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

A new high-pressure instrument has been developed with the pressure uncertainty characteristic of free-piston gauges in the 3000 MPa range. It consists of an end-loaded piston-cylinder apparatus modified to use argon as a pressure medium. Pressure is determined from the piston force and the piston seal area with minor corrections for vessel bore dilation and seal friction. Bore dilation is approximately 0.36% and double-valued friction for the piston seal ranges from about 1% to 2%. A force-balance analysis of the piston seal indicates that the friction is highly symmetric. The equilibrium pressure of the Bi I-II transition has been determined in this apparatus to serve as a reference point on the practical pressure scale. Transition pressure measurements were made on 99.9999% pure polycrystalline bismuth. Small piston force excursions were made with both phases present to determine the equilibrium pressure which is insensitive to reaction rate. 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].