Real-Time Observation of Crystal Evaporation in a Metal Phosphate at High Temperature

A number of experimental studies on crystal growth have been performed in connection with a variety of crystalline systems ranging from simple oxides to complex organic compounds. In contrast, little is known regarding how crystals evaporate. By using a combination of real-time high-resolution electron microscopy at high temperature, image simulations, and density functional theory calculations, we demonstrate the evaporation of metal-phosphate nanocrystals with flat surfaces at atomic resolution. In situ imaging and direct comparison with image simulation results reveal that, while a layer-by-layer lateral process is macroscopically maintained, the cations preferentially evaporate over the (PO₄)^3– tetrahedral anions from shrinking ledges. The present observations provide the first atomic-scale experimental details of the evaporation of complex oxides, emphasizing the value of direct visualization in real time

Capturing Heterogeneous Nucleation of Nanoscale Pits and Subsequent Crystal Shrinkage during Ostwald Ripening of a Metal Phosphate

It has been generally accepted that crystal shrinkage during Ostwald ripening can be understood simply as a reverse process of crystal growth, and as a result, little attention has been paid to shrinkage behavior. The entire microstructure of polycrystalline materials, however, forms as a consequence of both growing and shrinking crystals. Thus, scrutiny of shrinking characteristics in addition to growth aspects is essential for a complete understanding of the evolution of microstructure during Ostwald ripening. By capturing real-time <i>in situ</i> high-resolution electron micrographs at high temperature, we herein demonstrate the shrinkage behavior of nanocrystals embedded in a solid crystalline matrix during the ripening process of a metal phosphate. Unlike typical crystal growth behavior based on two-dimensional homogeneous nucleation, heterogeneous types of nucleation with nanoscale pits at solid–solid interfaces (or crystal edges) are observed to dominantly occur during shrinkage of the crystals. The findings of this study suggest that crystal shrinkage proceeds with a lower activation energy barrier than that of crystal growth, although both crystal growth and shrinkage take place at the same time during Ostwald ripening

Correlative microscopy of the constituents of a dinosaur rib fossil and hosting mudstone: Implications on diagenesis and fossil preservation

<div><p>We have applied correlative microscopy to identify the key constituents of a dorsal rib fossil from <i>Koreanosaurus boseongensis</i> and its hosting mudstone discovered at the rich fossil site in Boseong, South Korea, to investigate the factors that likely contributed to diagenesis and the preservation of fossil bone. Calcite and illite were the commonly occurring phases in the rib bone, hosting mudstone, and the boundary region in-between. The boundary region may have contributed to bone preservation once it fully formed by acting as a protective shell. Fluorapatite crystals in the rib bone matrix signified diagenetic alteration of the original bioapatite crystals. While calcite predominantly occupied vascular channels and cracks, platy illite crystals widely occupied miniscule pores throughout the bone matrix. Thorough transmission electron microscopy (TEM) study of illite within the bone matrix indicated the solid-state transformation of 1M to 2M without composition change, which was more evident from the lateral variation of 1M to 2M within the same layer. The high level of lattice disordering of 2M illite suggested an early stage of 1M to 2M transformation. Thus, the diagenetic alteration of both apatite and illite crystals within the bone matrix may have increased its overall density, as the preferred orientation of apatite crystals from moderate to strong degrees was evident despite the poor preservation of osteohistological features. The combined effects of rapid burial, formation of a boundary region, and diagenesis of illite and apatite within the bone matrix may have contributed to the rib bone preservation.</p></div

Composite optical micrographs of the main thin section under different light settings.

<p>(A) Cross-polarized light. (B) Polarized light with the lambda wave plate (530 nm) inserted. (C) Normal transmitted light. Although poorly preserved, osteohistological features are best observed under normal transmitted light as shown in the magnified images in (C).</p

Identified phases and their distributions in each region of the samples studied.

<p>Identified phases and their distributions in each region of the samples studied.</p

An area showing lateral variation of the 1M-type illite and the 2M-type illite within the same layer.

<p>The gradual transformation of the 1M to 2M illite in a lateral direction is apparent. Note the highly disordered {00l} lattices of 2M from the HRTEM image.</p

Composite cross-polarized optical micrograph of the main thin section (x40).

<p>The section was divided into three regions–hosting mudstone (yellow arrow), boundary (red arrow), and rib bone (blue arrow). Clusters of calcite microcrystals can be directly observed in all regions. The mudstone and boundary region primarily contains detrital clasts of quartz and feldspars. Due to the compressed nature of the bone matrix, specific osteohistological features were not discernible from the rib bone besides the vascularization pattern.</p

Unveiling Chemical Reactivity and Structural Transformation of Two‑Dimensional Layered Nanocrystals

Two-dimensional (2D) layered nanostructures are emerging fast due to their exceptional materials properties. While the importance of physical approaches (e.g., guest intercalation and exfoliation) of 2D layered nanomaterials has been recognized, an understanding of basic chemical reactions of these materials, especially in nanoscale regime, is obscure. Here, we show how chemical stimuli can influence the fate of reaction pathways of 2D layered nanocrystals. Depending on the chemical characteristics (Lewis acid (¹O₂) or base (H₂O)) of external stimuli, TiS₂ nanocrystal is respectively transformed to either a TiO₂ nanodisc through a “compositional metathesis” or a TiO₂ toroid through multistage “edge-selective structural transformation” processes. These chemical reactions can serve as the new design concept for functional 2D layered nanostructures. For example, TiS_2(disc)-TiO_2(shell) nanocrystal constitutes a high performance type II heterojunction which not only a wide range solar energy coverage (∼80%) with near-infrared absorption edge, but also possesses enhanced electron transfer property

The interplanar spacings, and chemistry of representative clay phases from each region.

<p>The interplanar spacings, and chemistry of representative clay phases from each region.</p

Simplified illustration of the boundary region.

<p>The haphazard, reticular orientation of clay and the relatively abundant inclusion of quartz and albite clasts is a notable feature of this region. Clay concentration progressively increases towards the mudstone region, and a thin line of calcite microcrystals covers the bone surface. I = illite, V = vermiculite.</p