Cubic lattice structure preferred by noble metals (e.g., Au, Ag, Pd and Pt) is thermodynamically highly stable. Lower symmetry crystal structures are not known to stabilize relative to the face-centered cubic (fcc) bulk even at high pressures. Observing a structural transformation in these metals is therefore fundamentally exciting and can be possibly useful in tuning their otherwise noble behavior. Recent explorations in this direction have focused on nanocrystals, as the energy needed for lattice distortion is relatively low when the size is only a few nanometers. Inducing lattice strain at larger length scales to cause structural transformation in the crystallite is indeed a herculean task. The same has been achieved in the laboratory by stabilizing a decahedral nanocrystal corrugated morphology through a simple synthetic method involving thermolysis of a metal-organic precursor in air.
The bipyramidal Au microcrystals thus synthesized possess penta-twinned tips, exhibiting nanofacets of unusually high indices. Using a laboratory X-ray source, the presence of non-fcc phases, body-centered tetragonal and body-centered orthorhombic, have been readily detected. The remarkable occurrence of non-fcc phases of Au is related to the geometrically induced strains in the bipyramids; annealing at high temperatures, application of high pressures or exposure to energetic ions relieves the stress thus destabilizing the non-fcc phases. Interestingly, the non-cubic phases are chemically highly stable; the microcrystals can withstand mercury treatment and exhibit low dissolution rate in aquaregia unlike the conventional gold. Nanofocus X-ray diffraction measurements on an individual crystallite at a synchrotron beamline have provided a deeper insight into the distribution of the various phases in relation to the crystal morphology. Further, the study allowed mapping of the fcc and non-cubic phases within the volume of the chosen crystallite based on diffraction microscopy. Accordingly, the orthorhombic phase seems to form the body of the crystallite (82% of the crystal volume), while the fcc resides in the tips.
(a) Unraveling the spatial distribution of catalytic non-cubic Au phases in a bipyramidal microcrystallite by X-ray diffraction microscopy.
(b) Nobler than the Noblest: the noncubic Au microcrystallite can stand harsh chemical treatments while the fcc crystallites dissolve away.
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