structure from rhombohedral R3c to monoclinic Cc with reducing grain size. The increase in dielectric permittivity with decreasing grain size observed in this material is caused by high domain wall density linked with a crosshatched domain pattern. This finding is consistent with the literature which identifies interactions between domain walls and antiphase boundaries or tetragonal platelets as the source of the Cc nanotwin defect structure. High grain boundary density was found to restrict the electric field induced Cc-to-R3c transition, maintaining the multi-domain defect structure. The depoling temperature Td associated with the R3c-to- Cc transition occurs at higher temperatures for larger micrograin size BNT ceramics but is independent of electric field strength. Grain boundaries are expected to have less impact on the structure of these BNT ceramics allowing the long-range R3c ferroelectric order to be retained to higher temperatures. Evidence of a critical grain size for ferroelectricity was not found within the investigated grain size range, however a decrease in dielectric permittivity with further reduction in grain size for ceramics with nanometer (≤ 100 nm) grains suggest a grain size limit may exist for the Cc defect structure in BNT. Alternatively, the Cc defect structure may still occur and instead the reduction in dielectric permittivity results from a dilution effect caused by the high density of grain boundaries.Grain size affects both the temperature and the permittivity value of the high temperature Tm peak, measured at 100 kHz, in BNT. Tm broadens and shifts to higher temperatures as the grain size is reduced while the increased stress exerted at the grain boundaries of the smaller grains hinders the domain wall motion suppressing the permittivity value. Although no static structural transition takes place at Tm in BNT, a gradual change in structure occurs as the P4bm phase increases at the expense of the R3c/Cc structure. A larger number of domain states are thought to be offered by the phase below Tm making it more stress accommodating. Room temperature crystal structure analysis reveal a pseudo-cubic distortion of R3c and P4bm symmetries in ceramic BNBT-6 which transform to a Pm m/P4bm mixed phase with decreasing grain size. While the Pm m(R3c)/P4bm structure provided the best fit, the XRD data is not wholly satisfied by this refinement. Mechanical impact has a similar effect as a weak poling field on the structure of the morphotropic phase boundary (MPB) composition and may account for the uncertainty surrounding the unpoled structure. A herring-bone domain pattern consistent with a P4mm structure was observed in the micrograin ceramics. As the grain size is reduced the dielectric permittivity decreases. Either fewer domain walls occur in the smaller grains or their movement is restricted. Electrical poling in strong electric fields promotes the R3c symmetry in the micrograin ceramic while the increased density of grain boundaries in the smaller grain ceramics opposes domain reversal limiting the decrease in dielectric permittivity induced by poling. The field-strength dependence of Td is consistent with the field-induced phase separation reported in the literature. Further evidence of a field induced P4bm-to-P4mm-to-R3c multiphase transition is provided by P-I-E loop tests. The position of the high temperature permittivity peak Tm is independent of grain size, particularly the range investigated in this study. The polar nanoregions forming the domain structure of the MPB composition are thought to be too small for their dynamics to be significantly affected by grain size. The increased stress exerted at the grain boundaries of the smaller grains, however is believed to hinder the domain wall motion suppressing the permittivity value at Tm. This result is consistent with the grain size effect observed in other MPB compositions, including Na0.5K0.5NbO3.