Local Atomic Structure of Perovskite and Layered Perovskite Ferroelectrics


Student thesis: Doctoral Thesis

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Award date7 Jun 2022


Ferroelectric materials are widely used in many electrical and electromechanical devices, such as capacitors, actuators, sensors, ultrasonic transducers, electrocaloric coolers, etc. Owing to their excellent functional properties, Pb-based ferroelectrics, such as PbZr1−xTixO3 (PZT), are widely used at present. However, due to concerns regarding Pb toxicity, international legislations aim to restrict the commercial applications of Pb-based materials in electrical/electromechanical devices in the near future. Therefore, efforts are mounted to develop alternate Pb-free ferroelectric ceramics. In this work, (K, Na)NbO3-based perovskite and Dion-Jacobson layered perovskites are explored as candidates for Pb-free ferroelectric ceramics.
In K0.5Na0.5NbO3, the local atomistic mechanism for phase transitions and ferroelectric polarization were elucidated based on a detailed analysis of temperature-dependent neutron total scattering. Analysis of the neutron diffraction patterns indicated that there are monoclinic-tetragonal-cubic structural transitions in K0.5Na0.5NbO3 with increasing temperature from 100 to 773 K. However, analysis of the neutron pair distribution function indicates that the local structure ∼10 Å stays monoclinic over the same temperature range. Importantly, we observed an order-disorder transition at T∼ 473 K, stemming from a competition between the polarizing force of the local monoclinic polar units and increased thermal vibrations at higher temperatures. Finally, based on an analysis of the local atomic structure, we show that large ferroelectric polarization in K0.5Na0.5NbO3 can be attributed to displacements of both A-site K/Na and B-site Nb ions relative to the oxygen cages.

In 0.06LiNbO3-0.94K0.5Na0.5NbO3, we have systematically studied the temperature-dependent evolution of local and average structures from 290 to 773 K using neutron total scattering. Analysis of the Bragg peaks indicates the coexistence of monoclinic and tetragonal phases at room temperature, but it transforms to a single tetragonal phase at "T ≥ 473 K" and a cubic phase at "T ≥ 673 K" . In contrast, the short-range structure ~ 10 Å is best described as a monoclinic phase within the same temperature range. Furthermore, the local octahedral distortion remains comparatively stable as a function of temperature, in contrast to larger temperature-dependent changes in the octahedral distortion of the average structure. These results indicate that 0.06LiNbO3-0.94K0.5Na0.5NbO3 ceramics exhibit length-scale dependent structural complexities.

In xLiNbO3-(1-x)Na0.5K0.5NbO3, analysis of the neutron Bragg diffraction data indicated an average structural transition from monoclinic to tetragonal phases at the morphotropic phase boundary for the composition of x ~ 0.06, which coincides with that determined by the best electrical properties. However, analysis of the neutron pair distribution function indicates that the local structure ~ 10 Å stays monoclinic for the entire compositional range of 0 ≤ x ≤ 0.08. Pair distribution function and Raman spectrum analysis indicated that the monoclinic-to-tetragonal phase transition in x Li-(1-x)Na0.5K0.5NbO3 is an order-disorder type whereby inhomogeneous AO12 polyhedral distortions compete with the polarization field of the monoclinic structural units. From density function theory (DFT) calculations, we furthermore conclude that changes in the local A-site bonding environment are most strongly affected when Li is substituted for K rather than Na, which is due to the greater mismatch between ionic radii of Li and K as compared to those between Li and Na.

In CsBiNb2O7, the neutron total scattering experiments and first-principles theory were employed to examine the local structure in the well-known example of Dion-Jacobson ferroelectric. It is shown that the instantaneous short-range atomic structure 7 Å in CsBiNb2O7 can be described by a modified Debye-Einstein model that accommodates atomic motions due to both acoustic and low-energy optic phonon modes. Our result demonstrates a necessary means to account for the effect of localized atomic dynamics on the short-range atomic structure of layered perovskite ferroelectrics.

In KCa2Nb3O10, the structure of cation-oxygen divacancies was obtained in this Dion-Jacobson layered perovskite by combining the x-ray pair distribution function with first-principles computation. Our results indicate that calcium-oxygen divacancy pairs are most likely to form in between the octahedral layers, which give rise to local structural distortions leading to large dipole moment density oriented within the 2-dimensional perovskite blocks. Most importantly, the current results demonstrate a strong polar nature of defect-induced local structural distortions in this Dion-Jacobson phase, which should motivate a broader investigation of defect dipoles in layered perovskites and their implication on polar interphases and other related macroscopic properties.