Discovery of the recoverable high-pressure
iron oxide Fe4O5
Barbara Lavinaa,b,1, Przemyslaw Derac, Eunja Kimb, Yue Mengd, Robert T. Downse, Philippe F. Weckf,
Stephen R. Suttonc, and Yusheng Zhaoa,b
a
High Pressure Science and Engineering Center, University of Nevada, Las Vegas, NV 89154; bDepartment of Physics and Astronomy, University of Nevada,
Las Vegas, NV 89154; cGeoSoilEnviroCARS, Center for Advanced Radiation Sources, University of Chicago, Argonne, IL 60439; dHigh Pressure Collaborative
Access Team, Carnegie Institution of Washington, Argonne, IL 60439; eGeosciences, University of Arizona, Tucson, AZ 85721-0077; and fDepartment of
Chemistry, University of Nevada, Las Vegas, NV 89154
Phases of the iron–oxygen binary system are significant to most
scientific disciplines, directly affecting planetary evolution, life,
and technology. Iron oxides have unique electronic properties and
strongly interact with the environment, particularly through redox
reactions. The iron–oxygen phase diagram therefore has been
among the most thoroughly investigated, yet it still holds striking
findings. Here, we report the discovery of an iron oxide with formula Fe4 O5 , synthesized at high pressure and temperature. The
previously undescribed phase, stable from 5 to at least 30 GPa,
is recoverable to ambient conditions. First-principles calculations
confirm that the iron oxide here described is energetically more
stable than FeO þ Fe3 O4 at pressure greater than 10 GPa. The
calculated lattice constants, equation of states, and atomic coordinates are in excellent agreement with experimental data, confirming the synthesis of Fe4 O5 . Given the conditions of stability and its
composition, Fe4 O5 is a plausible accessory mineral of the Earth’s
upper mantle. The phase has strong ferrimagnetic character comparable to magnetite. The ability to synthesize the material at
accessible conditions and recover it at ambient conditions, along
with its physical properties, suggests a potential interest in Fe4 O5
for technological applications.
solid Earth ∣ mineral physics ∣ extreme conditions ∣
density functional theory
I
ron oxides have a widespread occurrence, being composed of
the two most abundant elements on Earth. They are the most
common strong magnetic phases in nature and have vast technological uses including semiconductors, pigments, catalysts, and
iron extraction (1). Iron oxides are also important biominerals
and are used in biomedical applications (2). At ambient pressure,
six crystalline forms of iron oxides exist, including wüstite FeO,
magnetite Fe3 O4 , and four phases of Fe2 O3 (1, 3–7 and references therein). The basic atomic arrangements of these phases
are simple; however, they show extensive nonstoichiometry and
complex defect structures (8, 9) that complicate the understanding and modeling of their properties. The possibility to host
extensive amounts of defects explains their solid-state redox activity. For thousands of years iron oxides have attracted scientific
curiosity and challenged the modeling of their physical and
chemical properties, so it is compelling to report the discovery
of a phase that is not simply a polymorph of known iron oxides,
but a distinct compound with formula Fe4 O5 .
Synthesis and Structure of Fe4 O5
Fe4 O5 was first synthesized in the diamond anvil cell from the
breakdown products of siderite (FeCO3 ) at about 10 GPa and
1,800 K. A spherical crystal of about 8 μm in diameter grew in
less than a minute upon heating (Fig. 1). Diffraction patterns
were collected at high pressure and ambient temperature using
highly focused synchrotron X-rays. The single crystal diffraction
pattern (Fig. S1) was indexed with an orthorhombic C-centered
cell with a ¼ 2.8430ð7Þ Å, b ¼ 9.700ð8Þ Å, c ¼ 12.29ð6Þ Å.
