The wüstite enigma

PttYSI(S O F T t l E EARTH AN D PLAN ETARY I NTE R IORS

ELSEVIER

Physics of the Earth and Planetary Interiors96 (1996) 135-145

The wi stite enigma Ho-kwang Mao *, Jinfu Shu, Yingwei Fei, Jingzhu Hu, Russell J. Hemley Geophysical Laboratory and CenterJbr High Pressure Research, Carnegie Institution of Washington, 5251 Broad Branch Road. N. W.. Washington, DC 20015-1305, USA

Received 28 August 1995; revised 11 October 1995; accepted 13 November1995

Abstract

High-pressure energy dispersive X-ray diffraction of wi~stite has been obtained with three types of diamond cells; each one is designed to optimize a type of in situ study, namely single-crystal X-ray diffraction, deviatoric strain measurement, or simultaneous high-P-T experimentation. The results demonstrate that above 17 GPa at 300 K wiastite undergoes a displacive transition from B 1 to a rhombohedral structure, and above 90 GPa at 600 K, a second transition to the NiAs structure. Many of the earlier inconsistencies concerning the structure and properties of wiistite at high pressures can be attributed to the extreme softening of the C44 elastic modulus at pressures above 10 GPa.

1. Paradoxes Under ambient conditions, wiistite crystallizes in the NaC1 (B1) structure with a nonstoichiometric formula of Fe I _ xO where x is variable up to 0.12. (Hereafter Fel_xO is designated FeO.) Ringwood (1977) proposed a compositional model of the Earth's core in which FeO was the major light component. FeO is also an end-member of magnesiowtistite, an important phase in the lower mantle. This makes FeO possibly the only common major component of both the metallic core and the oxide mantle. Studies of the phase transitions, oxidation-reduction, disproportionation and metallization of wiistite are thus central to understanding core formation, core-mantle interaction, and inner-core solidification (Dubretsev and Pankov, 1972; Bullen, 1973; Mao, 1974; Stevenson, 1981). A number of apparent paradoxes have

' Correspondingauthor.

emerged from the extensive studies of wfistite (see Hazen and Jeanloz, 1984; Yagi et al., 1985; Jackson et al., 1990; Knittle and Jeanloz, 1991). Indeed, a comprehensive description of its high-pressure behavior has been elusive. Recent experimental results shed new light on the nature of this enigmatic material, and an overview is presented below. More extensive experimental data and detailed descriptions of the techniques will be published elsewhere. 1.1. Equation o f state The P - V equation of state of wiistite has been studied with various high-precision techniques, including single-crystal X-ray diffraction (Hazen, 1981), polycrystalline X-ray diffraction with hydrostatic and quasihydrostatic pressure media (Mao et al., 1969; Jeanloz and Sato-Sorensen, 1986; Liu and Liu, 1987), ultrasonic-wave velocity measurement (Sumino et al., 1980; Bonczar and Graham, 1982; Jackson et al., 1990), and shock-wave compression

0031-9201/96/$15.00 Copyright © 1996 Published by Elsevier Science B.V. All rights reserved. PII S0031-9201(96)03146-9

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(Jeanloz and Ahrens, 1980). The resulting values reported for the zero-pressure bulk modulus, K 0, however, range from 142 to 182GPa. Whether this large discrepancy is due to the material properties or to experimental techniques has been the subject of some debate. For example, Mao (1974) and McCammon (1993) proposed a correlation between K 0 of Fel_~O and its non-stoichiometry (x). Jeanloz and Hazen (1983) maintained that such correlation does not exist. The latter workers, instead, ascribed the difference to a finite bulk viscosity of wfistite that causes the value of K 0 obtained from dynamic (ultrasonic and shock-wave) measurements to be intrinsically higher than that obtained from static condition (P-V) measurements. Nevertheless, some recent dynamic ultrasonic measurements gave K 0 values as low as 151GPa (Jackson et al., 1990), and there are also some static data scattered in the P-V region corresponding to K 0 = 180GPa (Jeanloz and Sato-Sorensen, 1986).

