Comparison of the Bismuth I–II Phase Transformations in Liquid and Solid High-Pressure Systems

Bismuth specimens exposed to liquid and to solid pressure-transmitting media were examined simultaneously in a hydrostatic pressure system, and the measured initiation and equilibrium pressures of the I–II and II–I transitions were compared. The hydrostatic pressure-transmitting-medium was an equivolume mixture of normal- and iso-pentane, and the solid pressure transmitting media investigated were AgCl and Epoxy resin. Encapsulation by AgCl and Epoxy resin caused a lower I–II initiation pressure, but did not alter the equilibrium pressure. The bismuth I–II transition initiated near the equilibrium pressure in each of the solid pressure media. In the liquid pressure medium the equilibrium pressure was also found to be asymmetrically located toward the high-pressure side of the initiation pressure hysteresis. The region of indifference was 30 bar wide. The transformation behavior is explained in terms of the strain and kinetic dependences of solid-state nucleation. The solid pressure media accelerate the nucleation kinetics so that the transition initiates at the boundary of the strain hysteresis which is small for the forward transformation. For standard calibration of solid-media systems on increasing pressure, experimental justification is provided for the practice of calibrating the initiation of the I–II transition with the equilibrium pressure. The pressure of initiation is defined, and the importance of the pressurization rate is illustrated. Several common calibration methods used in solid-media systems are shown to be incorrect.