Electronic and Magnetic Structures of the Postperovskite- Type Fe[Subscript 2]O[Subscript 3] and Implications for Planetary Magnetic Records and Deep Interiors

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Electronic and magnetic structures of the postperovskite- type Fe[subscript 2]O[subscript 3] and implications for planetary magnetic records and deep interiors

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Citation Shim, Sang-Heon et al. “Electronic and magnetic structures of the postperovskite-type Fe2O3 and implications for planetary magnetic records and deep interiors.” Proceedings of the National Academy of Sciences 106.14 (2009): 5508-5512.

As Published http://dx.doi.org/10.1073/pnas.0808549106

Publisher National Academy of Sciences

Version Final published version

Citable link http://hdl.handle.net/1721.1/50247

Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. Electronic and magnetic structures of the postperovskite-type Fe2O3 and implications for planetary magnetic records and deep interiors

Sang-Heon Shima,1, Amelia Bengtsonb, Dane Morganb, Wolfgang Sturhahnc, Krystle Catallia, Jiyong Zhaoc, Michael Lerchec,d, and Vitali Prakapenkae aDepartment of Earth, Atmospheric, and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139; bDepartment of Materials Science and Engineering, University of Wisconsin, 1509 University Avenue, Madison, WI 53706; cX-ray Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439; dHigh Pressure Synergetic Center, Carnegie Institution of Washington, 9700 South Cass Avenue, Argonne, IL 60439; and eCenter for Advanced Radiation Sources, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637b;

Edited by Russell J. Hemley, Carnegie Institution of Washington, Washington, DC, and approved February 3, 2009 (received for review August 28, 2008)

