Hexaferrum and epsilon iron (ε-Fe) are
synonym
A synonym is a word, morpheme, or phrase that means precisely or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are a ...
s for the
hexagonal close-packed
In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement (or Lattice (group), lattice). Carl Friedrich Gauss proved that the highest average density – that is, the greatest fract ...
(HCP) phase of
iron
Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
that is stable only at extremely high pressure.
A 1964 study at the
University of Rochester
The University of Rochester is a private university, private research university in Rochester, New York, United States. It was founded in 1850 and moved into its current campus, next to the Genesee River in 1930. With approximately 30,000 full ...
mixed 99.8% pure
α-iron powder with
sodium chloride
Sodium chloride , commonly known as Salt#Edible salt, edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs a ...
, and pressed a 0.5-mm diameter pellet between the flat faces of two diamond anvils. The deformation of the NaCl lattice, as measured by
x-ray diffraction
X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. ...
(XRD), served as a pressure indicator. At a pressure of 13 GPa and room temperature, the
body-centered cubic
In crystallography, the cubic (or isometric) crystal system is a crystal system where the Crystal structure#Unit cell, unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals.
There ...
(BCC)
ferrite powder transformed to the HCP phase in Figure 1. When the pressure was lowered, ε-Fe transformed back to ferrite (α-Fe) rapidly. A
specific volume
In thermodynamics, the specific volume of a substance (symbol: , nu) is the quotient of the substance's volume () to its mass ():
:\nu = \frac
It is a mass-specific intrinsic property of the substance. It is the reciprocal of density (rho) ...
change of −0.20 cm
3/mole ± 0.03 was measured. Hexaferrum, much like
austenite
Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 ...
, is more dense than ferrite at the phase boundary. A
shock wave
In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...
experiment confirmed the diamond anvil results.
Epsilon
Epsilon (, ; uppercase , lowercase or ; ) is the fifth letter of the Greek alphabet, corresponding phonetically to a mid front unrounded vowel or . In the system of Greek numerals it also has the value five. It was derived from the Phoenic ...
was chosen for the new phase to correspond with the HCP form of
cobalt
Cobalt is a chemical element; it has Symbol (chemistry), symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. ...
.
[T. Takahashi & W.A. Bassett,]
High-Pressure Polymorph of Iron
" ''Science'', Vol. 145 #3631, 31 Jul 1964, p. 483–486.
The
triple point
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three Phase (matter), phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.. It is that temperature and pressure at ...
between the alpha, gamma and epsilon phases in the unary phase diagram of iron has been ''calculated'' as T = 770 K and P = 11 GPa, although it was determined at a lower temperature of T = 750 K (477 °C) in Figure 1. The
Pearson symbol
The Pearson symbol, or Pearson notation, is used in crystallography as a means of describing a crystal structure. It was originated by William Burton Pearson and is used extensively in Pearson's handbook of crystallographic data for intermetallic p ...
for hexaferrum is ''hP2'' and its
space group
In mathematics, physics and chemistry, a space group is the symmetry group of a repeating pattern in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of the pattern that ...
is ''P6
3/mmc''.
Another study concerning the ferrite-hexaferrum transformation
metallographically determined that it is a
martensitic rather than
equilibrium
Equilibrium may refer to:
Film and television
* ''Equilibrium'' (film), a 2002 science fiction film
* '' The Story of Three Loves'', also known as ''Equilibrium'', a 1953 romantic anthology film
* "Equilibrium" (''seaQuest 2032'')
* ''Equilibr ...
transformation.
While hexaferrum is purely academic in
metallurgical engineering
Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys.
Metallurgy encompasses both the ...
, it may have significance in
geology
Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...
. The pressure and temperature of
Earth's iron
core
Core or cores may refer to:
Science and technology
* Core (anatomy), everything except the appendages
* Core (laboratory), a highly specialized shared research resource
* Core (manufacturing), used in casting and molding
* Core (optical fiber ...
are on the order of 150–350 GPa and 3000 ± 1000 °C. An extrapolation of the austenite-hexaferrum phase boundary in Figure 1 suggests hexaferrum could be stable or metastable in Earth's core.
For this reason, many experimental studies have investigated the properties of HCP iron under extreme pressures and temperatures. Figure 2 shows the compressional behaviour of ε-iron at room temperature up to a pressure as would be encountered halfway through the outer core of the Earth; there are no points at pressures lower than approximately 6 GPa, because this allotrope is not thermodynamically stable at low pressures but will slowly transform into α-iron.
References
{{Reflist
Metallurgy
Iron
Steel