Structural solution and refinement converged to a structure isowww.pnas.org/cgi/doi/10.1073/pnas.1107573108
Fig. 1. The single crystal of Fe4 O5 synthesized in the diamond anvil cell at
high pressure after laser heating. The sample chamber, about 60 μm in
diameter, viewed through a diamond anvil at 10 GPa and its sketch show
the rounded opaque crystal of Fe4 O5 grown in the heated area. The sample
is well separated from the gasket, ruby, and the anvils by the inert neon
medium.
morphic to CaFe3 O5 (10), with space group Cmcm; therefore, we
propose the crystal to be a distinct oxide of stoichiometry Fe4 O5
(Table 1). Refined and ab initio calculated atomic coordinates
at 10 GPa are in excellent agreement (Table 2); occupancies of
all sites are full within uncertainty (about 5%), supporting the
inferred stoichiometry. The atomic arrangement of Fe4 O5 can
be described as a stacking along the c axis of layers of edge-sharing FeO6 octahedra and layers of face-sharing trigonal prisms,
hosting Ca in the prototype phase (Fig. 2). There is close similarity between the structures of Fe4 O5 and h-Fe3 O4 , the highpressure polymorph of magnetite (11, 12). In the latter, edgesharing octahedral layers are made of equivalent sites, whereas
in the former two nonequivalent octahedral sites are arranged
in a thicker layer. The similarity in the stacking perpendicular
to the octahedral layers accounts for the similarity of two of the
cell parameters of the two oxides; the thicker octahedral layer
along the c axis in Fe4 O5 accounts for a c-cell parameter that
is about 3 Å longer than the analogous direction in h-Fe3 O4 .
After synthesis and without further heating, the compressibility
of the single crystal was measured up to 30 GPa (Fig. 3). The
single crystal c lattice parameter shows a much greater decrease
compared to powder diffraction data and first-principles calculations that are described later in this paper. We attribute this
behavior to nonhydrostatic conditions developed as soon as presAuthor contributions: B.L. designed research; B.L. and E.K. performed research; B.L. and
P.D. analyzed data; and B.L., P.D., E.K., Y.M., R.T.D., P.F.W., S.R.S., and Y.Z. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
1
To whom correspondence should be addressed at: High Pressure Science and Engineering
Center–Department of Physics and Astronomy, 4505 South Maryland Parkway, BPB 209
Box 454002, Las Vegas, NV 89154-4002. E-mail: lavina.b1@gmail.com.
This article contains supporting information online at www.pnas.org/lookup/suppl/
doi:10.1073/pnas.1107573108/-/DCSupplemental.
PNAS Early Edition ∣
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GEOPHYSICS
Edited by Alexandra Navrotsky, University of California, Davis, CA, and approved August 29, 2011 (received for review May 13, 2011)
Table 1. Summary of instrumental parameters and refinement
statistical parameters for the single crystal structural analysis
Beamline
13 IDD, GSECARS, APS, ANL
Wavelength, Å
Pressure, GPa
Temperature, K
Composition
Symmetry
Lattice parameters a, b, c, Å
Volume, Å3
Z
Rint
Refl. range
θ range, °
N. reflections
N. independent
N. I > 2σðIÞ
Refinement
R
wR2
0.3344
10
298
Fe4 O5
orthorhombic, Cmcm
2.8430 (7), 9.700 (8), 12.29 (6)
339 (2)
4
0.032
−2 ≤ h ≤ 2, −7 ≤ k ≤ 7, −2 ≤ l ≤ 4
3.5 ≤ θ ≤ 9.1
34
20
20
F2
0.050
0.134
sure was increased after the synthesis. The single crystal grew
at 10 GPa after the load was released from about 40 GPa. The
sample chamber became very thin in the new compression cycle,
because the gasket was deformed during the first compression
and the crystal likely bridged the anvils. The single crystal c axis,
nearly parallel to the load axis, shows therefore an apparent
greater compressibility than in the powder samples.
In order to explore the stability range of Fe4 O5 in a pure Fe-O
system, several syntheses were performed starting from mixtures
of iron and Fe3 O4 in appropriate proportions. Fe4 O5 was readily
synthesized at 10 and at 20 GPa, upon heating at 1,500–2,200 K.
In most cases the high-pressure phase shows a rather coarse grain
size (Fig. S2). Two samples were decompressed without further
heating to ambient pressure. Remarkably, we found that Fe4 O5
is a retrievable phase. One of the samples was heated during
decompression; at about 5 GPa we observed its breakdown to
magnetite and wüstite, showing that the observed phase has indeed intermediate composition between the two cubic oxides.