the rhombohedral cell were not given by either Zou et al. (1980) or Yagi el al. (1985). They attributed the d spacing misfit to the effect of non-hydrostatic stress. However, the magnitude of the misfit is unprecedented. The systematic errors m d (the large observed value for 003 and small value for 102) were as great as 20 times the standard deviation (%Jd = 0.001). For other materials, non-hydrostatic stress effects on X-ray diffraction measurements on diamond-cell samples rarely exceed A d / d = 0.003. Yagi et al. (1985) proposed that the discrepancy might indicate a further distortion from the rhombohedral symmetry, i.e. the assignment of the rhombohedral cell is questionable. Above 40GPa, Yagi et al. (1985) observed a sign of further splitting of the rhombohedral 104 line. Does this signify a transition to yet another lower symmetry phase? The intensity misfit was attributed to a preferred orientation. Direct evidence is needed to verify the orientation dependence on the stress field.

1.2. Rhombohedral transition

1.3. C44 softening

Zou et al. (1980) reported a phase transition in wiJstite at 9GPa and room temperature based on polycrystalline X-ray diffraction and M6ssbauer spectroscopy measurements. The transition is characterized by a broadening and, at higher pressures, a splitting of the 111 diffraction line with the 200 line remaining a sharp singlet. Meanwhile a six-peak, hyperfine magnetic structure develops in the MiSssbauer spectrum. By analogy with the zero-pressure antiferromagnetic transition of FeO at 198K (Willis and Rooksby, 1953), the high-pressure phase was interpreted as a rhombohedral distortion of the face-centred cubic (fcc) unit cell by elongation of a body diagonal [111] and contraction of the other three body diagonals. Static compression of polycrystalline wiistite was later carried out by Yagi et al. (1985) at 25°C up to 120GPa using X-ray diffraction. They observed the same transition at 16GPa, and also attributed the high-pressure phase to a rhombohedral distortion based on the splitting of 111,220 and 311 lines, and non-splitting of 200. A serious problem arises with this assignment. The misfit of interplanar spacings (d) and intensities of the observed peaks to a rhombohedral phase were so large that high-pressure unit-cell parameters for

Jackson et al. (1990) pointed out that at low pressure wiastite has a very low value of the elastic modulus C44 (45.5GPa) in comparison with Cl2 (123.0GPa) and the difference increases with pressure with dCa4/dP = - 1 . 0 3 and d C j J d P = 2.77. This represents an extreme deviation from the Cauchy relation (C12 C44 at zero pressure) and elastic isotropy (Cll = C~2 + 2C44). As C44 measures the body-diagonal stiffness but Ct~ and C~2 measure the axial stiffness under compression, the 'soft' C44 favors the deformation along the body diagonals. Would the progressive softening of the C4a eventually destablize the B1 structure and facilitate the transition to the rhombohedral phase? Would the 'soft' C~ continue to dominate the elastic properties of FeO after the transition? =

1.4. Electronic transition At low pressures, non-metallic FeO forms an immiscible liquid with metallic iron. Ringwood (1977) and Dubretsev and Pankov (1972) proposed that metallization of FeO at high pressures will make it readily soluble in liquid iron. Indeed, shock-wave and laser-heated diamond-cell experiments indicate

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H.-k. Mao et al./ Physics of the Earth and Planetary Interiors 96 (1996) 135-145

that FeO transforms to a denser metallic phase above 70GPa (Jeanloz and Ahrens, 1980; Knittle and Jeanloz, 1986; Knittle et al., 1986). The mechanism of the metallization has been debated; i.e. whether it is due to pressure-induced spin-pairing (Jackson and Ringwood, 1981; Jackson et al., 1990) or a band overlap Mott transition (Knittle et al., 1986). M~Sssbauer spectroscopy shows a loss of magnetic moment at the Fe site between 65 and 100GPa at 300K (Pasternak et al., 1993; Jeanloz et al., 1995). The nature and relationship between the low-temperature magnetic transition and the high-temperature metallization still need to be clarified. 1.5. B 2 vs. B 8 structure

