Tuning Noncubic Phases of Au at Microscale using Low-Energy Ar⁺ Ion Irradiation

Abstract

The structural stability and tunability of nano-and microscaled noble metals with unconventional lattices are a subject of widespread interest for their novel functional properties. Recently, we have been actively exploring penta-twinned Au microcrystallites that host extraordinarily stable noncubic body-centered orthorhombic and tetragonal (bc(o,t)) phases with strains in the elastic regime. Their structural stability remains unaltered with common perturbants, namely, temperature, pressure, X-ray, and electron beam, in moderate ranges. Here, we consider these Au microcrystallites as the model system and investigate their structural changes with low-energy Ar + ion irradiation (1.2−5 keV). Our results highlight a distinct body-centered tetragonal phase, termed bct-I, along with the pristine bc(o,t) phases at intermediate exposure times. Prolonged irradiation results in the spatial homogenization of elastic strain along the [001]-direction and irreversible transformation of all noncubic phases into a family of bct-I structures. These structural changes can be precisely controlled with beam energy and time of exposure as the modulating parameters. Theoretical analysis shows the role of electronic excitations in the stabilization of anisotropic elastic strain and the correlation between electronic energy and irradiation time. Our work will stimulate further studies on microstructural changes in geometrically frustrated systems with routinely used ion irradiation as a perturbant.