The sample homogeneity and chemical gradients were monitored
by collecting diffraction patterns in 2D grids across the sample
area before and after most heating cycles. The analysis of individual patterns shows variable mixtures of fine unreacted material and more coarse high-pressure phases, not systematic chemical
gradients. Because of the challenges in recovering and preparing
the samples synthesized at high pressure as well as in measuring
the Fe∕O ratio, we could not independently determine the com-
Fig. 2. Structure of Fe4 O5 . Green and blue octahedra (sites Fe1 and Fe2,
respectively) illustrate nonequivalent edge-sharing FeO6 groups forming
layers perpendicular to the c axis. Layers are alternated by channels where
iron, represented as red spheres (Site Fe3), is arranged in larger triangular
prisms sharing faces along the direction of the a axis.
position of the proposed phase. The bulk Fe∕O ratio was, however, monitored from phase abundances determined by Rietveld
refinement of the averaged patterns. Throughout the experiments the calculated Fe∕O ratio shows random variations
between 0.81 and 0.77, suggesting that no appreciable redox reaction occurred. Wide oscillation patterns were collected where
Fe4 O5 appeared to be the dominant phase and to have a finer
size. The pattern of a fine grain powder collected at 10.5 GPa
Table 2. Atomic fractional coordinates of Fe4 O5
Atom
Wyckoff position
Fe1
Fe2
4a
8f
Fe3
4c
O1
4c
O2
8f
O3
8f
Method
x
SXD
DFT
XRPD
SXD
DFT
XRPD
SXD
DFT
XRPD
SXD
DFT
XRPD
SXD
DFT
XRPD
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
y
0
0.2616(4)
0.2623
0.2657 (4)
0.5048(6)
0.4995
0.5038 (5)
0.158(3)
0.1567
0.174 (2)
0.365(3)
0.3681
0.3566 (12)
0.085 (3)
0.0838
0.090 (1)
z
0
0.1131(11)
0.1161
0.1160 (3)
1/4
1/4
1/4
1/4
1/4
1/4
0.55(1)
0.5375
0.5389(10)
0.643 (9)
0.6422
0.6372 (9)
SXD, X-ray structural refinement of single-crystal (10 GPa); XRPD, powder
diffraction data (10.5 GPa); DFT, ab initio calculation (10 GPa).
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Fig. 3. Compressibility of Fe4 O5 . Pressure dependence of lattice parameters
and volume of the unit cell. Data are from single crystal (open diamonds),
two separate powder diffraction experiments (red and green circles), and
from first-principles calculations (blue squares). Dashed black trends, interpolated from powder diffraction data, are reported as a visual aid. Standard
deviations are smaller than the symbol sizes.
Lavina et al.
Energetics and Stability of Fe4 O5
To the best of our knowledge, Fe4 O5 has not been previously
described. Therefore, complementary first-principles calculations
were carried out to examine the viability of Fe4 O5 in terms of its
energetics and phase stability under pressure. The enthalpies and
volume-compression data of FeO, h-Fe3 O4 [the high-pressure
polymorph of magnetite (11, 12)] and Fe4 O5 were calculated
using an effective Hubbard parameter of Ueff ¼ 2 eV for Fe-d
electrons (see Materials and Methods, Fig. S4, and Table S2).
The calculated lattice constants for Fe4 O5 at 0 K and 0 GPa
are a ¼ 2.88 Å, b ¼ 9.75 Å, c ¼ 12.58 Å, in excellent agreement
with the experimental values (Table S2). The calculated bulk
moduli of FeO and h-Fe3 O4 are 186.3 GPa and 184.4 GPa, respectively, in close agreement with previous studies (13, 14).
The calculated bulk modulus of Fe4 O5 is 185.7 GPa, in excellent
agreement with the predicted value using Vegard’s law (ca. 0.5%
deviation).
