복잡계 산화물에서 공간 반전 대칭성 제어를 통한 기능성 향상에 대한 연구

Abstract

Spatial inversion symmetry is defined as a single point in a crystal symmetry where, upon placing a mirror, its reflection will look the same as the original. The control of the inversion symmetry breaking is of great importance for enhancing the functional properties in oxide materials due to the fundamental scientific development and promising device applications. In the case of complex oxides, manipulating the inversion symmetry can lead to enhancement of the ionic displacement (spontaneous electric-polarization) upon the structural phase transition, resulting in an enhancement in the functional properties, like piezoelectricity, ferroelectricity, flexoelectricity, and multi-ferroelectricity. In this thesis, using various complex oxide materials using different experimental approached such as anion doping, stress at the grain boundary of two different materials, interfacial strain to enhance functional properties in the complex oxides with controlled spatial inversion symmetry. We controlled the spatial inversion symmetry in the perovskite Pb(Zr, Ti)O3 via quantitively sulfur substitution at the apical oxygen site. By substituting the sulfur at the apical oxygen site, a structural phase transition from the monoclinic Pb(Zr, Ti)O3 to tetragonal Pb(Zr, Ti)O3-xSx is noticed in the reciprocal space mapping (RSM), resulting in an enhancement in ferroelectricity. The origin of the enhanced ferroelectric polarization is explained via Raman spectroscopy. The sulfur (S) makes a strong covalent bond with the titanium (Ti) element (Ti 3d-S 3p) compared with the Ti 3d-O 2p bonding due to the lower electronegativity of the sulfur element than the oxygen atom. The strong covalency of the Ti-S (apical) bonding lifted the spatial inversion symmetry uniformly in PZT oxysulfide.

In this study, by adding the ferroelectric Bi1/2(Na0.78,K0.22)1/2TiO3 (BNKT) particles (volume percent fraction) to the relaxor 0.72Bi1/2Na1/2TiO3-0.28SrTiO3 (BNT-28ST), the spatial inversion symmetry in perovskite BNT-28ST is uniformly lifted resulting in a high piezoelectric response at a low applied field in the BNT-28ST/BNKT ceramic composite. At a given applied electric field in BNT-28ST/BNKT composite, the BNKT ferroelectric particles compressively stressed the BNT-28ST relaxor ceramic and lifted the spatial inversion symmetry. Consequently, a structural transition from the pseudocubic BNT-28ST to tetragonal is identified with the spatial inversion symmetry controlled, leading to the high piezoelectric response under the low applied electric field in the BNT-based ceramic composite. In the Al-doped SrMnO3, we aimed to produce the ferroelectric polarization in 6H-hexagonal SrMnO3 ceramics with controlled spatial inversion symmetry. The origin of the ferroelectric/polar-ordering in the hexagonal oxides (like YMnO3) is strongly connected to the buckling/tilting of a trigonal MnO5 bipyramid rather than the off-centering displacement in perovskites. Herein, we constructed the hexagonal SrAl2O4 grains locally in the proximity to the 4H-hexagonal SrMnO3 grains by doping the Aluminum (Al) to the SrMnO3 ceramic. Using the interfacial strain mechanism, a 6H-hexagonal polymorph is realized in the SrMnO3 polymorph identified in the x-ray diffraction analysis and Raman spectroscopy results. We plan to generate the ferroelectric ordering by octahedral tilting in a 6H-hexagonal SrMnO3 oxide. Then, the inversion symmetry in 6H-hexagonal SrMnO3 is expected to be broken and generate a new polar symmetry. A free-standing epitaxial SrRuO3 (SRO) membrane with nanoscale thickness is synthesized. The heteroepitaxial sacrificial layer of SrCuO2 (SCO) followed by an encapsulated SRO film is grown on the SrTiO3 (001) substrate via pulsed-laser deposition (PLD) technique. A free-standing SRO epitaxial-membrane is attained by chemical etching of the sacrificial SCO layer in the acidic potassium chloride (KCl) solution. The structural, chemical, surface, and electrical characterization of the SrRuO3 epitaxial membrane were demonstrated via x-ray diffraction (XRD), scanning electron microscopy (SEM) measurements, Atomic force microscopy (AFM), and the four-prob method. A better understanding of the enhanced functional properties in complex oxides can play a promising role in device applications like non-volatile memories, actuators, and flexible electronics.