Pressure‐temperature‐volume relationship for hexagonal close packed iron determined by synchrotron radiation

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Abstract
The equations of state of both body‐centered cubic (bcc) and hexagonal close packed (hcp) iron have been determined in a high‐temperature diamond anvil cell using synchrotron radiation as the X ray source. Four isothermal runs, 25°, 150°, 300°, and 450°C were carried out using gold as the pressure calibrant. The compressibility of bcc iron was determined up to 12 GPa at each temperature. The equation of state data for iron were found to be in reasonable agreement with measurements made by earlier diamond cell work and by the ultrasonic methods. The compressibility of hep iron was determined in the pressure range 14–24 GPa at various temperatures. By fitting the compression data at room temperature to the Birch‐Murnaghan equation, we determined the bulk modulus to be 212 ± 14 GPa assuming Ko′ = 4. Volume thermal expansion of hep iron is determined to be 1.001, 1.011, and 1.021 (±0.005) at 150°, 300°, and 450°C, respectively, in this pressure range. The da ratios of the hep phase remained constant at 1.60 ± 0.02 up to 300°C in this experiment.