Phase transitions, Grüneisen parameter, and elasticity for shocked iron between 77 GPa and 400 GPa

Abstract

Sound velocities determined in iron, shock compressed to pressures between 77 GPa and 400 GPa, indicate that two phase transitions exist on the Hugoniot. A discontinuity in sound velocities at 200 +_ 2 GPa may mark the transition of e iron to 7 iron. A second discontinuity at 243 _+ 2 GPa is believed to indicate the onset of melting. The calculated temperature at melting lies between 5000 K and 5700 K. When extrapolated from the Hugoniot melting point, the Lindemann criterion yields an estimate of 5800 +_ 500 K for the melting of pure iron at the inner core boundary pressure of 330 GPa. The product of density times the thermodynamic Grfineisen parameter in liquid iron, calculated from the present data, is 19.6 +_ 0.8 Mg m-3. A temperature profile ranging from 3800 K at the core-mantle boundary to 5000 K at the earth's center is calculated using the present data. Sound velocities for e iron provide a better match to seismic velocities for the earth's inner core than do those of 7 iron. A comparison between liquid iron velocities and the velocity profile through the outer core provides further evidence for alloying of iron with "lighter elements" in the core.

Quenching of bcc-Fe from high to room temperature at high-pressure conditions: a molecular dynamics simulation

“…The melting temperature of Fe at the pressure of the outer core-inner core boundary (330 GPa), according to the EAM, is consistent with the temperature computed from first principles [18] (later revised to a somewhat lower temperature) within the mutual error bars. The EAM melting temperature is in agreement with the shock-wave data [3] at a pressure of 243 GPa ( figure 1). The most recent melting data by Ma et al [19] was predicted almost exactly back in 2000 [6].…”

“…Except for the measurements of velocities of rarefaction waves behind the shock-wave in iron [3,4], no other experiments have either confirmed or ruled out the stability of another, different from the hcp, iron phase at the IC conditions. Recently, the hcp-bcc transition was observed in iron slightly alloyed with nickel at pressures around 200 GPa on temperature increase.…”

“…conditions corresponding to the IC, iron is stable in the hexagonal close-packed (hcp) phase [2]. However, the experimental work by Brown and McQueen [3] suggested that iron might transform to another, different from the hcp, phase at the conditions of IC. That discovery [3] was not confirmed by later shock-wave experiments [4]; however, the difference in compositions of iron samples could be the reason for that (see [5] for discussion).…”

“…However, the experimental work by Brown and McQueen [3] suggested that iron might transform to another, different from the hcp, phase at the conditions of IC. That discovery [3] was not confirmed by later shock-wave experiments [4]; however, the difference in compositions of iron samples could be the reason for that (see [5] for discussion). It was suggested, on the basis of non-empirical [6] molecular dynamics (MD) simulations, that under high pressure iron transforms on heating to the body-centered cubic (bcc) phase [7].…”

The stability of bcc-Fe at high pressures and temperatures with respect to tetragonal strain

“…Diamond-anvil cell (DAC) studies have been interpreted as showing that hcp Fe transforms at high temperatures to a phase which has variously been described as having a double hexagonal close packed structure (dhcp) (Saxena et al, 1996) or an orthorhombically distorted hcp structure (Andrault et al, 1997). Furthermore, high pressure shock experiments have also been interpreted as showing a high pressure solid-solid phase transformation (Brown and McQueen, 1986;Brown, 2001). It has been suggested that this transition could be due to the development of a bcc phase (Ross et al, 1990;Matsui and Anderson, 1997).…”