More accurate determination of the BiI-BiII phase transition pressure — The reference point of the international scale of high pressures

New determination of the bismuth I-II equilibrium pressure: a proposed modification to the practical pressure scale

“…The Bi I-II equilibrium pressure is (2520 ± 5) MPa at 25 C. This value is 30 MPa (1.2 %) lower than Heydemann's [1] value of (2550 ± 6) MPa. It is 8 MPa (0.3 %) higher than the value of (2512 ± 3) MPa from Sekoyan et al [2].…”

“…Unfortunately, the situation with Bi is much worse than that implied by Heydemann's claimed uncertainty. Another free-piston determination of the Bi I-II transition is reported by Sekoyan et al [2]. They and others, e.g.…”

“…The piston-insertion hysteresis loop from Figure 4 is superimposed for perspective. The upper row of points contains the two equilibrium values determined in free piston gauges, by Heydemann [1] and Sekoyan et al [2], along with the value determined in this work.…”

“…The double-valued friction is expressed by the width of the piston insertion hysteresis loop. The excursion starts at the lowest pressure plotted and approaches the onset of the Bi I-II transition at a pressure rate of 0.011 MPa/s, characteristic of freepiston rates [1,2]. The compressive onset of the Bi I-II transition is labelled in the lower right of the figure, at 2575 MPa.…”

“…The force-balance equations for inward piston motion are, for axial forces (1) and, for radial forces (2) For outward piston motion, the frictional forces change sign. For the axial forces we have (3) and for the radial forces (4) The frictional forces on the bearing faces are related to the normal forces by (5) The frictional forces ( 5) are substituted into the four force-balance equations (1-4), which are then solved for the force normal to the bore and the force normal to the piston chamfer for inward and outward piston motions.…”