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

## TL;DR

Scientists used X-ray microscopy to observe how gold microcrystals change shape and structure when heated, revealing stable noncubic lattices.

## Contribution

The study demonstrates real-time observation of lattice untwisting in gold microcrystals under high temperatures using X-ray diffraction microscopy.

## Key findings

- Gold bipyramids retain ∼83% of their noncubic bc-(o,t) lattice after annealing up to 800°C.
- High temperature causes untwisting of domains with minimal change in lattice proportions.
- Thermal annealing produces metastable lattices with altered lattice parameters.

## Abstract

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.