Recent studies have shown that high pressure (P) induces the important to investigate how these electronic and magnetic transi- metallization of the Fe2+–O bonding, the destruction of magnetic tions will be influenced by structural transitions and compositional ordering in Fe, and the high-spin (HS) to low-spin (LS) transition variations existing in the deep Earth’s interior. of Fe in silicate and oxide phases at the deep planetary interiors. Some iron oxides and sulfides carry important paleomagnetic Hematite (Fe2O3) is an important magnetic carrier mineral for deci- records. However, impacts can disturb or even erase the records. phering planetary magnetism and a proxy for Fe in the planetary For example, magnetic fields can be generated during a shock interiors. Here, we present synchrotron Mössbauer spectroscopy event, providing a possible explanation for high magnetic rema- and X-ray diffraction combined with ab initio calculations for Fe2O3 nence found in young lunar glasses (14). In addition, a recent study revealing the destruction of magnetic ordering at the hematite → suggested that randomization of paleomagnetic records by impact- Rh2O3-II type (RhII) transition at 70 GPa and 300 K, and then the generated fields may be responsible for the weak magnetic field revival of magnetic ordering at the RhII → postperovskite (PPv) intensity observed at the impact basins of Mars (15). As shown for transition after laser heating at 73 GPa. At the latter transition, pyrrhotite (16), impacts can demagnetize minerals, providing an 3+ at least half of Fe ions transform from LS to HS and Fe2O3 alternative interpretation for the weak fields at the impact basins. changes from a semiconductor to a metal. This result demonstrates Yet, high-P behaviors of most magnetic carrier minerals are not that some magnetic carrier minerals may experience a complex well studied. sequence of magnetic ordering changes during impact rather than Several intriguing changes have been observed in Fe2O3 at a monotonic demagnetization. Also local Fe enrichment at Earth’s 40 ≤ P ≤ 60 GPa: a large increase in density (14%) and a dras- core-mantle boundary will lead to changes in the electronic struc- tic drop in resistivity (17–19), a phase transition to a Rh2O3-II ture and spin state of Fe in silicate PPv. If the ultra-low-velocity (RhII) type structure (20), disappearance of magnetic hyperfine zones are composed of Fe-enriched silicate PPv and/or the basaltic fields during the hematite → RhII transition (19), and the HS → materials are accumulated at the lowermost mantle, high electrical LS transition of Fe (21). conductivity of these regions will play an important role for the Although Fe end member is stable in the MgO–FeO system that electromagnetic coupling between the mantle and the core. forms the second most abundant mantle phase, it is not stable and therefore not accessible experimentally for PPv in the MgSiO3– spin ordering | electrical conductivity | high pressure | phase transition FeSiO3 system that forms the most abundant phase in the D region. It has recently been found that the PPv phase is stable at 2,000 K and 60 GPa with an Fe2O3 stoichiometry (1). Furthermore, he origin of the magnetism in hematite (Fe2O3) has long some studies suggested that a substantial amount of Fe in Mg- + T drawn interest. It is a ubiquitous mineral on the oxidizing sur- silicate Pv and PPv are Fe3+ (Fe3 / Fe = 0.1 ∼ 0.7) at mantle faces of the terrestrial planets, such as Earth and Mars, and is an P–T conditions due to crystal chemistry effects (22, 23). There- important magnetic carrier mineral for deciphering the magnetic fore, Fe2O3 provides opportunities to understand the properties history of the planets. Furthermore, it serves as a proxy for the of Fe end-member PPv. phases in the Earth’s core and mantle, as oxygen may be an impor- Here, we report the synchrotron Mössbauer spectroscopy tant alloying elements in the core and as Fe enters the dominant (SMS) and X-ray diffraction (XRD) of Fe2O3-PPv as well as the silicate and oxide phases in the mantle. Fe2O3 has a stability field other Fe2O3 polymorphs at high P. We find that Fe in both the for the postperovskite (PPv) - type crystal structure (1) to which octahedral (hereafter B) and bipolar-prismatic (hereafter A) sites the dominant lower mantle silicate transforms at the pressure are magnetically ordered and, combined with our ab initio cal- (P)–temperature (T) conditions corresponding to the D region culations, Fe in at least one of the two sites is in a HS state in (bottom 200- to 400-km-depth region of the mantle) (2–4). PPv. Therefore, Fe2O3 undergoes a series of transitions: mag- A previous study (5) showed a large increase in electrical con- 3+ → 2+ netic insulator with HS Fe nonmagnetic semiconductor with ductivity in FeO at mantle related P, suggesting that the Fe –O LS Fe3+ → magnetic metal with at least half of Fe3+ in HS, during bonding character changes from covalent/ionic to metallic. An ab initio study (6) predicted a magnetic collapse of Fe in silicates and oxides at high P, perhaps within the deep mantle. A high-spin → (HS) low-spin (LS) transition has been observed in silicates Author contributions: S.-H.S. designed research; S.-H.S., A.B., D.M., and K.C. performed and oxides at mantle related P (7–9). These transitions can induce research; W.S., J.Z., M.L., and V.P. contributed new reagents/analytic tools; S.-H.S., A.B., changes in important properties, such as elasticity, optical prop- D.M., and W.S. analyzed data; and S.-H.S., A.B., and D.M. wrote the paper. erties, electrical conductivity, and element partitioning (7, 10, 11). The authors declare no conflict of interest. Recent ab initio studies suggested that structure and composition This article is a PNAS Direct Submission. can have enormous influence on the spin transition, e.g., yielding 1To whom correspondence should be addressed. E-mail: [email protected]. large and opposite trends in transition pressure with composition This article contains supporting information online at www.pnas.org/cgi/content/full/ for perovskite (Pv) and ferropericlase (Fp) (12, 13). Therefore, it is 0808549106/DCSupplemental.

