Chaitali Sow

Au bipyramids hosting body-centered orthorhombic and tetragonal lattices (bc(o,t)) exhibit extraordinary stability at ambient conditions and even under high-temperature/high-pressure conditions. The phases undergo conversion to a conventional face-centered cubic (fcc) lattice only during annealing at 700 °C due to the unlocking of the geometrically induced stresses. The spatial distribution of the phases in the crystallite volume has revealed fcc capped bc(o,t) lattices with two halves of the bipyramid twisted by ∼6° along the length with approximately ± 5% strain. Understanding the spatial distribution and dynamics of these phases at high temperatures can provide detailed information on their thermal stability. Herein, using nanoprobe scanning X-ray diffraction microscopy (SXDM), in situ annealing of the bc(o,t) Au bipyramid (∼1.5 μm long and 300 nm wide) has been performed at different temperatures (up to 800 °C). The study reveals untwisting of the domains assisted by the supplied high temperature, while the existing lattices undergo variation in parameters with negligible changes in proportion. The study reveals and picturizes the dynamic change in diffracting volumes across a wide temperature range. Notably, despite annealing, ∼83% of the bc(o,t) content is still retained (with different lattice parameters), proposing the annealing route to produce unusual metastable lattices of gold.

Properties of metal crystallites are governed by their morphologies and inherent crystal structures. In this work, bipyramidal Au microcrystallites hosting non‐cubic lattices, body‐centered orthorhombic and tetragonal (together termed as bc(o,t)), are investigated for their stability in aqua regia. Specifically, microcrystallites comprising 92 % of bc(o,t) have been subjected to aqua regia of different concentrations and the changes in morphology and lattice phases have been monitored using scanning electron microscopy and X‐ray diffraction techniques. The dissolution process was found to be crystal structure dependent and begin at the bipyramidal tips enriched with fcc lattice while retaining the bc(o,t) rich body. Interestingly, with increasing the reaction times, the remaining core was found to be highly reluctant to dissolution and instead, transformed to tetragonal lattices which with increasing treatment, exhibited lattice parameters closer to that of fcc. The study reveals the presence of a bc(o,t)‐fcc core‐shell structure with the tips enriched with fcc.

Noble metals (Ru, Os, Rh, Ir, Pd, Pt, Ag, and Au) are known for their extraordinary oxidant-resistant behavior, good electrical and thermal conductivity, high work function, and brilliant luster. All occur in close-packed crystal structures: Ru and Os in hexagonal close-packed (hcp) and the rest in face-centered cubic (fcc) structures, both possessing high-symmetry structures and, therefore, a high degree of stabilization. Numerous studies in the literature have attempted to stabilize these metals away from their conventional crystal structures with the aim of realizing new properties. While obtaining conventional fcc metals in hcp structure or vice versa has been the subject of most studies, there are also examples of fcc metals crystallizing in lower-symmetry structures such as monoclinic. The nonnative crystal structures are generally realized during the crystallite growth itself, with a few exceptions in which a posttreatment was required for lattice transformation. In most cases, the new crystal structures pertain to the nanometer-length scale in the form of nanoparticles, nanoplates, nanoribbons, and nanowires, but there are good examples from microcrystallites as well. In this article, we review this active area of research, focusing on ambient stable crystal systems with some account of their interesting properties as reported in recent literature.

Tuning of crystal-structures and shapes of sub-micron-sized noble metals have revealed fascinating catalytic, optical, electrical and magnetic properties that enable developments of environment-friendly and durable nano-technological applications. Several attempts have been made to stabilize Au, knowing its extraordinary stability in its conventional face-centered cubic (fcc) lattice, into different lattices particularly to develop Au-based catalysis for industry. Here, we report the results from scanning X-ray diffraction microscopy (SXDM) measurements on an ambient-stable penta-twinned bipyramidal Au microcrystallite (about 1.36 µm in length and 230 nm in diameter) stabilized in non-cubic lattice, exhibiting catalytic properties. With more than 82% of the crystal volume, the majority crystallite structure is identified as body-centered orthorhombic (bco) while the remainder, the standard fcc. A careful analysis of the diffraction maps reveals that the tips are made up of fcc while the body contains mainly bco with very high strain. The reported structural imaging technique of representative single crystallite will be useful to investigate the growth-mechanism of similar multiphase nano- and micron-sized crystals.