The enthalpy of Fe4 O5 is computed and compared with the
sum of the enthalpies of its possible breakdown products (FeO,
cubic and orthorhombic Fe3 O4 , Fig. 4), revealing that Fe4 O5 is
about 0.096 eV less stable than h-Fe3 O4 þ FeO and about
0.783 eV less stable than c-Fe3 O4 þ FeO at 0 GPa. However, as
pressure increases, Fe4 O5 becomes energetically more favorable
than the breakdown products (Fig. 4), providing complementary
evidence that the synthesis of Fe4 O5 is energetically plausible at
high pressures. At 11 GPa, the average lengths of the Fe-O bonds
are 1.944 Å (4a) and 2.076 Å (8f ) in the octahedra and 2.089 Å
(4c) in the trigonal prism. There is a significant charge transfer of
about 1.6 e from Fe atoms to O atoms. Density functional calcu-
Fig. 4. Enthalpies of Fe4 O5 and of the plausible breakdown oxides FeO and
h-Fe3 O4 . To examine the relative stability of Fe4 O5 with respect to its possible
breakdown products (FeO and Fe3 O4 in the ambient and high-pressure structures), the enthalpy differences, ΔHc ¼ HðFe4 O5 Þ − ½HðFeOÞ þ Hðc-Fe3 O4 Þ
and ΔHh ¼ HðFe4 O5 Þ − ½HðFeOÞ þ Hðh-Fe3 O4 Þ, were calculated as a function
of pressure. The solid, dashed, and dot-dashed lines represent the enthalpies
of Fe4 O5 , ΔHh (the reference), and ΔHc , respectively. ΔHc is extracted from
the literature (15). The Fe4 O5 structure is favored over the sum of breakdown
products at 10 GPa.
Lavina et al.
lations show that the proposed iron oxide is strongly ferrimagnetic (SI Text). The calculated lattice constants are a ¼ 2.84 Å,
b ¼ 9.55 Å, and c ¼ 12.34 Å at 11 GPa, close to experimental
values. The structural changes of Fe4 O5 under pressure can be
closely followed by comparing the volume-compression data
theoretically obtained from the first-principles calculations to the
experimental data. In order to simulate the compression under
hydrostatic conditions, all atoms and lattice constants were
allowed to relax at each pressure point. The agreement between
theory and experiment is remarkably good (cf. Fig. 3 and Table 2),
providing a clear confirmation of Fe4 O5 . Nevertheless, further
confirmation of its composition by direct methods will be
pursued.
Physics, Geophysics, Geochemistry, and Material Science
Implications.
The stoichiometry of the phase here described falls within the
most puzzling region of the Fe-O phase diagram, between wüstite
and magnetite (7, 16–19). With increasing temperature, at ambient pressure, the Fe-O phase diagram at 44 at. % Fe shows
coexistence of iron and magnetite up to about 900 K, wüstite and
magnetite up to 1,700 K, magnetite and liquid up to 1,900 K, and
liquid at higher temperature. Coexisting wüstite and magnetite
exhibit extensive degrees of nonstoichiometry and complex defect
structures, depending on the synthesis temperature and cooling
path. The two cubic phases share the same oxygen array and can
grow coherently (8, 20, 21), providing a nearly continuous range
of structures with random defect distributions, clustered defects,
and coherently grown distinct phases. The high-pressure behavior
of the binary phase diagram is relatively unexplored. With the
exception of the Fe-rich portion (22–24), relevant to planetary
cores, only the polymorphism of known oxides is well investigated. Wüstite is stable to very high pressure and temperature
(25), whereas magnetite undergoes a phase transition at about
10 GPa (26) into a nonrecoverable high-pressure phase having
strong similarities with the phase presented in this work. Our
experiment suggests that Fe4 O5 is stable from about 5 GPa to at
least 30 GPa. This phase, with lower symmetry, layered structure,
and all six-coordinated iron, is favored over the largely nonstoichiometric framework structures hosting four- and six-coordinated iron, showing that pressure changes the character of atomic
ordering of iron oxides in the most complex, intriguing region of
the binary phase diagram.
