High Pressure Iron Phases

There are two known phases for iron at the high pressure and temperature conditions relevant to the deep Earth.
Widely Accepted Phases
*Hexaferrum (ε-Fe)
(see article on hexaferrum iron)
Hexaferrum occurs at pressures greater than 10 GPa and temperatures greater than a few hundred K. It has a hexagonal close packed structure, so it is also known as HCP iron.
*Melt
Iron can also be found in the liquid form. The melting temperature is established to be around 2700-3000K at 100 GPa. Values at higher pressures and temperatures are generally extrapolated from this point. The stability of other phases, including face centered cubic (FCC) and high pressure body centered cubic (BCC) iron affect the location of the melting curve at more extreme conditions.
Possible Phases
*High Pressure BCC
A high pressure body centered cubic (BCC) phase was found to be stable at pressures around 225 GPa and temperatures around 3400K. It has been suggested to be responsible for a solid-solid transition seen in some studies ( Brown and McQueen), and was suggested to be a metastable transformation from HCP iron. It is also a possible component of the inner core.
Background
Phases of iron, that have been determined from experimental and theoretical studies, have powerful implications for understanding the composition and properties of the Earth’s lower mantle, inner, and outer core. These properties include mysterious features such as the core anisotropy, the amount and identity of light elements contained within, and ferromagnetism. Since these locations cannot be directly sampled, other methods to determine these properties have been used. Seismological evidence is an important source of information. It is complicated because it requires deconvolution due to seismic waves passing through multiple layers before detection. Changes in seismic velocities reflect changes in structure and chemical composition. They can be detected as heterogeneities in the data. Experiments attempt to reproduce the hydrostaticity, temperature uniformity, thermodynamic and chemical properties (Boehler) at the extreme conditions of the earth’s lower mantle, inner and outer core. Experiments may also define bounds for properties, such as the melting point of iron, and the stability of some phases.
Experimental Evidence
Methods
Two types of experiments can produce conditions necessary to determine the structure and properties of iron allotropes at high pressure and temperature conditions. While multi-anvil experiments are used to create high pressure conditions, they cannot reach the pressure-temperature regime necessary for experimentation (Boehler). Theoretical calculations suggest possible phases at particular conditions as well as phase changes, either through thermodynamic, first principles, or other approaches such as the embedded atom method. Diamond anvil cell experiments and shock wave experiments can reach the conditions necessary for experimentation. Shock experiments occur on short time scales and allow accurate measurement of mineral properties, such as sound velocities and temperatures. Due to the nature of experimentation, the pressure and temperature conditions are related. Experimental conditions are limited to following the Huguenot, an isentropic path. The Huguenot rises faster than the melting temperature, which makes determining the slope of melting curves demanding from this method. Shock experiments may have difficulty in measuring temperature due to the short time scales of experimentation. Laser-heated diamond anvil cell (DAC) experiments can reach temperatures around 7000K and pressures around 300 GPa, somewhat less than core conditions. DAC experiments have a limited sample size. Gradation in the temperature curves across the sample during experimentation can also produce overestimation of the melting temperature (Boehler). High pressures in DAC’s may also cause loss of hydrostaticity. DAC experiments have the advantage that pressure and temperature can be kept constant. Both types of experiments have the shortcoming that metastable states may be observed during experimentation. The inclusion of lighter elements, necessitated by density calculations, would alter the properties of mineral structures.
Applications
Transitions
*Melting Curve - The outer core has been observed to be liquid due to the attenuation of seismic shear waves passing through this layer. Therefore, the melting curve of iron at high pressures can suggest the temperature conditions at the liquid outer core - solid inner core boundary. The pressure at the boundary is suggested to be around 330 GPa. To determine the temperature at the boundary, the location of the second triple point must be determined (see Figure 1). This triple point is the transition between gamma (FCC) iron, epsilon (HCP) iron, and iron melt allotropes. It has been suggested to be at 88 GPa and 2800K. The location of the iron melting curve must be accurate because it is used to extrapolate to more extreme conditions via a Lindemann equation. A number of melting points varying from 3800 K at a pressure of 200 GPa from static experiments to 6720 K at 300 GPa from shock experiments have been found. Computational data generally agrees with the higher values from shock experimentation. However, some authors (Kombayashi) suggest the discrepancies may be due to problems with the equilibrium melting temperatures.
*FCC to HCP transition - The location of the FCC to HCP transition is controversial due to variation in the maximum stable pressure of FCC iron. The slope of the transition provides the location of the second triple point, but the value is controversial in static experiments. Mikhaylushkin’s calculations and DAC experiments suggest the transition can parallel the melting curve at higher pressures. Their quenched DAC experiments found mixed FCC and HCP phases at higher pressures. Free energy calculations suggest this is likely a metastable phase. These results are not included in the diagram.

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