Ambient Stable Tetragonal and Orthorhombic Phases in Penta-Twinned Bipyramidal Au Microcrystals

摘要

Face-centered cubic (fcc) lattice is the only known crystal structure of bulk gold. In the present work, we report the presence of body-centered tetragonal (bct) and body-centered orthorhombic (bco) phases in bipyramidal Au microcrystals with penta-twinned tips. These microcrystals have been obtained by thermolysis of (AuCl_4)~- stabilized with tetraoctylammonium bromide (ToABr) in air at about 220 ℃ for 30 min. Using a laboratory monochromatic X-ray source, the non-fee phases could be readily detected. The remarkable occurrence of non-fcc phases of Au grown in the temperature window of 200-250 ℃ results from the geometrically induced strains in the bipyramids. Having derived first-principles theoretical support for the temperature-dependent stability of non-fcc Au structures under stress, we identify its origin in soft modes. Annealing at high temperatures relieves the stress, thus destabilizing the non-fcc phases.