Atomic-scale control of grain-boundary dopant segregation and premelting observation at 2D Defects in Ba-based perovskite oxides

Abstract

The structure of real materials always deviates from the perfect configuration and they contain structural defects. By the second law of thermodynamics and the kinetics of processing, some degree of disorder and imperfection should be present in any material at finite temperatures. Among various types of defects, the two-dimensional lattice defects, of which the surface and interface are the most prevalent, contain phenomenological and functional differences that are significantly different from bulk in composition and structure. Furthermore, the heterogeneous feature of the corresponding lattice defects strongly affects the behavior of crystalline solids. Therefore, a broad understanding and observation of two-dimensional lattice defects is considered crucial from the perspective of materials science and engineering. In the present study, phenomena arising from two-dimensional defects as well as changes in physical properties through compositional and structural control have been examined in oxide systems, particularly in perovskite oxide systems, with atomic-scale direct observation. As the first research part, the improved dielectric properties through the control of grain-boundary composition and microstructure in oxide dielectrics were examined. Multilayer ceramic capacitors are important core component in electronic devices and aim for high relative permittivity. As a result, they typically consist of polycrystalline BaTiO$_3$ with chemically heterogeneous core-shell type grains with multiple dopants. Instead of using this conventional duplex grain structure, here the nonequilibrium atomic‐scale grain‐boundary segregation of an appropriate single acceptor additive in nanocrystalline BaTiO3 for developed dielectric functionalities is utilized. Composition analyses directly demonstrate that the distribution of each additive is strongly confined to grain boundaries within several unit‐cell width together with no chemical heterogeneity in the bulk grains. In contrast to fairly high dissipation factor and serious reduction of permittivity under DC bias in current ceramic capacitors, the resulting dielectric energy loss of our samples is identified to be enormously low up to high frequencies and, in particular, the nominal high relative permittivity with exceptional temperature stability is preserved even under a large DC bias field. This result emphasizes that the suppression of grain growth and the control of additive segregation at the atomic level are necessary for achieving unprecedented reliability. As the second research part, the formation of segregation-induced superstructure at the grain boundaries and its effect on grain boundary migration or resultant dielectric performance are verified by atomic level observation. when aliovalent foreign cations are doped into ABO$_3$-type perovskite oxides, they are usually substituted for either dodecahedral A or octahedral B sites, depending on their relative ionic size. In addition, their effective charge can be ionically compensated through the creation of positively charged oxygen vacancies for acceptor doping and negatively charged cation vacancies for donor doping. In stark contrast to this well-known formation of Schottky defects in perovskite oxides, we directly uncover the presence of Frenkel-type indium dopants occupied in the square-planar interstices (Ini•••) and charge-compensating Ba vacancies (VBa) at grain boundaries in polycrystalline BaTiO$_3$. Moreover, this significant chemical heterogeneity by the defect combination of Ini••• and VBa at grain boundaries appears to act as a substantial barrier impeding grain-boundary migration during sintering. The highly densified fine-grain microstructure attained by strong inhibition of grain growth in In-added BaTiO3 enables unprecedented dielectric properties, encompassing remarkably low dissipation loss and DC-field-insensitive and temperature-independent high permittivity, applicable for multilayer ceramic capacitors. This study thus highlights that precise identification of atomic-scale defects structures can make a critical contribution to in-depth understanding of unexpected physical properties in complex oxides. In the last research part, the first atomistic experimental evidence of premelting initiated at the two-dimensional lattice faults inside perovskite oxides are investigated. since two major criteria for melting were proposed by Lindemann and Born in the early 1900s, many simulations and observations have been carried out to elucidate the premelting phenomena largely at the crystal surfaces and grain boundaries below the bulk melting point. Although dislocations and clusters of vacancies and interstitials were predicted as possible origins to trigger the melting inside a crystal, experimental observations demonstrating the correlation of premelting with lattice defects remains elusive. Using atomic-scale direct imaging with scanning transmission electron microscopy in polycrystalline BaCeO$_3$, the initiation of melting at two-dimensional faults inside the crystals below the melting temperature was clarified. In particular, melting in a layer-by-layer manner rather than random nucleation at the early stage was identified as a notable finding. Emphasizing the value of direct atomistic observation, the study of this chapter suggests that lattice defects inside crystals can be preferential nucleation sites for phase transformation including melting.