We may well ask whether the crystal structure changes at the electronic transition. What is the nature of the high-pressure structure? It may be analogous to that of most alkali halides, whereby the NaCI (B1) structure transforms to the CsCI (B2) structure at high pressures (Bassett et al., 1968; Jeanloz, 1982). On the other hand, for transition metal chalcogenides, the high-pressure structure is often NiAs (B8) type, and FeO may therefore behave likewise. The two alternatives have been debated on the basis of thermochemical systematics (Navrotsky and Davies, 1981), the mechanism for metallization and bonding (Knittle and Jeanloz, 1991), and the volume change at the transition (Jackson and Ringwood, 1981). Jeanloz and Ahrens (1980) reported a 4% density increase at the transition. Jackson and Ringwood reassessed the shock-wave data of Jeanloz and Ahrens and obtained a zero-pressure density

increase of the transition ( A g / p ) 0 of at least 10- 16% and possibly as great as 18-28%. They concluded that the density increase appeared too large for B1B2 and therefore favored the B1-B8 interpretation.

2. Critical tests Three sets of high-pressure X-ray diffraction experiments have been designed to answer the above questions--to understand the wt~stite enigma. In one set, we eliminated the effect of non-hydrostatic stress by studying a single-crystal wtistite sample immersed in an essentially hydrostatic helium medium. In the second set, we characterized the non-hydrostatic stress effect by measuring the uniaxial strain ellipsoid for a polycrystalline wfistite sample. In the third set, we studied polycrystalline samples over an extended P - T range. Energy dispersive X-ray diffraction (EDXD) measurements were performed by employing polychromatic X-radiation from the superconducting wiggler beamline X-17C of the National Synchrotron Light Source (NSLS). The intriguing transition sequence under investigation, B l-rhombohedral-B8, is illustrated in Fig. 1. The B l-rhombohedral transition is purely distortional. By compressing three [111] body diagonals relative to the fourth one, the cubic symmetry is lowered to rhombohedral. The fourth body diagonal becomes the unique c-axis which is perpendicular to alternating Fe and O layers with the stacking sequence of ABCABCA (Fig. 1). The shortest Fe-Fe or O - O distance in the hexagonal close-packed layers is parallel to the new a-axis.

~c

~c

C

-C -A

i

-B

-A-I

-A

I .a~_a---i~

Face-centered cubic

Rhombohedral

NiAs

Fig. 1. Progressivetransformationsin the high-pressurephases of FeO (Q, Fe, O, O). From left to right: cubic NaCI (B1), rhombohedral. highly distortedrhombohedral,and NiAs (B8) structures.

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The rhombohedral-B8 transition requires displacements between layers. In the NiAs (B8) structure, the stacking sequence is ABACA with Fe layers fixed at the A and O layers alternating between B and C (Fig. 1). The transition reduces the Fe-Fe distance not only within a layer but also between layers because Fe atoms stack directly on top of each other.

3. Single-crystal wiistite in near-hydrostatic helium medium Previously the B l-rhombohedral transition was studied with polycrystalline X-ray diffraction, which only provides information on d values and intensities of the diffraction peaks. Four lines for the cubic (111, 200, 220, 311) and seven lines for the nominally rhombohedral (003, 101, 102, 104, 110, 105, 113) phases were observed by Yagi et al. (1985). As both d values and intensities fit poorly to a rhombohedral unit cell, this structure assignment has been questionable. On the other hand, single-crystal diffraction can be used to determine the structure unequivocally, because it provides information on reciprocal lattice angles as well as d spacings. Different reflections of the same class (e.g. 111 and 111) have the same d value, but are separable because they appear at different orientations. Previous study of single-crystal diffraction of wi]stite was