5508–5512 PNAS April 7, 2009 vol. 106 no. 14 www.pnas.org / cgi / doi / 10.1073 / pnas.0808549106 yefiefil ( field hyperfine htFe that 34 ie104()3.36 .02 0.6 0.30(2) 39.73(6) 0.48(4) 0.84(1) 1 Site 73(4) 70(4) (AFM) ( ifrn esites). Fe different 55(3) 47(3) 33(2) n hs auscag nysihl with 25) slightly (19, only phase change AFM values the these of and results Mössbauer conventional with 1 (Fig. GPa 55 and 33 between al .Fte ösae aaeeso Fe of parameters Mössbauer Fitted 1. Table Fe in transition Morin The Discussions and Results structures magnetic and Fe. electronic of the on structure crystal of ence hematite 1. Fig. Shim B GPa Pressure, eigcnb eadda ouae ea uvsrsligfrom resulting curves decay modulated as regarded be phase can RhII tering the for (20). spectrum studies 1 Mössbauer (Fig. previous simple with very a consistent found phase, We RhII K. the 300 to at GPa mation 55 to up phase hematite the in remains sample ersn bevdadfitdseta respectively. spectra, fitted and observed represent QS hf ubr nprnhssae2 are parentheses in Numbers t7 P n 0 ,orXDcnrsacmlt transfor- complete a confirms XRD our K, 300 and GPa 70 At antchprn field; hyperfine magnetic , tal. et = C 0.4 M fFe of SMS 2 → .Tetm pcr bandfo ula owr Scat- Forward Nuclear from obtained spectra time The ). O → 3 →− F,rslsi ag hnei uduoesplitting quadrupole in change large a in results AFM, ean F o5 P.OrXDsosta the that shows XRD Our GPa. 55 to AFM remains hItype RhII ie2 Site 2 . ms t15Ga(4.Our (24). GPa 1.5 at mm/s) 0.8 O B 3 hf oyopsa high at polymorphs bandfo h pcrlfitn fSMS of fitting spectral the from obtained ) QS − − − − 0.31 1.15 1.09 0.93 mm/s , → ( ( ( ( P rniin,sgetn toginflu- strong suggesting transitions, PPv 7 1 2 1 ) ) ) ) 2 IS σ O smrsitdfeec ewe h two the between difference shift isomer , A oteosietype Postperovskite 3 netite ( uncertainties 49.86(3) 50.23(3) 51.11(2) hr atdantiferromagnetic canted where , 92(1 0.4 19.21(11) and B Rh hf Hematite ,T 2 P B O h pncrlsadtikcurves thick and circles open The . r ncmlt agreement complete in are ) 3 I type -II 2 P IS QS O msSt occupancy Site mm/s , Tbe1,indicating 1), (Table 3 uduoesplitting; quadrupole , thg pressures high at QS n magnetic and ( 1 ) pcrlfitn sahee hnw nld w ifrn esites Fe different two finite include with we both when achieved is fitting spectral ioa rsai,wt : oa ratio. and molar octahedral sites, 1:1 cation a different with two prismatic, of has bipolar structure which regarded crystal 27) proposed be 2, (1, the can PPv with result agreement the reasonable (26), a sites as Fe frac- different recoil-free we the as that from fact such tions, the responses, Considering Mössbauer 1). same (Table 6:4 the be assumed to found is sites two Fe apecmltl oPv ehae o200K(i.2 (Fig. K the 2,000 transform to heating To after heated of PPv. remains we PPv min of PPv, growth 10 to strong After completely observed K. sample we 1,250 K, at 1,500 2 appears at PPv (Fig. of trans- phase line has RhII intense sample the the to that formed indicate patterns diffraction conducted heating, also We high (1). con- situ 0.6% also within in are report parameters previous unit-cell a fitted fits with The sistent which type. min. of PPv 30 pattern the another for diffraction to to GPa well the transformed 70 2), completely at (Fig. phase K is orthorhombic 2,000 sample to the up heating, sample After the of heating laser consistent is model structure. conventional single-site type a RhII the with the Also, with agreement (19). in is result is which Mössbauer ordering GPa, magnetic 70 the at that destroyed indicates This field. hyperfine netic ihasnl est with site Fe a single for a spectra with time the in modulation less reasonable much hyper- in magnetic results a of nuclear field absence fine the the i.e., general, In transitions, splitting. nuclear level energy the in differences energy the nls hssi Fe in phases angles) 2. Fig. ugsigfiiemgei oet o Fe strongly for 1), moments (Fig. hematite magnetic of that finite than less suggesting but phase, RhII the of hs,idctn httePvpaei hroyaial stable RhII thermodynamically the GPa. is to 70 phase transformation at PPv reverse the a that of indicating sign phase, any observe not did rhp c tutrdA;adN irgn e rhenium; obtained we D, Re, in phase nitrogen; PPv a the N, For subtracted. and were backgrounds The Ar; lines). structured hcp Ar-hcp, t7 P,w etFe heat we GPa, 70 at = h finite The fe bevn h rniint h hIpae econducted we phase, RhII the to transition the observing After h M fFe of SMS The 2 2.6507 O 3 Pvi antc oudrtn h antcodrn and ordering magnetic the understand To magnetic. is -PPv R atrso h hI(rytinls n P-ye(lc tri- (black PPv-type and triangles) (gray RhII the of patterns XRD ( 7 ) QS Å, P B – b hf ag.Ide,tebs pcrlfitn a achieved was fitting spectral best the Indeed, range. T B = 2 o h w ifrn ie lal niae that indicates clearly sites different two the for hf ifato yuigasprt ape Before sample. separate a using by diffraction 8.598 O 2 T O and 3 unh(i.2 (Fig. quench 3 Pvsosmc oemdlto hnthat than modulation more much shows -PPv PNAS 2 thigh at ( O 3 QS ) 3 QS Pvt low a to -PPv ,and Å, h eocpnyrtobtenthe between ratio occupancy Fe The . = pi ,2009 7, April P – 0.84 c T = Xryeeg 0kV r argon; Ar, keV; 40 = energy (X-ray D 6.413 A ± .I h eodhaigcycle heating second the In ). .Drn etn h most the heating During ). .1m/ u ihu mag- without but mm/s 0.01 T ( 2 ,0 ,fr3 i.We min. 30 for K, 1,200 , ) Å. o.106 vol. 3 + nPv h best The PPv. in ∗ o 14 no. unidentified , C .Pure ). 5509