Density functional theory (DFT) calculations show that Fe4 O5
is ferrimagnetic. Although the high-pressure polymorph of magnetite is known to be a semiconductor (26, 27), a detailed account
of the electronic structure of Fe4 O5 is beyond the scope of this
study and will be the subject of a forthcoming investigation along
with experimental efforts. As in the isostructural calcium ferrite
(28), electron delocalization is expected among the edge-sharing
iron sites with short Fe-Fe distances (Fig. S5). Furthermore, in
Fe4 O5 strong interactions should be expected among the facesharing Fe3 sites. Given its distinctive physical properties, the relatively low pressure of synthesis and its recoverability, Fe4 O5 is a
potentially interesting material for technological applications.
The isomorphism with CaFe3 O5 suggests that the doping with
large cations is likely to decrease the synthesis pressure while
concurrently affecting physical properties.
The stability of oxides in the deep Earth is a complex function
of pressure, temperature, oxygen fugacity, bulk composition, and
partitioning equilibria among coexisting minerals. It is therefore
difficult to predict the occurrence of Fe4 O5 in the deep interior
without extensive investigations of systems with realistic composition. Nonetheless, it is important to note that Fe4 O5 , stable at
the P-T conditions of the Earth’s upper and lower mantles, is
more likely to occur than Fe3 O4 from the viewpoint of oxygen
fugacity given its more reduced composition. As geophysical research advances, the description of the Earth’s interior becomes
PNAS Early Edition ∣
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GEOPHYSICS
and that of a slightly textured sample collected at ambient pressure were selected for powder diffraction structural analysis
(Fig. S3). Atomic fractional coordinates were refined for the
pattern at 10.5 GPa; they were instead fixed to the values found
for the single crystal in the analysis of the ambient conditions
pattern. The Rietveld analysis converged with very satisfactory
statistical parameters (Table S1) in both cases. Calculated atomic
coordinates at 10.5 GPa are in good agreement with those from
the refinement of the single crystal diffraction data and from
first-principles calculations (Table 2).
increasingly complex, including structural, mineralogical, and
chemical lateral heterogeneity. Oxygen fugacity and bulk chemical fluctuations might likely determine variations in the stabilities
of accessory oxide minerals. The occurrence of mixtures of
wüstite and ferrite inclusions in diamonds (29) provides direct
evidence of the existence of compositions close to Fe4 O5 in
the Earth’s mantle. It follows that Fe4 O5 is a plausible accessory
phase of the Earth’s interior at least in locally Fe-rich portions at
highly zoned shallow depths. Being isostructural with CaFe3 O5 ,
Fe4 O5 and h-Fe3 O4 would likely host significant substitutions of
calcium and other large cations, not common in spinels and wüstite, representing a possible different scenario for the element
partitioning of iron oxides at depth. It should be noted that the
geophysical and geochemical importance of Fe4 O5 goes beyond
its occurrence or abundance, revising the iron–oxygen phase diagram, which is a fundamental reference for the thermodynamics
and oxygen fugacity of the deep Earth.
As all oxides of elements of multiple closely accessible valence
states, we expect Fe4 O5 to show defect structures depending on
pressure, temperature, and oxygen fugacity. In Fe4 O5 , in addition
to vacancies and interstitials that are common in the cubic oxides,
stacking disorder might occur. The Fe4 O5 and h-Fe3 O4 structures
have similar trigonal-prism units that are intercalated by octahedral layers of different thickness, and therefore stacking disorder
involving Fe4 O5 and h-Fe3 O4 octahedral layers is plausible. As
the structures of iron oxides are common to many binary systems
and complex solid solutions, we anticipate that a rich set of isostructural compounds and solid solutions with tunable properties
may be synthesized.
and 16IDB of High Pressure Collaborative Access Team, at the Advanced
Photon Source, Argonne National Laboratory. Double-sided heating was performed with fiber lasers (32, 33). The structure was probed in situ with X-ray
beams of 37 and 30 keV focused to 5 × 5 μm. Single crystal and powder
diffraction patterns were collected with MAR165CCD detectors. Diffraction
images were collected while the samples were rotated around the vertical
axis by 30° to 70° according to the limitations imposed by the angular opening of the diamond anvil cell. GSE_ADA, RSV (34), Shelxl97 (35), FIT2D (36),
General Structure Analysis System (GSAS) (37), and Endeavour (38) were used
for data processing.
