Alloy decomposition under pressure: Bi-Sb incommensurate phase as a case study

The high-pressure phase behavior of the Bi-Sb system was investigated experimentally and modeled thermodynamically to study the effect of the transition to the incommensurate Bi-III type structure, which is adopted by the high-pressure phases Bi-III and Sb-II of the pure elements. The thermodynamic model of the Bi-Sb phase diagram including the effect of pressure in the isomorphous regime was extended to include solid-solid phase transitions of the pure elements. Consequently, the alloy phase diagram was found to transform to the eutectic form with pressure and is predicted here in the pressure range of 3-6 GPa. The alloy was found to transform from the ambient, rhombohedral A7 structure to the incommensurate phase. The transformation occurred at intermediate pressures between those observed in elemental Bi and Sb. The region of thermodynamic stability was determined by calculating the spinodal curves. To verify these predictions, the structure of a Sb-rich alloy was determined as a function of pressure by X-ray diffraction measurements in a diamond anvil cell, demonstrating the transition from A7 to the incommensurate Sb-IIlike structure. It was found that the decomposition of the alloy under pressure is determined by the thermodynamic instability located at the spinodal curve. In contrast, alloy compositions located in the metastable region between the binodal phase line and the spinodal curve did not decompose under pressure, thus indicating a new region of phase space to be explored in high-pressure studies of alloys. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Experimental and thermodynamic studies were conducted on the high-pressure phase behavior of the Bi-Sb system, predicting the transformation and stability region of the alloy under pressure. The alloy was found to transform from ambient A7 structure to an incommensurate phase, with the stability region determined by spinodal curves.