GEOPHYSICS Fig. 3. The spin ordering in the PPv-type Fe2O3. The red and blue spheres represent the bipolar-prismatically (site A) and octahedrally (site B) coordinated Fe3+ ions, respectively. The gray spheres represent oxygen atoms. The crystallographic orientations of the models are shown at A and B, Lower Left. We denote magnetic arrangements using + and − for up and down moments, respectively.

+ + − − spin state in Fe2O3-PPv, we have conducted ab initio calculations A A B B , demonstrating that the AFM arrangements stabilize [GGA+U, see supporting information (SI) Appendix for calcula- HS Fe3+ on the B site to higher P. The spin transition to LS Fe3+ tion details]. We focus on three symmetry distinct collinear HS on the B site brings A+A+B−B−, A+A−B+B−, and A+A+B+B+ all ferromagnetic (FM) and AFM arrangements in the Fe2O3-PPv within 25 meV of each other, suggesting that different magnetic primitive unit cell, and three corresponding structures with LS on orderings with LS B site are nearly degenerate in energy and all the B site, because we found the A site does not go LS (see below). more stable than the same arrangements with HS Fe3+ at high An additional supercell magnetic arrangement was explored and pressures. The stable spin arrangements in our ab initio calcula- is discussed in the SI Appendix. tions can be also rationalized by considering the crystal structure of First, we consider the two symmetrically distinct AFM arrange- PPv combined with superexchange theory (28) (see SI Appendix). ments and one FM arrangement in the primitive cell: A+A+B−B−, In our SMS, the QS observed for both sites (−0.31 and 0.48 A+A−B+B−, and A+A+B+B+, where + and − are spin up and mm/s) in PPv are consistent with Fe3+ ions in the HS state (26). down, respectively, and A and B represent bipolar prismatic and The QS of LS Fe3+ is expected to be higher than what we observed + + − − octahedral sites, respectively (Fig. 3). In A A B B , all Fe atoms for PPv (29), as best shown in the high QS found in Fe2O3-RhII in the A site are + and all Fe atoms in the B site are − through (Table 1), which has all Fe3+ in LS according to X-ray emission AFM interaction along the [111] direction. All spin arrangements spectroscopy (21). We obtain small differences in isomer shift, along the edge-sharing, [100], and corner-sharing, [001], direc- IS = 0.30 ± 0.02 mm/s, between the two different Fe sites, indi- tions are FM in A+A+B−B−. Arrangement A+A−B+B− is AFM cating that the spin and oxidation states of the two Fe sites are along [001] and [101] in the B and A sites, respectively, and FM likely to be the same (29). From these results and the ab initio along the edge-sharing direction, [100], in both the A and B sites. calculation, we conclude that the magnetic ordering observed in 3+ + + − − These arrangements are modeled with both HS and LS Fe on SMS is likely to be AHSAHSBHSBHS. 3+ the B site. Because we found no stable LS state on the A site The high Bhf of site 1 is also consistent with HS Fe (Table 1). up to 150 GPa, we only consider LS in the B site. Note that the However, the Bhf of site 2 (19.2 T) is lower than expected + + + + + + − − AHSAHSBLSBLS (FM) and AHSAHSBLSBLS arrangements (the sub- scripts indicate the spin state of Fe) have a net magnetic moment per primitive cell 12µB and 8µB, respectively. Therefore, the lat- + − + − ter arrangement is ferrimagnetic. However, AHSAHSBLSBLS has no net magnetic moment, i.e., AFM (see SI Appendix for details). The calculated enthalpies of the different magnetic arrangements and spin states are shown in Fig. 4 for PPv. At low P, arrangements with HS Fe3+ are more stable than those with LS Fe3+ on the B site. Among the HS structures there is a significant gain in stability for AFM over FM arrangements by up to 100 meV per atom. There is no magnetic ordering transition at high P if Fe3+ remains HS. However, we find the A+A+B−B−, A+A−B+B−, and A+A+B+B+ (FM) arrangements all undergo a P-induced spin transition on the B site from HS to LS. For the most stable magnetic arrangement the spin transition occurs at a critical pressure, Pc, of 68 GPa. Below Pc, the most stable mag- + + − − netic arrangement is AHSAHSBHSBHS. Above Pc, the most stable + − + − arrangement is AHSAHSBLSBLS. The Pc is very close to the values of 70 GPa where PPv has been measured in SMS work. Because the choice of the Coulomb interaction (U) is somewhat ambigu- ous and influences Pc (U = 4 eV lowers Pc to 50 GPa and = + + − − U 7 eV raises Pc to 110 GPa), we consider both AHSAHSBHSBHS + − + − A A B B and HS HS LS LS magnetic arrangements as candidates for the Fig. 4. Stability of different magnetic arrangements and spin states in experimentally observed phase. + Fe2O3-PPv at high P. A reference state consisting of a polynomial fit to the The pressure at which the B site Fe3 undergoes a spin tran- + + − − enthalpy of (AHS AHS BHS BHS ), which is the stable state at ambient pressure, is sition depends on the magnetic ordering: it occurs at 25 GPa for subtracted off each curve. The gray area indicates the P where we measured + + + + + − + − A A B B (FM), at 55 GPa for A A B B , and at 75 GPa for the SMS of PPv. See the text for the notations.

