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1. SAMPLE PREPARATION AND EXPERIMENTAL DETAILS
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The samples were prepared in a gas-mixing furnace by reduction of commercially available
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Fe2O3 (99.999% purity). To obtain the desired compositions, CO/CO2 gas mixtures were usedRS1.
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After heating the samples for 24 h, we quenched them in water to avoid precipitation of
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magnetite. The phase purity of the prepared material was tested using x-ray diffraction and
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Mössbauer spectroscopy. The samples contained a single phase of wüstite. We estimated the
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composition using a previously reported relation between the composition and the lattice
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constantsRS2. The lattice parameters of 4.2983(10) and 4.3056(10) Ǻ for the two different
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samples correspond to Fe0.925O and Fe0.94O, respectively. Some iron was added initially to the
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Fe0.94O sample as a marker for powder-diffraction measurements.
The samples along with a 4:1 deuterated methanol-ethanol mixture, used as a
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pressure-transmitting medium, were loaded into the sample chamber of a Paris-Edinburgh press
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mounted in a cryostatRS3 At 300 K, the liquid (methanol-ethanol mixture) provides satisfactory
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hydrostatic conditions in the pressure range under investigation (0 - 8 GPa). The data were
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collected at the D20 high-intensity diffractometer at the Institute Laue Langevin, Grenoble
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(France) using the wavelength λ = 1.36 Ǻ and an oscillating radial collimator to avoid a
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background signal from the cryostat. All pressure changes were carried out at ambient
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temperature where the pressure transmitting medium is known to be fluid. Data were then
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collected upon cooling at a rate of 0.126(1) K/min under constant load. The pressure values cited
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here were determined from the known equations of state of wüstite for different compositionsRS4.
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The uncertainty in the temperature determination was 5 K.
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Fig. S1 The Fullprof plot of the neutron powder diffraction data of Fe0.925O at 4.2 GPa and 140
K (gray dots). The simulated diffraction data based on the crystallographic model are shown as a
solid line. The tickmarks represent structural (upper tickmarks) and magnetic (lower tickmarks)
contributions in the diffraction pattern. The arrow marks the strongest magnetic peak.
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2. NEUTRON-DIFFRACTION DATA
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Collected neutron-diffraction spectra were analyzed using the FullprofRS5 software with a
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minimal set of variable parameters, namely, the lattice parameters, the overall temperature factor
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(Biso), the shape parameter U, the Lorentzian isotropic strain parameter X, and the value of the
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magnetic moment at iron sites. Fitting the data for the paramagnetic phase was accomplished
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using both cubic and rhombohedral (in hexagonal setting) models. The structure of the
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antiferromagnetic phase, ordered along the [111] direction, was described only by the
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rhombohedral model. Figure S1 shows an example of the Fullprof plot of the diffraction data of
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Fe0.925O (at 140 K and 4 GPa) after Rietveld refinement.
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For several sets of data collected for different compositions and P-T conditions, we
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determined TS (temperature of structural transition) and TN (temperature of magnetic transition),
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which can clearly be seen in the gray-color maps of 2Θ vs T for Fe0.925O (Fig. S2). At 8 GPa
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[Fig. S2(a)], the rhombohedral splitting almost coincides in temperature with the onset of the
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magnetic transition, while at 4.2 GPa [Fig. S2(b)], TN and TS are obviously different. The
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separation of TN and TS becomes even more pronounced after quenching from high to ambient
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pressure [Fig. S2(c)], when we observe a strong magnetic signal below about 198 K, but there is
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no sign of the structural transition at temperatures down to 110 K.
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Fig. S2 Gray color maps prepared based on
Fe0.925O diffraction patterns for selected 2θ
regions at different pressures: (a) 8 GPa, (b) 4.2
GPa, (c) 0 GPa. Lattice-plane indices are given
for the cubic structure. The magnetic reflection
(3/2 1/2 1/2) is marked as well.
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3. MAGNETIZATION MEASUREMENTS
The antiferromagnetic transition temperature of Fe0.925O (for an “as prepared” sample,
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prior to compression) was measured by use of a SQUID magnetometer (Quantum Design) at
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the Helmholtz-Zentrum Dresden-Rossendorf. The estimated temperature is TN = (200±5) K
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(Fig. S3).
Fig. S3 Temperature dependence of the magnetization of an “as prepared” Fe0.925O sample.
Red curve – data obtained during heating in an applied magnetic field of 50 Oe (fh) after
zero-field cooling (zfc). Blue curve – data measured during cooling in 50 Oe (fc).
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4. DEFECTS IN THE FexO STRUCTURE
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The cubic structure of FexO contains two major types of primitive defects (Fig. S4)RS6. (1)
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An interstitial Fe ion in a tetrahedral environment created by oxygen (blue sphere in Fig. S4),
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and (2) four octahedral vacant sites (red crosses) forming a tetrahedron filled with the interstitial
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Fe atom (red crosses on Fig. S4).
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In case of the order-disorder transition under high pressure, the blue atom occupies one of
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the empty neighboring octahedral sites. Upon pressure release the blue atom may return back to
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the interstitial site.
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Fig. S4 Structure of cubic FexO
with defects [after RS1]. Red
spheres mark oxygen, greenish
spheres denote iron on
octahedral sites. The blue sphere
is an interstitial Fe atom, the red
crosses mark surrounding
vacancies.
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5. REFERENCES
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RS1
L.S. Darken, and R.W. Gurry, J. Am. Chem. Soc., 67,1398-1412 (1945)
68
RS2
C.A. McCammon, and L.-G. Liu, Phys. Chem. Miner., 10, 106-113 (1983)
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RS3
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S. Klotz, Th. Strässle, G. Rousse, G. Hamel, and V. Pomjakushin, Appl. Phys. Lett., 86,
031917 (2005)
RS4
A. Kantor, I. Kantor, A. Kurnosov, L. Dubrovinsky, M. Krish, A. Bossak, and S.
Jacobsen, Appl. Phys. Lett., 93, 034106 (2008)
RS5
J. Rodríguez-Carvajal, Physica B, 192, 55 (1993);
http://www-llb.cea.fr/fullweb/powder.htm
RS6
P.D. Battle, and A.K. Cheetham, J. Phys. C.: Solid State Phys., 12, 337-345 (1979)