-

limited to 15 GPa, and no phase transition was observed (Hazen et al., 1981). Above 15GPa, Hazen et al. observed irreversible changes in wi~stite, probably because the neon pressure medium became less hydrostatic at higher pressures, or because the wi]stite single crystal was too large and bridged the diamond anvils when the gasket thinned above 15 GPa. We studied single-crystal X-ray diffraction of wiistite under improved hydrostatic conditions in a diamond cell up to 30GPa. The sample was a 15 txm x 151~mX51xm single crystal of Fe0.940 wtistite surrounded by a helium pressure-transmitting medium filling the sample chamber (100 I~m diameter × 20 ~m thickness). The chamber is sufficiently larger than the sample that bridging did not occur, and helium remains softer than neon at high pressures (Bell and Mao, 1981). As the diamond cell is rotated around the × and o~ axes, EDXD peaks are obtained through a 90 ° conical opening of the cell. The experimental setup is shown in Fig. 2. The wfistite sample remained a single crystal with the cubic B1 structure up to 17GPa. Forty-five diffraction lines were measured in 16 classes: 111, 200, 220, 311, 222, 400, 331, 420, 333, 440, 600, 622, 444, 800, 662, and 840. The cubic lattice parameters calculated from each of the 45 diffraction lines were identical within the experimental uncertainty (Aa/a=O.O005). Above 17GPa, the 16 classes of the cubic phase split into the 32 classes for a rhombohedral phase (hexagonal index) as 003-101,

20

S1 Y A

--Z

.- X

Fig. 2. Single-crystal energy dispersive X-ray diffraction (EDXD) at high pressures is collected with the above layout. A diffracted beam at a variable 20 angle is collimated by two slits, DI and D2. The FeO single crystal in a diamond cell is brought to the center of the diffraction axis by X - Y - Z translations, and moved to diffraction orientations by X-to rotations.

H.-k. Mao et al. / Physics of the Earth and Planetary Interiors 96 (1996) 135-145

139

cubic unit cell becomes the c-axis of the rhombohedral cell, and 220 becomes the a-axis (Fig. 1). The rhombohedral distortion proceeds with the c:a ratio starting at the ideal cubic value of ~/6 and increasing continuously with increasing pressure. The distortion greatly reduces the a-axis, which corresponds to the Fe-Fe distance in a layer. We conjecture that the shortening of Fe-Fe distance increases the electrical conductivity and reduces the magnetic moment of FeO (Isaak et al., 1993; Sherman and Jansen, 1995).

102, 104-110, 105-113-201, 006-202, 204, 107205-211, 116-212, 009-303, 208-220, 306, 2 0 10 -226-402,0012-404,408,2014-4010 - 4 2 2, 2 2 12 - 4 2 4; the classes connected by dashes denote multiplets originating from the same class in the cubic phase. Multiple diffraction peaks belonging to different classes in the rhombohedral phase differ in energy (or d value) and in X and to angles. As shown in Fig. 3 by way of example, 111 of the cubic phase splits into 003 and 101 of the rhombohedral phase. The observed and calculated d values are in very good agreement ( A d / d = 0.0005), thus establishing that transition to the rhombohedral phase has indeed occurred under these nearly hydrostatic conditions. The transition is completely reversible and appears to be second order. The 111 spacing of the

4. The effect of uniaxial stress

In the absence of hydrostatic media, increasing load in a diamond cell exerts a uniaxial stress with the maximum ((ra) in the axial direction (A in Fig.

d-spacings (A) 3.167 T

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10

15

20

25

30

35

_1

0

15

20

25

30

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Energy (Key)

