Noblest of All Metals Is Structurally Unstable at High Pressure

In a series of experiments in externally electrically heated diamond anvil cells we demonstrate that at pressures above $\sim 240\mathrm{GPa}$ gold adopts a hexagonal–close-packed structure. Ab initio calculations predict that at pressures about 250 GPa different stacking sequences of close-packed atomic layers in gold become virtually degenerate in energy, strongly supporting the experimental observations.

Figure 1

Variation of unit cell volume of fcc-Au at ambient temperature (blue or dark gray symbols) and at temperatures 1020 K to 1070 K (red or light gray symbols) (data collected in six different experiments; all data points at intermediate temperatures omitted for clarity). In agreement with previous studies [8, 9, 10] we found that at pressures up to at least 150 GPa and temperatures up to 1050 K Anderson et al. [19] thermal equation of state (TEOS) (solid lines—lower one for 300 K, upper one for 1050 K) describes our data within the error-bars of the measurements of pressure, temperature, and volume. In the experiments above 150 GPa and high temperatures, in order to avoid any possible chemical reactions, we loaded only gold (99.9999% purity; Goodfellow, Inc.) in the pressure chamber and relied on fcc-Au TEOS as the internal pressure standard.

Figure 2

Examples of diffraction pattern collected from gold (a) compressed at ambient temperature to 275(20) GPa, (b) heated at 860(10) K at 248(20) GPa, (c) slowly cooled down to room temperature at 236(20) GPa, and (d) heated again to 1050(15) K at 237(20) GPa. The inset shows part of 2D diffraction image of the sample at 236(20) GPa and room temperature. The pattern is dominated by diffraction lines of the high-pressure hcp-Au phase.

Figure 3

Phase relations between fcc (blue triangles) and hcp (inverse red triangles; amount of hcp-Au above 80%) gold phases as was observed in three independent multimegabar pressure experiments. Green diamonds show conditions of phase coexistence (fcc-Au above 20%). The solid line indicates the tentative phase boundary between the fcc- and hcp-Au phases.

Figure 4

Calculated enthalpies of different arrangements of the close-packed atomic layers in gold under compression. The enthalpy of the fcc phase is taken as reference and is shown by a horizontal full line.