5510 www.pnas.org / cgi / doi / 10.1073 / pnas.0808549106 Shim et al. eivsiae nld h rtcl(ui rNe)temperature Néel) or (Curie to Fe critical questions for the Important include mineral. preexisting investigated investi- carrier the be the affect be would in to changes records remains of magnetic sequence It this ordering. how the gated of destruction complete opigo h ateadtecr.W xlr h lcrclcon- electrical Fe the of explore We ductivity core. the electromagnetic and the mantle for the role of important coupling an play may which man- (33), the tle of part lowermost the in conductivity electrical metal-like interiors. aeo eaie u td eosrtsta antcordering high magnetic the at that revived in be demonstrates However, can study (16). intensi- our Mars field hematite, on magnetic of basins case low impact the the at explaining observed for ties used been has this some has Fe applications. of geophysical and transitions planetary electronic important and transitions structural A P hs bevdi h xeiet nfc,tea initio PPv ab for SMS the measured fact, we among In the pressure differences the enthalpy experiment. in the at the states that in indicate spin results observed different phase with PPv orderings magnetic different hs ilb ealc l h hsswt SFe LS with phases the All metallic. be will phase n hnbc oH a es o afo Fe of half for least (at HS LS to to HS back from changes then state and spin the and reconstructed, then and mea- spin we competing parameters these of Mössbauer all or the some arrangements. case, over averages the be could is sured this indeed if epciey rvddta pncnrbto ssildominant still is contribution spin for that provided respectively, lo illkl eprmgei P)a oe mantle lower of at (PM) difference paramagnetic energy be likely an phase will alloy HS have the AFM in enough and weak (FM is mantle coupling lower magnetic the at conduction. for However, that electrons thermal enough many be small will there is temperatures gap this moment magnetic low at stable efudta te Smgei tutrshv ogp(e.g., gap no have structures magnetic HS A other that found we oeto Fe of moment ln svr eki h Ssae h O for DOS The state. LS the in weak very cou- magnetic is the since pling conditions, mantle lower under PM be will tae httedge fsi arn a o nraemonoto- increase not may pairing with spin nically of degree the that strates → niomnsfrF ntoepae 3)adcnicuespin include can Fe on and study Our (29). (32) sites of phases subset a diverse those just in the on of transition Fe in because of Fe complicated more state for lot spin a environments be the may However, PPv and mantle. Pv the in structural exists significant transition no with where increases It compositions monotonically mantle-related (31). with pairing conductivity spin electrical density that and as appears (11), such properties properties optical physical HS (30), important a man- undergoes influences in Fe oxides which that and shown silicates been has tle it Recently properties. material during minerals carrier the of ordering impacts. magnetic high- of the structure influence our crystal Nevertheless, the demag- induce and unloading. necessarily netization not fast does pressure during that highlight field results stability PPv the n 1 and Fe HS for Shim B rbto sapoiaey1 e pnfrFe for spin per T 11 approximately is tribution u biii aclto.Arrangement calculation. initio ab our hf HS + HS + oegoei bevtos uain a eepandb a by explained be can nutation, observations, geodetic Some yroiehsbe hw ob eantzda high at demagnetized be to shown been has Pyrrhotite nsmay u bevtosso htmgeimi destroyed is magnetism that show observations our summary, In rnetr atepae n ly motn oe o the for roles important plays and phases mantle enters Iron RhII A A B tal. et au o h SadL ttswudb 3ad1 T, 11 and 33 be would states LS and IS the for value HS − HS − hf µ 2 O B n osblt o h low the for possibility One . B B epciey osdrn httemxmmsi con- spin maximum the that Considering respectively. , → 3 HS + LS + Pvadtebhvoso h pnodrn omdin formed ordering spin the of behaviors the and -PPv B 3 B P hs rniin.Ti togculn between coupling strong This transitions. phase PPv + P LS − HS − nitreit pn(S n Ssae,the states, LS and (IS) spin intermediate In . 