Observation of hexagonal (4H) phase in nanocrystalline silver

Abstract

We report the observation of an unusual hexagonal (4H) phase in silver nanoparticles deposited by high-pressure dc magnetron sputtering. X-ray-diffraction and transmission electron microscopy studies clearly reveal the presence of a hexagonal 4H phase in the silver nanoparticles in addition to the cubic 3C phase— which is the equilibrium crystal structure for bulk silver. The smaller the average particle size, the larger was the volume fraction of the 4H phase. The hexagonal phase can be made to revert to the usual cubic structure by heating the nanocrystalline sample and allowing the grains to grow. This rare hexagonal phase had previously been observed only in certain mineral deposits. We show that there is a size-induced stabilization of this phase in nanocrystalline silver below ≈30 nm.