Fig. 3. Single-crystal EDXD patterns for the cubic (BI) phase of wiistite at 5.38GPa are show at the left. The 111 and 111 diffractions observed at different to and × have identical energy (or d value, Ed = 47.5 keV-A). When the pressure is raised to 22.60GPa as shown at the right, these two peaks become the 003 and 101 of the rhombohedral phase, respectively. Their d values are clearly different, although the orientation of the crystal in the diamond cell has changed slightly (approximately 5 °) between the two pressures. The relative intensities of the fundamental and its overtone (e.g. I 11 and 222) reflect the energy profile of the synchrotron and the absorption characteristics of the diamond cell. Other unmarked weak peaks are either escape peaks that occur at 9.886 keV (GeK,~) below the intense single crystal lines or polycrystalline gasket diffraction peaks which are observed to be independent of to and X.

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H.-k. Mao et a l./Physics of the Earth and Planet.ry Interiors 96 (1996 ) I35-145

4) and the minimum (or R) in the radial direction. The corresponding lattice strains measured by X-ray diffraction of polycrystalline samples have been treated on the basis of anisotropic elasticity theory by Singh (1993) for cubic crystals and by Singh and Balasingh (1994) for hexagonal crystals. In previous studies of polycrystalline wtistite, X-ray diffraction was obtained with the incident X-ray beam coaxial with the diamond cell axis (parallel geometry in the study by Singh (1993)), i.e. the direction of diffraction vector (D) nearly perpendicular to A (the angle between A and D, X = 9 0 ° - 0, where 0 = 3-20°). This geometry reveals only a limited segment of the strain field in the vicinity of the radial direction (X = 70-87°) • Kinsland and Bassett (1976) and Funamori et al. (1994) have developed techniques with the incident X-ray beam perpendicular to A so that the strain field can be sampled from X = 0 to X = 90°. In the present study, we used a new technique to sample the entire strain field (Figs. 4 and 5). The diamond cell is mounted with R coaxial with the x-rotation,

A !

Z=0 o A

R~

20

.J

i -/-

\,D

2o

Fig. 5. The angles (X) between the diffraction direction (D) and the uniaxial stress direction (A) can be varied continuously by rotation of the diamond cell around the R axis.

A

R

inc x

cted Ly

incident x-ray Fig. 4. Sketch of diamond-anvil cell for polycrystalline X-ray diffraction studies.

and the x-axis is set (by rotating to) to bisect the 20 angle between incident and diffracted X-ray beams (see Fig. 2 for the relation of X and to). As the diamond cell is rotated around the R-X axis, strains of each hkl can be measured directly as a function of X- A 100 Ixm × 100 Ixm × 25 Ixm platelet of polycrystalline Fe0.940 wiistite was compressed without a pressure medium in the diamond cell up to 30GPa. (The sample is surrounded by a solid boron gasket which is transparent to the synchrotron X-radiation.) A gold foil of 2 Ixm thickness was placed between the diamond and the sample. EDXD patterns of FeO and gold are collected at every 10° step of X. Representative d values calculated from various diffraction lines as a function of X are shown in Fig. 6. The maximum differential stress, 2~cr =~r A -orR, depends upon the shear strength of the material. Because of the low strength of gold, it sustains low Act. As shown in Fig. 6(a), the differential strains between 0 ° and 90 ° and among 111, 200, and 311 directions are negligible (hd/d = 0.002) for gold at 12.2GPa. Pressures were determined based on the

H.-k. Mao et al. / Physics o f the Earth and Planetary Interiors 96 (1996) 135-145

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Fig. 6. Values of d spacings of polycrystalline samples are shown as functions of × (defined in Fig. 5): (a) gold at 12.2 GPa; (b) B l-w~stite at 12.2GP~ (c) rhombohedrai FeO at 25.8GPa. The gli~hes at 70 ° and 80° in the doo3 and d]0 n of (c) are due to measurement uncertainties.

