Effect of Pressure on Crystal Structure and Lattice Parameters of Iron up to 300 kbar

TLDR

X‑ray diffraction measured bcc and hcp iron lattice parameters up to 300 kbar at ~23 °C. The c/a ratio of hcp iron is 1.603±0.001 and pressure‑independent between 80–300 kbar, the extrapolated zero‑pressure volume is 6.72±0.06 cm³/mol, the bcc‑hcp transition at 130 kbar involves a −0.34±0.01 cm³/mol volume change, and these values satisfy the bcc‑hcp‑fcc triple‑point conditions.

Abstract

Lattice parameters of the bcc and hcp phases of iron have been determined as a function of pressure up to 300 kbar at 23°±3°C by means of x-ray diffraction techniques. The c/a ratio for hcp iron has been determined to be 1.603±0.001 at pressures between 80 and 300 kbar and is independent of pressure. Based upon the extrapolation of the high-pressure data using an exponential form of the Murnaghan equation of state, the volume of hcp iron at zero pressure is 6.72±0.06 cm3/mole. The volume change for the bcc-hcp transformation at 130 kbar is −0.34±0.01 cm3/mole. This value satisfies the triple point conditions for the bcc-hcp-fcc triple point when published values for the other phase transformations are used.