First-principles total energy calculations were performed using spin-polarized DFT as implemented in the Vienna ab initio simulation package (39).
Structural and magnetic properties of Fe, FeO, h-Fe3 O4 , and Fe4 O5 were
calculated using the generalized gradient approximation (GGA) with the
parameterization of Perdew and Wang (PW91) (40). The plane-wave cutoff
energy for the electronic wave functions was set to 500 eV, ensuring the convergence of the total energy of the system to within 2 × 10−6 eV∕atom. A
periodic unit cell containing four formula units was used in the calculations
for FeO (B1), h-Fe3 O4 (Cmcm), and Fe4 O5 (Cmcm). The Brillouin zone was
sampled using the Monkhorst–Pack special k-point scheme (41) with a 3 × 3 ×
3 mesh for structural optimization and total energy calculations. In general,
the PW91 functional tends to underestimate the lattice constants of iron
and iron oxides; the GGA+U method was introduced in the total energy
calculations (42). The resulting lattice constants of Fe and magnetite are
close to the measured values when Ueff ¼ 2 (SI Text).
The first experiment, where the single crystal of Fe4 O5 was obtained, was
performed using a crystal of siderite (FeCO3 ) from Ivigtut, Greenland, as starting material. Several further syntheses were performed from mixtures of
pure commercial magnetite and iron fine powders. Samples were loaded
in diamond anvil cells along with the pressure gauges, e.g., gold (30) and
a ruby sphere (31), and with Ne as a pressure medium. Ruby and gold were
placed at the edge of the sample chamber; they were neither in contact with
the sample nor were they heated. Experiments were conducted at the stations 13IDD, of GeoSoilEnviroCARS, Center for Advanced Radiation Sources,
ACKNOWLEDGMENTS. We thankfully acknowledge the Department of Mineral Sciences, Smithsonian Institution for providing sample NMNH 17893-2 for
research. The authors thank the anonymous reviewers for their valuable
comments and suggestions. The University of Nevada, Las Vegas (UNLV)
High Pressure Science and Engineering Center is supported by Department
of Energy-National Nuclear Security Administration (NNSA) Cooperative
Agreement DE-FC52-06NA262740. This work was performed at GeoSoilEnviroCARS (Sector 13), and at High Pressure Collaborative Access Team (HPCAT)
(Sector 16), Advanced Photon Source (APS), Argonne National Laboratory
(ANL). GeoSoilEnviroCARS is supported by the National Science Foundation—Earth Sciences (EAR-0622171) and Department of Energy (DOE)–Geosciences (DE-FG02-94ER14466). HPCAT is supported by Carnegie Institution of
Washington, Carnegie/DOE Alliance Center, UNLV, and Lawrence Livermore
National Laboratory through funding from DOE-NNSA, DOE-Basic Energy
Services (BES), and National Science Foundation. APS is supported by
DOE-BES, under Contract DE-AC02-06CH11357. E.K. and P.F.W. acknowledge
funding through Subcontract 135947 with Los Alamos National Lab within
the Department of Energy’s Fuel Cycle Research and Development program.
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Supporting Information
Lavina et al. 10.1073/pnas.1107573108
SI Text
Single Crystal Structural Analysis of Fe4 O5. Diffraction data of the
single crystal synthesized at 10 GPa and 1,800 K were collected
at ambient temperature with the rotation method, ω-scan
(Fig. S1). The distribution of the peaks in the reciprocal space was
determined with GSE_ADA (1); indexing was performed with
RSV using 37 reflections.
Because of the small number of observed structure factors,
structural solution with direct methods failed. We therefore used
a simulated annealing strategy with equal weights of diffraction
data and simple repulsion potentials (2). The solution readily
converged to the structure type of CaFe3 O5 , space group Cmcm.