142

H.-k. Mao et al. / Physies of the Earth and Planetary Interiors 96 (19961 135 145

averaged lattice parameter of gold calculated from all diffraction lines at all X values (Anderson et al., 1989). Wiistite has sufficient strength to produce a significant A~r, which causes a differential strain between different × angles. For elastically isotropic materials, the maximum differential strain, A~ = (d90odoo)/d, is identical for all hkl values. For a given angle of X, the unit-cell parameters calculated from the observed d of different [hkl] should be in good agreement. However, for elastically anisotropic materials such as wi~stite, large systematic deviations are observed for different hkl values. Fig. 6(b) shows wiastite data at 12.2GPa gold pressure (which corresponds to the ~rA of wiistite). Clearly, the maximum differential strain varies greatly with hkl, e.g. the strain in the body diagonal of the cubic crystal, Ael~ ~, is five times greater than the strain along the crystallographic axis, Ae200. Although calculations of the absolute value of Cij require an independent determination of the stress ellipsoid, the relative values of Cij can be calculated based on measurement of the strain fields of different hk! values. The results indicate an extremely soft diagonal shear modulus, C44, and a stiff axial modulus, C~l, consistent with the extrapolation of the low-pressure observation of low C44 and negative dCaa/dP (Jackson et al., 1990). We conclude that the drastic softening of the C44 shear mode creates a large difference among the body diagonals in a non-hydrostatic stress field and drives the rhombohedral distortion. The orientation of the crystal in the rhombohedral phase is strongly related to the stress direction that drives the transition. Intensities of reflections of planes close to the c-axis (e.g. 003 and 104) are highest near × = 90 °, decrease rapidly with decreasing X, and diminish at X = 40°, indicating that the elongated c-axis is preferably oriented along the direction of minimal stress. Fig. 6(c) shows differential strains at crA = 25.8 GPa. The very large A~ in 003 and in 101 diffractions (split from the cubic 111) indicate that the 'cubic' C44 continues to soften after the phase transition. (The cubic C44 corresponds to the rhombohedral C~ and C33 after the transformation of axes.) Calculation of unit cell parameters based on d values observed at X = 900 - 0 would account for the very large discrepancy between the results of Zou et al. (1980) and Yagi et al. (1985).

This effect could also cause large uncertainty in P-V measurements and in reported values of K 0 in the cubic phase. At X = 0°, where the diffraction direction coincides with A, the strain of each [hkl] is uniquely defined by the uniaxial stress field. At other angles of ×, however, the measured strain of each [hkl] also depends upon the orientation of the crystallite relative to A. For example, for a cubic crystal with 220 diffraction at X = 90 °, the stress axis, A, could be parallel to either [111] or [002]. Owing to the strong elastic anisotropy of wfistite, the [220] strains at 90 ° under the two scenarios are different, and under extreme anisotropy may even split into two lines. This behavior provides an alternative explanation of the observed splitting of the rhombohedral 104 line (originating from the cubic 220) above 40GPa, which has been interpreted as a further distortional transition of the rhombohedral unit cell (Yagi et al., 1985).

5. High-P-T phase relations Using a resistance-heated diamond cell (Mao et al., 1991), the phase diagram of FeO (Fig. 7) has been determined up to 100GPa and 1100K (Fei and Mao, 1994). Phases were identified via polycrystalline EDXD measurements. The B l-rhombohedral boundary, which is characterized by the splitting of 111, 220, and 311 diffraction lines, fits the linear relation P(GPa) = - 5.0 + 0.070T(K) between 300 and 700K. Extrapolated to zero pressure, this line yields 71 K. This value is much lower than that observed for the antiferromagnetic rhombohedral transition at 198 K (Willis and Rooksby, 1953). The difference indicates that either the Clapeyron slope d P / d T flattens significantly below 300 K or that the observed high-pressure and the low-temperature rhombohedral phases are intrinsically different. At higher P-T, a first-order phase transition was observed. The d values of the new diffraction pattern are consistent with FeO adopting an NiAs structure (Fei and Mao, 1994). The relative intensities, however, deviate from randomly oriented polycrystalline FeO with an NiAs structure, indicating that the preferred orientation of the rhombohedral phase carries through to the NiAs phase. As the transition