2 P ntepeec fpaetastosi planetary in transitions phase of presence the in 3 and , O a adgpo . V vnwt fixed a with Even eV. 0.4 of gap band a has , + and 3 Pvb sn h est fsae DS from (DOS) states of density the using by -PPv sepce ordc oapoiaey3 approximately to reduce to expected is A A P HS + HS + hog tutrltasto vnafter even transition structural a through A A HS + HS + B B HS + LS + B A B HS + HS + LS + ,w xetta h SPM HS the that expect we ), A r eysal Therefore, small. very are HS + B A B hf HS + HS − → steceitneof coexistence the is A P B 3 Stasto (7–9) transition LS HS + + HS − hs a strongly can phase < uighematite during ) B , 3 . V htthe that eV) 0.1 3 + A A HS − + h expected the , HS + HS + B nteBsite B the on 2 A A HS − T O HS + HS + hc is which , Because . 3 P B B demon- LS − LS − P nFp in B B and µ LS − LS − B , , ag lcrclcnutvt n rpsdta 200-km-thick a that proposed (Mg of and layer conductivity electrical large a ae napoiaeP ntcl yuigteSSapoc (see references). approach and SQS the details using for by cell unit PM approximate an lated a endcmne nFe in documented been has been not have impurities and defects of included. effects mobility hole possible or the electron the and determine to made been has attempt mantle lower under epmantle deep o h lcrclcnutvt.Asuyo yoii composi- pyrolitic a on study Fe A that showed conductivity. tion electrical critical is is the Fe concentration Fe how for because know phases to the important among is distributed it Yet (35). observations geodetic aeil tteCBi loacniaefrtehg electrical high the for candidate Fe a as CMB, also the is at conductivity CMB basaltic of the trans- accumulation are at possible likely materials Therefore, and CMB. (41) the mantle to become lower ported layers the basaltic subduct- in the the addition, concentration buoyant that In negatively high (40). suggested basaltic a CMB have the reach has studies slabs that slab seismic ing notable some subducting is and the Al It of of 23). top the (22, at PPv layer and Pv mantle both A h RhII the nig rma ntocluain,ti niae htteelectri- the Fe that of indicates conductivity this cal calculations, initio ab from findings admsrcue(Q)apoc 3) h O o h two the for with DOS GPa 40 The at (34). found were approach SQS quasi- (SQS) special the structure using by random cell unit PM approximate well. an as calculated metallic be will alloy PM LS the that suggesting ihteF nihet h lcrclcnutvt fF remains high Fp at of conductivity low electrical quite the enrichment, not Fe may the and with even Furthermore, low coupling. very electromagnetic the be for should important be PPv elec- the mantle then of case, the conductivity is trical former the If (37). composition opposite olivine the an proposed on study another whereas (36), PPv mantle aesonehneeti h ocnrto fFe of concentration the studies in recent enhancement be Furthermore, can shown (39). (38) have PPv CMB Fe-enriched the by at explained seismologically ultra-low- observed the zones that proposed velocity been has It (CMB). boundary mantle (31). conductivity the decreases further efre oepoesaiiya ucinof function a ambient as stability at explore to relaxed performed fully is structure PPv P Fe-substituted The properties. state A atom. to respect per with convergence Energy cell. unit A Pure Methods (42). electron species two through these conductivity between electrical hopping the increase would ratio Maly hrfr,w rdc ealcbhvo