Real-Time Monitoring of Thermally Induced Twisting-Untwisting of Noncubic Domains in Au Microcrystallites using X-ray Diffraction Microscopy.

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), 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.