Ljubomira Ana Schmitt

On the phase identity and its thermal evolution of lead free (Bi1/2Na1/2)TiO3-6 mol% BaTiO3

Temperature-dependent dielectric permittivity of 0.94(Bi1/2Na1/2)TiO3-0.06BaTiO3 (BNT-6BT) lead-free piezoceramics was studied to disentangle the existing unclear issues over the crystallographic aspects and phase stability of the system. Application of existing phenomenological relaxor models enabled the relaxor contribution to the entire dielectric permittivity spectra to be deconvoluted. The deconvoluted data in comparison with the temperature-dependent dielectric permittivity of a classical perovskite relaxor, La-modified lead zirconate titanate, clearly suggest that BNT-6BT belongs to the same relaxor category, which was also confirmed by a comparative study on the temperature- dependent polarization hysteresis loops of both materials. Based on these results, we propose that the low-temperature dielectric anomaly does not involve any phase transition such as ferroelectric- to-antiferroelectric. Supported by transmission electron microscopy and X-ray diffraction experiments at ambient temperature, we ...

Nanodomains in morphotropic lead zirconate titanate ceramics : On the origin of the strong piezoelectric effect

The outstanding piezoelectric properties of lead zirconate titanate (PZT) ceramics with compositions close to the morphotropic phase boundary of the quasibinary phase diagram of lead zirconate and lead titanate are still under debate. A combination of ex situ and in situ transmission electron microscopy and high resolution x-ray diffraction revealed that the extrinsic piezoelectric effect in morphotropic PZT is closely connected to the existence of nanodomains. The in situ transmission electron microscopy investigations with applied electric field show that mainly the nanodomains respond to the electric field while the microdomain structure does not change noticeably in our experiments.

Composition dependence of the domain configuration and size in Pb(Zr1−xTix)O3 ceramics

The composition dependent variation of domain configuration and size in Pb(Zr1−xTix)O3 (PZT) has been investigated in a detailed transmission electron microscopy study in the range of 0.40⩽x⩽0.55. Single phase composition, Pb(Zr0.45Ti0.55)O3 and Pb(Zr0.60Ti0.40)O3, the former belonging to the tetragonal, the latter to the rhombohedral phase, feature small microdomain widths coupled with a pronounced bimodal domain distribution. Samples with compositions around the morphotropic phase boundary exhibit a decrease of bimodal distribution and an increase in microdomain width associated with nanodomain formation. The investigation of micro- and nanodomains, as well as the bimodal distribution of microdomains in undoped PZT ceramics, with respect to composition, is reported. We define nanodomains as “domains arranged within microdomains possessing a width of a few nanometers.” The strict alternation of the two orientation variants of microdomains is denoted as “bimodal domain distribution,” and is characterized ...

SINGLE GRAINS HOSTING TWO SPACE GROUPS — A TRANSMISSION ELECTRON MICROSCOPY STUDY OF A LEAD-FREE FERROELECTRIC

In this letter, conclusive experimental evidence is presented for the coexistence of two phases within individual grains in the system (Bi0.5Na0.5TiO3)0.91-(BaTiO3)0.06-(K0.5Na0.5NbO3)0.03. Electron diffraction data unequivocally reveal the presence of a rhombohedral and a tetragonal phase with space group R3c and P4bm, respectively, both on the nanoscale level. Superlattice reflections, observed in the corresponding selected area electron diffraction, were used to identify the rhombohedral and tetragonal phase present at different regions within one grain. It is thought that the rhombohedral phase acts as a nucleation site for the phase transformation monitored under applied electrical field. The coexistence of these two phases in conjunction with the triggered tetragonal to rhombohedral phase transformation rationalizes the high bipolar and unipolar strain recorded for this lead-free ferroelectric material.

Comparative study of two lead-free piezoceramics using diffraction techniques

A comparative study of two distinct lead-free piezoceramics, (Bi0.5Na0.5TiO3)0.92–(BaTiO3)0.06–(K0.5Na0.5NbO3)0.02 and (Bi0.5Na0.5TiO3)0.94–(BaTiO3)0.05–(K0.5Na0.5NbO3)0.01, termed 92-06-02 and 94-05-01, respectively, is presented. The samples were investigated by complementary diffraction techniques, namely X-ray, neutron and electron diffraction. Transmission electron microscopy (TEM) and powder diffraction experiments clearly revealed the presence of both rhombohedral and tetragonal phases in space groups R3c and P4bm, respectively. Superlattice reflections observed in the diffraction patterns were used to identify the two phases. It was found that sample 92-06-02, with a high proportion of the nonpolar tetragonal phase, shows a grainy contrast, whereas specimen 94-05-01 features domain-like contrast, related to a higher rhombohedral phase fraction. The combination of local scale analyses via TEM with X-ray and neutron diffraction provides the experimental basis for further structural investigations.

