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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, experimental direct observations demonstrating the correlation of premelting with lattice defects inside a crystal remain elusive. Using atomic-column-resolved imaging with scanning transmission electron microscopy in polycrystalline BaCeO3, here we clarify the initiation of melting at two-dimensional faults inside the crystals below the melting temperature. 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, our study suggests that lattice defects inside crystals should not be overlooked as preferential nucleation sites for phase transformation including melting. No experimental observations have been reported to clarify how a melting transition proceeds inside a crystal. Here the authors demonstrate that melting is initiated at two-dimensional faults inside BaCeO3 crystals below the melting temperature in a layer-by-layer manner. [ABSTRACT FROM AUTHOR]

In article number 1802481, Sung‐Yoon Chung and co‐workers demonstrate that grain boundaries (GBs) in LaCoO3 and LaMnO3 are exceptionally active sites for oxygen evolution reaction (OER) electrocatalysis. By using a combination of bicrystal thin films, atomic‐scale direct observations, and theoretical calculations, this work elucidates the strong correlation between the symmetry‐broken atomic displacements at GBs and the variation of electronic structure to facilitate the charge transfer and thus the OER activity. [ABSTRACT FROM AUTHOR]

A grain boundary forms as an internal interface when two crystalline grains with mutually different crystallographic orientations are in direct contact with each other. As a result, atomic arrangement at grain boundaries differs from that of the bulk, showing serious displacements deviating from the original symmetric positions. As these symmetry‐broken configurations are difficult to achieve in the bulk crystals, grain boundaries are considered distinctive platforms that can exhibit new physical properties. By using both sintered polycrystals with various grain sizes and thin films on bicrystal substrates, it is directly verified that surface‐terminating grain boundaries in LaCoO3 and LaMnO3 are exceptional in oxygen evolution electrocatalysis, showing more than an order of magnitude higher activity. A combination of atomic‐scale structure observation and density functional theory calculations demonstrates that the displacement of atoms in metal–oxygen octahedra correlates with significant splitting of the degenerate transition‐metal 3d orbitals, and subsequently much easier charge transfer between metals and oxygen is attained. In addition to identifying the grain boundaries as strikingly active sites, the findings suggest that symmetry breaking by atom displacements in metal–oxygen octahedra is an efficient approach to remarkably enhance the oxygen electrocatalytic efficiency in perovskite oxides. Surface‐terminating grain boundaries in perovskite oxides are identified to be exceptional in oxygen evolution electrocatalysis. Atomic‐scale direct observation and density functional theory calculations demonstrate that symmetry‐broken atom displacement in oxygen octahedra at grain boundaries strongly correlates with easier charge transfer between metals and oxygen for remarkable enhancement of the oxygen electrocatalytic efficiency. [ABSTRACT FROM AUTHOR]

Proton conduction in the hexagonal perovskite-related oxide Ba 7 Nb 4 MoO 20 (BNM) holds great potential for high-temperature fuel cells and electrolysis cells, especially at intermediate temperatures below 700°C. However, sintering BNM electrolytes leads to the formation of secondary phases that deteriorate proton-conducting properties. In this study, we propose facile microwave sintering combined with annealing to achieve high purity and structural integrity. We examine the sintering behavior of BNM and the effects of annealing on its density, microstructure, and purity, aiming to optimize the process. We successfully achieve a 98 % relative density in BNM after just 5 minutes of MWS at 1350°C. Furthermore, annealing at 1050°C significantly improves proton conduction in both the bulk and grain boundaries, highlighting the importance of purity when using BNM as an electrolyte. This approach markedly reduces processing time and energy consumption, offering a viable way for fabricating high-density, high-purity BNM ceramics. [ABSTRACT FROM AUTHOR]

Microstructural optimization was performed on Mg alloy produced by high-energy milling and spark plasma sintering (SPS) through systematic investigation of mechanical properties and corrosion resistance. The SPS Mg alloys degassed under low vacuum condition led to continuous and thick magnesium oxide stringer consisted of MgO crystals at particle boundary even at high sintering pressure of 130Mpa. Simultaneously, large and irregularly shaped oxide particles at grain boundary and grain inside was identified from TEM observation. Meanwhile, for fully degassed at high vacuum and SPS Mg samples, particle boundaries were discontinuous and the cuboidal MgO particles were uniformly distributed within the grain. Sufficient degassing prior to sintering is critical for forming strong interfaces by facilitating the easy breakup of the oxide film during SPS because Mg powders is highly reactive and form easily Mg oxide on their surfaces, which makes sintering rather difficult. The width of particle boundary in the SPS samples from current experiment appears to be critically depended on degassing vacuum and sintering condition. The removal of continuous or thick oxide stringers with attainment of theoretical sintering density may be a critical parameter for ensuring better mechanical and electrochemical performance in Mg alloys fabricated by high-energy milling SPS process. A fine grained Mg alloy with high toughness and corrosion resistance is obtained by using this degassing process prior to SPS. In Brief : Mechanical properties and corrosion resistance were systematically investigated on different types of grain boundary (GBs) and particle boundaries (PBs) from high-energy-milled and spark plasma sintered Mg alloy. Image 1 • The width of PB in the degassing samples appears to be dependent on vacuum level used in degassing treatment. • Adequate degassing condition is critical for forming strong interfaces by facilitating the easy breakup of the oxide film. • The removal of continuous or thick oxide stringer at PB with attainment of theoretical sintering density may be a critical parameter. • The removal of oxide stringer at PB is ensuring better mechanical and electrochemical performance in Mg alloys fabricated by high-energy milling SPS process. [ABSTRACT FROM AUTHOR]