Increasing Atomic Electron Cloud Density Leads to Formation of Body Centered Cubic (BCC) Gold

Most metallic elements possess one relatively close packed structure of body-centered cubic (BCC), face-centered cubic (FCC) and hexagonal close packed (HCP) structures under ambient conditions. Access to these structures as a function of temperature and pressure varies for different groups in the periodic table. Under low pressures, the copper group elements (copper (Cu), silver (Ag) and gold (Au)) only have a FCC solid structure regardless of temperatures. Recently, BCC Au was detected under extreme compressive pressures. The structural preference for a given metal element is still not understood. In this work, we surprisingly observed formation of BCC Au from amorphous Au as well as FCC Au in vacuum at room temperature within a Au nanoparticle with electron self-transfer enabled by our synthesized zirconium hydroxide nitrate tube support. We propose a theory behind this unexpected phase formation: the increase in atomic electron cloud densities of Au atoms resulting from the electron transfer contributes more free electrons over the Au nanoparticle except its interfacial part, enhancing metallic bonding strength to hold Au atoms closer to form a BCC structure that can store denser electron states within a Fermi sphere in first Brillouin zone as compared to FCC Au. It can also explain formation of BCC Au under compression. Our theory here can be a supplement to the metallic bonding theory and our strategy via building heterogenous structures can guide how to expand the structure world of metals and their alloys under ambient conditions for beneficial applications.