Refinements were performed using SHELXL97 (3); 37 reflections were merged into 20 symmetry independents, with excellent
agreement between equivalents ðRint ¼ 3.2%Þ. As input coordinates we used both those from the solution and those from the
density functional theory (DFT) calculations, in both cases the
refinement readily converged to the same solution. Because of
the limited number of observed structure factors, positional
parameters of oxygen and iron were refined in alternate cycles.
Refinement of occupancies, at fixed atomic position, indicated
full occupancy within uncertainty of all sites; their values were
therefore constrained to one in the final refinement. Displacement parameters were refined after convergence of the fractional
coordinate: The average values for Fe (0.04 Å2 ) and O (0.06 Å2 )
were fixed in the final refinement, where the scale factors and the
three symmetry unconstrained iron coordinates were variable.
Statistical parameters indicate good agreement between observed and calculated structure factors (R1 ¼ 5.0%, wR2 ¼ 13%,
Goof ¼ 1.32). The correction for extinction factor was unnecessary.
Effect of U on Iron Oxides Calculated Properties. We have carried out
a systematic study to investigate the effect of the U values for Fe,
Fe3 O4 , and Fe4 O5 using the spin-polarized DFT as implemented
1. Dera P (2007) GSE-ADA data analysis program for monochromatic single crystal diffraction with area detector (GeoSoilEnviroCARS, Argonne, IL).
2. Crystal Impact (2009). Endeavour 1.7. Crystal Impact GbR, Bonn, Germany.
3. Sheldrick GM (2008) A short history of SHELX. Acta Crystallogr A 64:112–122.
4. Kresse G, Furthmuller J (1996) Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys Rev B Condens Matter 54:11169–11186.
5. Dudarev S, Botton G, Savrasov S, Humphreys C, Sutton A (1998) Electron-energy-loss
spectra and the structural stability of nickel oxide: An LSDA+U study. Phys Rev B Condens Matter 57:1505–1509.
Lavina et al. www.pnas.org/cgi/doi/10.1073/pnas.1107573108
in the Vienna ab initio simulation package (4). The structural
optimization and total energy calculations were carried out using
the GGA+U (5) with the parameterization of Perdew and Wang
(PW91) (6) and the Monkhorst–Pack special k-point scheme (7)
with a 3 × 3 × 3 mesh. The results are summarized in Table S2.
The pressure dependence of lattice parameter of magnetite
(Fe3 O4 ) was calculated using the Ueff values in the range of
0–2 eV and compared to experimental data (8, 9) as shown in
Fig. S4. Here, Ueff ¼ U − J when J ¼ 0 eV.
Synthesis and Powder Diffraction Analysis of Fe4 O5. The samples
were analyzed after heating by collecting diffraction patterns
in 2D grids covering the entire sample area in order to test
the samples homogeneity. Wide angle exposures were collected
where finer particle size was found in order to obtain the most
reliable intensities. Fe4 O5 was the dominant phase in every synthesis although minor iron oxides (FeO, Fe3 O4 , and/or Fe2 O3 ) were
also found. Some extent of sample heterogeneity is to be expected
when loading a small amount of powder mixture in the diamond
anvil cell. We selected two single-phase patterns of the sample
quenched to ambient conditions (Fig. S2) and of a different sample at 10.5 GPa for the Rietveld analysis performed with General
Structure Analysis System (GSAS) (10). The background function was fitted with a 11-parameter polynomial. Peak profiles
were fitted with the pseudo-Voigt function. Peak profile parameters were allowed to vary one by one until convergence. Lattice
parameters and atomic positional parameters were allowed to
vary simultaneously in the final refinement. The refinement of
displacement parameters was unstable; therefore isotropic displacement parameters of all atoms were set to U ¼ 0.02 Å2 .
The final refinement converged to Rp ¼ 1.2% and Rwp ¼ 2.5%.
Refined atomic coordinates (Table 2) are in good agreement with
the results from single crystal structural refinement and ab initio
calculations, taking into account the pressure difference.
6. Perdew J, Wang Y (1992) Accurate and simple analytic representation of the electrongas correlation-energy. Phys Rev B Condens Matter 45:13244–13249.
7. Monkhorst HJ, Pack JD (1976) Special points for Brillouin-zone integrations. Phys Rev B
Condens Matter 13:5188–5192.