De-aging of Fe-doped lead-zirconate-titanate ceramics by electric field cycling: 180°- vs. non-180° domain wall processes

Acceptor-doped ferroelectrics tend to show pronounced aging behavior. The microscopic effects of aging are commonly related to oxygen vacancies, however, there are still open questions with respect to their impact on domain wall movements. To elucidate the latter, the reverse process of de-aging by electric field cycling is investigated here on Pb(Zr0.54Ti0.46)O3 doped with iron in different concentrations. Measurements of the hysteretic behavior of large-signal parameters, i.e., polarization and strain, as well as small-signal parameters, i.e., electrical permittivity and piezoelectric coefficient, are used to distinguish between reversible and irreversible movement of 180°- and non-180° domain walls. The results indicate that for low doping concentrations, the de-aging behavior of 180° domain wall motion is governed by irreversible domain wall motion and a coarsening of the domain structure, while for non-180° domain walls the change in reversible domain wall mobility is the dominant de-aging mechanism....

In situ electric field induced domain evolution in Ba(Zr0.2Ti0.8)O3-0.3(Ba0.7Ca0.3)TiO3 ferroelectrics

In this work, the lead-free Ba(Zr0.2Ti0.8)O3-0.3(Ba0.7Ca0.3)TiO3 piezoelectric ceramic was investigated in situ under an applied electric field by transmission electron microscopy. Significant changes in domain morphology of the studied material have been observed under an applied electric field. During the poling process, the domain configurations disappeared, forming a single-domain state. This multi- to single-domain state transition occurred with the formation of an intermediate nanodomain state. After removing the electric field, domain configurations reappeared. Selected area electron diffraction during electrical poling gave no indication of any structural changes as for example reflection splitting. Rather, a contribution of the extrinsic effect to the piezoelectric response of the Ba(Zr0.2Ti0.8)O3-0.3(Ba0.7Ca0.3)TiO3 was found to be dominant.

Experimental measurement of stress at a four-domain junction in lead zirconate titanate

A junction between two lamellar bands of ferroelectric domains in a lead zirconate titanate ceramic is analyzed using Kikuchi diffraction patterns in the transmission electron microscope. Indexing of the diffraction patterns allowed the determination of the three-dimensional relative orientation of the four different domains at the junction and thus the characterization of the domain boundaries. The local c/a ratio could also be determined from the misorientations at the domain boundaries. Analysis of the data showed that large stresses were concentrated at the junction, and that this is inevitable at such band junctions. Such stress concentrations could act as nuclei for cracking of the ceramic under additional loading in service, perhaps particularly as a consequence of extended electromechanical cycling. Moreover, the stresses would increase with increasing c/a, making the issues all the more serious for Ti-rich compositions having larger c/a ratios.

The phase diagram of K0.5Na0.5NbO3–Bi1/2Na1/2TiO3

The phase diagram of the lead-free piezoelectric (1 − x)K0.5Na0.5NbO3–xBi1/2Na1/2TiO3 system has been studied by high-resolution synchrotron powder diffraction, neutron powder diffraction and selected area electron diffraction. The two lead-free piezoelectric materials K0.5Na0.5NbO3 and Bi1/2Na1/2TiO3 form an infinite substitution solid solution. The orthorhombic (O), monoclinic (M), tetragonal (T) and rhombohedral (R) phases and the phase coexistence of M (Pm) + T (P4mm) for 0.02 < x ≤ 0.14, T (P4bm) + pseudocubic (Pm{\overline 3}m) for 0.14 < x ≤ 0.87 and T (P4bm) + R (R3c) for 0.87 < x ≤ 0.96 have been investigated at room temperature, with a subtle change in the structure observed. The oxygen octahedral tilt system has been mapped as a function of composition and temperature. The results indicate that K0.5Na0.5NbO3–Bi1/2Na1/2TiO3 does not display a morphotropic phase boundary like lead zirconate titanate, and that the most significant structural changes as a function of composition occur near x = 0.14 and x = 0.87 as a result of ionic disorder at the A and B sites in the perovskite ABO3 structure at room temperature.

A-site occupancy in the lead-free (Bi1/2Na1/2TiO3)0.94-(BaTiO3)0.06 piezoceramic: Combining first-principles study and TEM

The crystal structure of the lead-free piezoelectric ceramic (Bi1/2Na1/2TiO3)0.94–(BaTiO3)0.06 was investigated by first-principles calculations and high-resolution transmission electron microscopy (HRTEM) imaging. Structures with different A-site occupation were relaxed by total energy calculations within density functional theory and then used for simulating the corresponding HRTEM images. Simulated and experimental HRTEM images were compared and the closest match selected for structure interpretation. By combining these techniques, we have identified the Bi(Ba)/Na distribution on the A-site to be homogeneous. We exclude the possibility that regions visible in HRTEM images within one grain can be attributed to different ordering but to a slight tilting of the structure with respect to the electron beam.