Determination of symmetry breaking transitions and polymorphism in Au Cu nanostructures by nano-beam electron diffraction

Alloyed nanostructures of Au Cu in varying compositions were synthesized by wet chemistry routes. Synthetic products in all the compositions are FCC solid solutions of Au Cu with slightly varying lattice parameter. Variation in shape, size, and nature of attachment has been studied through electron microscopy and localized surface plasmon resonance measurements. FCC solid solutions upon heat-treatment at ~400 °C for 1 h under N2 atmosphere, transform to orthorhombic AuCu (oP8) and many superlattice structures based on it. Nano-beam electron diffraction reveals multiplication of periodicities along [3 1 ¯ 0], [210] and [201] directions, mostly originating due to the chemical or vacancy ordering. Symmetry breaking transitions of orthorhombic AuCu (oP8) from Pbam to P21212 has been observed. The phase with P21212 space group also shows polymorphic transition to P21221. All these transitions have been confirmed through nano-beam electron diffraction coupled with computed diffraction patterns based on Wyckoff positions. Commonly observed orthorhombic (oI40) AuCu upon heat-treatment has not been found. The multiplication of lattice periodicity along certain crystallographic directions, symmetry breaking transition and polymorphism in the same sub-group are important findings of this work. These might have far-reaching implications in electron transport properties of such nanostructures.