Extraordinarily Stable Noncubic Structures of Au: A High-Pressure and -Temperature Study

Although the stability of Au in the face-centered cubic (FCC) phase at high temperatures and pressures has been well studied, the stability in other lattice phases rarely encountered in crystallite domains in microscopy studies has not been explored much because of their nanometric extensions. A recent report on Au microcrystallites crystallized in body-centered tetragonal (BCT) and body-centered orthorhombic (BCO) phases prompted the work presented here, in which we have investigated for the first time the structural stability of the BCT and BCO phases at high temperatures and separately at high pressures using high-energy synchrotron X-ray diffraction. A reversible phase transition was observed for pressures of up to ∼40 GPa, indicating unusual stability of the non-FCC Au phases. However, during a high-temperature treatment at ∼700 °C, the transformation to FCC was irreversible.