8. Kittel C (1986) Introduction to Solid State Physics (Wiley, New York).
9. Klotz S, Rousse G, Strässle Th, Bull CL, Guthrie M (2006) Nuclear and magnetic structure
of magnetite under pressure to 5.3 GPa and at low temperatures to 130 K by neutron
scattering. Phys Rev B Condens Matter 74:012410.
10. Larson A, Von Dreele R (2000) General structure analysis system (GSAS). Los Alamos
National Laboratory Report LAUR 86–748.
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Fig. S1. Diffraction pattern of the single crystal of Fe4 O5 in the diamond anvil cell at 10 GPa collected with the rotation method (32° oscillation around the
vertical ω-axis). Data were collected at GSECARS (station 13IDD), APS with a beam focused to 5×5 μm of 0.3344 Å wavelength.
Fig. S2. Ambient conditions diffraction image of the sample of Fe4 O5 synthesized in the diamond anvil cell at 20 GPa and 2,000 K. The pattern was collected
rotating the sample by 70° around the vertical ω-axis. The diamond anvils are responsible for the strong single crystal diffraction peaks. The pattern was
collected at HPCAT (station 16IDB) with an X-ray beam of 0.393545 Å wavelength.
Lavina et al. www.pnas.org/cgi/doi/10.1073/pnas.1107573108
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Fig. S3. The powder patterns were integrated after masking diamond peaks. Observed intensities are shown with black symbols; calculated patterns are
shown in red; the difference patterns are shown in blue.
Fig. S4. Compressibility of magnetite. The experimental data were extracted from Klotz et al. (9).
Lavina et al. www.pnas.org/cgi/doi/10.1073/pnas.1107573108
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Fig. S5. In this representation of the Fe4 O5 structure, iron–iron distances shorter than 3.2 Å (calculated for the single crystal data at 10 GPa) are shown. (light
gray: oxygen atoms; blue, red, and green: Fe atoms in 8f, 4c, and 4a, respectively).
Table S1. Summary of instrumental parameters and refinement statistical parameters of the
powder diffraction structural analysis
Beamline
Wavelength, Å
Pressure, GPa
Temperature, K
Composition
Symmetry
Lattice parameters a, b, c, Å
Volume, Å3
θ-range, °
Z
Rp
Rwp
16 IDB, HPCAT, APS, ANL
16 IDB, HPCAT, APS, ANL
0.393545
1 10−5
298
Fe4 O5
orthorhombic, Cmcm
2.8933 (3), 9.8092 (12), 12.583 (2)
357.1 (1)
2 ≤ θ ≤ 10.6
4
0.012
0.026
0.397728
10.5
298
Fe4 O5
orthorhombic, Cmcm
2.8326 (1), 9.6276 (5), 12.2920 (6)
335.22 (2)
1.5 ≤ θ ≤ 10.2
4
0.006
0.008
Table S2. The calculated lattice constants, bulk modulus, and magnetic moments of Fe, magnetite (Fe3 O4 ), and Fe4 O5
Ueff , eV
Fe
Magnetite (Fe3 O4 )
Fe4 O5
lattice constant, Å
bulk modulus, GPa
magnetic moment, μB
lattice constant, Å
bulk modulus, GPa
magnetic moment, μB
lattice constant, Å
0
2
4
Expt.
2.83
207
2.16
8.37
173
3.51 (8a) −3.37 (16d)
2.89
169
2.73
8.43
175
3.80 (8a) −3.78 (16d)
2.88 9.75 12.58
2.94
141
3.01
8.47
2.87*
168*
2.22*
8.39†
180†
4.20 (8a)†-3.97(16d)†
2.89 (this work); 9.81 (this work);
12.58 (this work)
bulk modulus, GPa
magnetic moment, μB
185.7
1.20 (4a) 3.80 (8b) 3.89 (4c)
Here, Ueff ¼ U-J when J ¼ 0 eV.
*Kittel (8).
†
Klotz et al. (9).
Lavina et al. www.pnas.org/cgi/doi/10.1073/pnas.1107573108
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