H.-k. Mao et al. / Physics of the Earth and Planetary Interiors 96 (1996) 135-145 2300

FeO

1800

NiAs type (B8)

1300

• NaCI type

(a

I

800

30(

(B1)

Pasternak et al. (1993) and Jeanloz et al. (1995) between 65 and 100GPa is a transition within the rhombohedral phase, rather than a transition to the metallic NiAs phase. This explanation is consistent with the decrease in magnetic moment predicted for rhombohedral FeO under pressure (Isaak et al., 1993).

shock wave data

E /

/

•

AN(X

.

6. Conclusion

,"h*mboh''a!2 \ 20

40

60

143

80

100

120

140

Pressure, GPa Fig. 7. Phase diagram of FeO. The boundary between B l ( [] ) and rhombohedral (zx) phases is established with reversible transitions. The boundary between rhombohedral and B8 ( 0 ) phases is determined only with transitions on the paths of increasing P - T. Both boundaries are determined by using a resistance-heated diamond cell (Mao et al., 1991; Fei and Mao, 1994. The highertemperature B 1-B8 boundary is inferred from shock compression (C), Jeanloz and Ahrens, 1980) and of laser-heated diamond-anvil cell experiments (arrowheads, Knittle and Jeanloz, 1986).

can occur by sliding alternating layers of Fe and O perpendicular to the c-axis (Fig. 1), the orientation of the c-axis is likely to be preserved. The transition further reduces the Fe-Fe distance, which apparently leads to metallization. Although our conclusion agrees with Jackson and Ringwood (1981) that the high-pressure phase is B8, our measurement of Ap/p= 4% at the phase boundary (7% at zero pressure by extrapolation) is consistent with the original assessment of a small Ap/p by Jeanloz and Ahrens (1980). The observation of rhombohedral to NiAs transition at 74GPa-900K and 90GPa-600K (Fig. 7) define a linear phase boundary: P(GPa)= 118.00.051T(K). Extrapolation to 300 K gives a transition pressure of 103GPa. On the other hand, Yagi et al. (1985) reported that the rhombohedral phase persisted to 120GPa at 300K. We performed a separate X-ray diffraction study at 300K to 220GPa and found only the rhombohedral phase. It is likely that the transition does not occur because of slow kinetics at low temperatures. The result suggests that the low-temperature magnetic transition reported by

Our results for FeO support the transition sequence of Bl-rhombohedral-B8 at low temperatures and B l-B8 at high temperatures. Most of the previous paradoxical observations can be explained by the extremely soft C44 of FeO at high pressures. The metallic B8 phase of FeO is likely to partition as a major component in the core, not only because of its metallic bonding nature but also because of its structural compatibility with other major components of the core (Fe, S, H). Recent experiments found that the high-P-T phase of FeS also has NiAs structure (Fei et al., 1995). Iron hydride has a double hexagonal close-packed (dhcp) structure (Badding et al., 1991) in which the iron layers have the ABACA stacking sequence similar to the Fe and O layers in the NiAs structure in Fig. 1. Most recently, pure iron has been found to crystallize in a dhcp structure at high P-T (Saxena et al., 1995; Yoo et al., 1995). The stability of dhcp-Fe could be greatly expanded by solid solutions with FeO, FeS and Fell.

Acknowledgements We thank Ronald E. Cohen, Thomas S. Duffy, Robert M. Hazen and two anonymous reviewers for their comments. We also thank NSLS for providing the synchrotron X-radiation, and NSF, NASA and CIW for supporting the research.

References Anderson, O.L., Isaak, D.G. and Yamamoto, S., 1989. Anharmonicity and the equation of state for gold. J. Appi. Phys., 65: 1534-1543. Badding, J.V., Hemley, R.J. and Mao, H.K., 1991. High-pressure

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