We have investigated the melting of hcp Fe at high pressure by employing molecular dynamics simulations in conjunction with the full potential linear muffin tin orbital method. Apart from being of fundamental value, the melting of iron at high pressure is also important for our understanding of the Earth. The subject of iron melting at high pressures is controversial. The experimental data for the iron melting temperature can be separated into two regions, "low" and "high." Here we present an ab initio simulated iron melting curve which is in agreement with the low temperatures at lower pressures, but is in excellent agreement with the high-mostly shockwave-temperatures at high pressures. A comparison with available data lends support to the presented iron melting curve.

Iron is thought to be the main constituent of the Earth's core, and considerable efforts have therefore been made to understand its properties at high pressure and temperature. While these efforts have expanded our knowledge of the iron phase diagram, there remain some significant inconsistencies, the most notable being the difference between the 'low' and 'high' melting curves. Here we report the results of molecular dynamics simulations of iron based on embedded atom models fitted to the results of two implementations of density functional theory. We tested two model approximations and found that both point to the stability of the body-centred-cubic (b.c.c.) iron phase at high temperature and pressure. Our calculated melting curve is in agreement with the 'high' melting curve, but our calculated phase boundary between the hexagonal close packed (h.c.p.) and b.c.c. iron phases is in good agreement with the 'low' melting curve. We suggest that the h.c.p.-b.c.c. transition was previously misinterpreted as a melting transition, similar to the case of xenon, and that the b.c.c. phase of iron is the stable phase in the Earth's inner core.

No abstract

No abstract

The Earth solid inner core is mostly iron 1,2 , therefore, the question-what is the structure of iron in the Earth inner core-is central to our understanding of the Core. However, the stable phase of iron in the Core is still unknown. Currently, two major candidates are considered-hexagonal close-packed (hcp) and body centered cubic (bcc) structures. Neither of these structures received unanimous support 3,4. Here we demonstrate stability of the bcc phase under conditions in the center of the Core by performing novel constant pressure-temperature ab initio molecular dynamics simulations with varying shape and volume of the computational cell. The bcc phase is stabilized by the discovered unique diffusion mechanism that originates in the low temperature dynamical instability of the bcc phase. It appears that the bcc phase has already been observed in the recent experiments, however, the experimental data was misinterpreted. The diffusion of iron atoms in solid state is quite unique and might allow us to explain both the anisotropy and the low shear modulus of the inner Core. The Earth solid inner and liquid outer cores consist mainly of iron 1. The pressure range of the inner core is from 330 to 365 GPa 2. The temperature is known less precisely, since the melting temperatures of iron have not been measured at the core pressures. When extrapolated, the data from shockwave (SW) 3,4 and diamond anvil cell (DAC) 5 experiments place melting temperature of iron somewhat above 6000 K at the pressure of the inner core outer core (ICOC) boundary (330 GPa). The melting temperature of iron in the center of the Earth then could be close to 7000 K. Some DAC studies 6 place this number considerably lower, around 5000 K. Theory supports higher melting temperatures 7-9. Structure and properties of a crystal are intimately related. Therefore, elasticity of the core, its seismic response, density, heat capacity and heat conductivity, to name a few, critically depend on the structure of iron in the core. The interpretation of seismic data, our major source of information about deep Earth, is completely dependent on the phase diagram of iron at the core conditions 10-12. At high pressure and low temperature the stable phase of iron is hcp 13. There are two experimental studies that support transition of iron to another, yet unknown phase at pressures above 150-200 GPa and temperatures higher than 3000 K 3,14. The SW data 3 was interpreted as a transition from the hcp to the bcc phase. The DAC data 14 is consistent with stabilization of the bcc phase 15. Later SW studies 4 claim that the Brown and McQueen data 3 is not an evidence for a phase transition. We note, however, that a similar situation was recently resolved for the case of Mo. It was suggested 16 that there is no phase transition in Mo contrary to the discovery made by Hixson et al. 17 earlier. Recently, it was demonstrated 18 that the 'no-transition' scenario 16 is not consistent with theory and likely the original data by Hixson et al. 17 is indeed an evidence for a tran...

No abstract

No abstract

Earth's solid-iron inner core is elastically anisotropic. Sound waves propagate faster along Earth's spin axis than in the equatorial plane. This anisotropy has previously been explained by a preferred orientation of the iron alloy hexagonal crystals. However, hexagonal iron becomes increasingly isotropic on increasing temperature at pressures of the inner core and is therefore unlikely to cause the anisotropy. An alternative explanation, supported by diamond anvil cell experiments, is that iron adopts a body-centered cubic form in the inner core. We show, by molecular dynamics simulations, that the body-centered cubic iron phase is extremely anisotropic to sound waves despite its high symmetry. Direct simulations of seismic wave propagation reveal an anisotropy of 12%, a value adequate to explain the anisotropy of the inner core.