o Fe for behavior metallic predict and we metallic gap Therefore, a alloy. no of claim PM show the supports pressures further both This metallic. at therefore are structures SQS both in ments AS.FrSS -aswt aditso e eetndt h nuclear the Source to tuned Photon were Advanced meV of 1 at of energy XRD bandwidths with transition and X-rays SMS SMS, For conducted (APS). We medium. lation P GA yuigasf xgnpedptnil h rloi oewssam- was projector- zone Brillouin The the pseudopotential. Monkhorst–Pack a oxygen and by soft pled a Approximation (DFT) treated using Gradient by was Generalized theory (GGA) (PBE) correlation Perdew–Burke–Ernzerhof Exchange functional the method. in density (PAW) plane-wave using augmented (VASP), monochromatic a age using by cell diamond beam. laser-heated X-ray double-sided conducted the was diffraction in Angle-dispersive detector. photodiode avalanche HS HS + n hnasre ffie ouecluain,ec nenlyrlxd are relaxed, internally each calculations, volume fixed of series a then and esrmns ro a odda rsuetasitn n insu- and pressure-transmitting a as loaded was Argon measurements. ag rpi lcrclrssiiyt ee fasemiconductor a of level a to resistivity electrical in drop large A eety thsbe eotdta (Mg that reported been has it Recently, ofrhrvrf httehigh- the that verify further To natraiepsiiiyi oa niheto ea h core- the at Fe of enrichment local is possibility alternative An acltoswr efre ihteVen biii iuainPack- Simulation Ab-initio Vienna the with performed were Calculations A A 57 HS HS − Fe B B 2 O → LS LS + 3 U B B a oddi h imn-ni elwt – uycisfor chips ruby 2–3 with cell diamond-anvil the in loaded was 0.9 P transition. PPv P au f5e ( eV 5 of value LS LS − Fe sfudi hssuy lo nehne Fe enhanced an Also, study. this in found as h O ftedsree antcarrange- magnetic disordered the of DOS The . and , 0.1 P 57 )SiO 2 P + e h iesetawr bandb sn an using by obtained were spectra time The Fe. n h esi rniina midmantle at transition spin Fe the and 2 A – PNAS O rfrnilyetr padis and Fp enters preferentially T k HS + pitms f7 of mesh -point 3 3 odtos hsrsl sqaiaiea no as qualitative is result This conditions. Pv .. h D the i.e., -PPv, A J sfrhrehne ota famtlat metal a of that to enhanced further is = HS + 2 O V sue ooti h orc ground- correct the obtain to used is eV) 1 B pi ,2009 7, April 3 LS + 3 RI 1–9.Cmie ihour with Combined (17–19). -RhII + P A B Obnigbcmsmtli at metallic becomes bonding –O HS Msaei ealc edirectly we metallic, is state PM LS + A l hwmtli behaviors metallic show all × HS k P 7 pit a etrta meV 1 than better was -points o high For . B × ae,wudepanthe explain would layer, HS o h rmtv 10-atom primitive the for 4 o.106 vol. 0.9 B HS Fe P 0.1 n 0Gawith GPa 70 and Msae ecalcu- we state, PM )SiO o 14 no. < 3 + IAppendix SI o%in mol% 3 3 Pvhas -PPv 2 yA in Al by 3 O + 3 /Fe -PPv 5511 2 P +

GEOPHYSICS ACKNOWLEDGMENTS. We thank B. Weiss and T. L. Grove for discussions that Fellowship. Measurements were performed at Sectors 3 and GeoSoilEnviro- improved this article and the editor and two anonymous reviewers for help- Cars (GSECARS) at Advanced Photon Source. GSECARS is supported by the NSF ful comments and suggestions. This work was supported by the National and DOE. Use of Sector 3 was supported in part bythe Consortium for Mate- Science Foundation (NSF) Grants EAR0738655 (to S.H.S.) and EAR0738886 rials Properties Research in Earth Sciences under NSF Cooperative Agreement (to D.M.), respectively. K.C. is supported by a Departmentof Energy (DOE) EAR 06-49658. Use of the APS was supported by the DOE, Ofﬁce of Science, National Nuclear Security Administration Stewardship Science Graduate Ofﬁce of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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5512 www.pnas.org / cgi / doi / 10.1073 / pnas.0808549106 Shim et al.