Equation of state of bismuth to 222 GPa and comparison of gold and platinum pressure scales to 145 GPa

Abstract

Unit cell volumes of Bi, Pt, and Au have been measured simultaneously to megabar pressures by x-ray powder diffraction using a diamond anvil cell and a synchrotron radiation source. The body-centered cubic (bcc) phase of Bi was found to be stable up to 222 GPa. The equation of state (EoS) of bcc-Bi was determined using the Pt pressure scale. A fit of the Vinet EoS to the volume compression data gave $B_0\text{=35.22(19)\hspace{0.17em}}\mathrm{GPa},$ $B_0^{\prime }\text{=6.303(18),}$ and 1 atm atomic volume $V_0\text{=31.60(4)\hspace{0.17em}}{\mathrm{A}}^3.$ Because of the high compressibility, the use of bcc-Bi as a pressure marker is expected to give improved precision in pressure measurement. The Pt and Au pressure scales were compared up to 145 GPa. The Au pressure scale gave lower pressure than the Pt pressure scale. The deviation between the two scales became noticeable at $\text{∼30\hspace{0.17em}}\mathrm{GPa}$ and diverged with increasing pressure, reaching $\text{∼20\hspace{0.17em}}\mathrm{GPa}$ at 145 GPa. A fit of the Vinet EoS to Au compression data on the Pt pressure scale gave: $B_0\text{=166.34(77)\hspace{0.17em}}\mathrm{GPa},$ and $B_0^{\prime }\text{=6.244(33).}$