Phase transformations and metallization of magnesium oxide at high pressure and temperature

7/30/2019 Phase transformations and metallization of magnesium oxide at high pressure and temperature

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MgO is among the simplest oxides constituting the rocky mantles of

terrestrial planets such as Earth, and the cores of Jupiter and other giant

planets. Present in the Earths mantle as an end-member component of

the mineral (Mg, Fe)O magnesiowstite, it can be abundant in larger

planets due to the theoretically expected dissociation of (Mg, Fe)SiO3

perovskite (1). Among common planetary constituents, MgO is thought

to be especially resistant to transformation under pressure and tempera-

ture, having a high melting temperature (27), a wide electronic band

gap under pressure (1, 8, 9), and a simple phase diagram featuring three

phases at the high pressures explored in the present study: the B1-

structured solid found at ambient conditions, the liquid state, and an as-

yet unobserved high-pressure crystalline phase having the B2 structure(15, 9, 10).

To characterize MgO at the elevated pressures and temperatures of

planetary interiors, we measured its pressurevolume (P, V) equation of

state to 0.6 TPa (6 Mbar), and its temperature (T) and optical reflectivity

(R) to beyond 1.4 TPa (14 Mbar) and about 50,000 K, under shock load-

ing (11). We determined shock temperature and corresponding shock

velocity (US) using the method of decaying shocks (1114), in which an

optically thick and reflecting shock wave decreases in amplitude as it

propagates, revealingthrough time resolved diagnostics of emission,

velocity, and reflectivitythe properties of a continuum of shock states

(Fig. 1). Determining the corresponding particle velocity (UP) in separate

experiments (Fig. 2 and fig. S1) allow

for the derivation of pressure and vo

ume from our measurements (11, 15).

We find that the shock temperatu

does not change monotonically wi

pressure, as expected for a single pha

(15, 16), but instead shows a larg

anomaly with a temperature minimuat about 0.45 TPa and 8500 K (Figs.

and 2). Two additional regions

anomalous behavior (at about 0.65 TP

and beyond 1 TPa) are most evide

when examining the specific heat, C

of the shock states (Fig. 2B) (1113

In broad regions of our data, CV clos

ly matches the Dulong-Petit limit (C

= 6Rgas, where Rgas is the gas consta

per mole of atoms), consistent with th

presence of a pure MgO phase abov

the Debye temperature (~760 K) (1

13, 15); in three distinct regions, how

ever, there is substantial deviation fro

this value. The anomalous values ofCsuggest the influence of latent hea

associated with structural, bonding

electronic transitions.

Indeed, beginning at the secon

transition (0.60 TPa) we find an i

crease in optical reflectivity from

0.5%a value consistent with th

optical properties of low-pressur

insulating MgO (17) (figs. S2 an

S3)toward ~ 20%, indicating su

stantial changes in electronic prope

ties. A simple (Drude-semiconducto

model of the measured reflectiviti

(1113) is consistent with the energgap between valence and conductio

electronic bands nearly vanishing du

ing the transition (Fig. 2C), resulting

metallic conductivities >100 ( cm)1 above 0.65-0.7 TPa (fig. S4) (11

To interpret these experimental results, we constructed a Mi

Grneisen-Debye, finite-strain equation-of-state for MgO (16, 18) co

strained by the shock data (11). From this model, we assess phas

transition properties, such as the volume and entropy of transformatio

(Vtrand Str, respectively). We find that the low-pressure behavior

MgO (to 0.35 TPa and 9500 K) is well described with this model, takin

parameters solely from measurements on B1-MgO near ambient cond

tions (e.g., pressures below 0.005 TPa) (19). Beyond 0.35 TPa, a s

quence of phase transformations is required in order to explain th

anomalies in shock temperature, consistent with our specific heat analy

sis (Table 1).Several interpretations are possible for the first, most promine

transition, although the observed decrease in shock temperature indicat

a positive entropy change (positive latent heat) from low- to high

pressure phases. If the transformation occurs at thermodynamic equili

rium, the data follow a phase boundary having a negative Clapeyro

slope and implying a volume change as negative as Vtr/V~ 7% (Tab

1) (11, 15). This suggests that the first transition corresponds with soli

solid transformation, and the second (consistent with positive volum

and entropy change, Table 1) with melting. Indeed, pressure-temperatu

conditions of the first and second transitions, and Clapeyron slopes o

tained assuming equilibrium transformation, are close to theoretical

R. Stewart McWilliams,1,2* Dylan K. Spaulding,3 Jon H. Eggert,4 Peter M.Celliers,4 Damien G. Hicks,4 Raymond F. Smith,4 Gilbert W. Collins,3 RaymondJeanloz31Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington,DC 20015, USA.2Howard University, 2400 Sixth Street NW, Washington, DC 20059, USA.3Department of Earth and Planetary Science, University of Cal ifornia, Berkeley, CA, 94707, USA.4Physics Division, Physical and Life Science Directorate, Lawrence Livermore National Laboratory,Livermore, California 94550, USA.

*To whom correspondence should be addressed. E-mail: [email protected] address: Grupo de Geociencias, Departamento de Fsica, Universidad de Los Andes, Bogot,

ColombiaPresent address: Commissariat L'nergie Atomique, Dpartement de Physique Thorique et Applique,Bruyres le Chtel, 91297 Arpajon Cedex, France.

Magnesium oxide (MgO) is representative of the rocky materials comprising themantles of terrestrial planets, such that its properties at high temperatures andpressures reflect the nature of planetary interiors. Shock-compression experimentson MgO to pressures of 1.4 TPa reveal a sequence of two phase transformations:from B1 (NaCl) to B2 (CsCl) crystal structures above 0.36 TPa, and from electricallyinsulating solid to metallic liquid above 0.60 TPa. The transitions exhibit large latentheats that are likely to affect the structure and evolution of super-Earths. Togetherwith data on other oxide liquids, we conclude that magmas deep inside terrestrialplanets can be electrically conductive, enabling magnetic field-producing dynamoaction within oxide-rich regions and blurring the distinction between planetary

mantles and cores.
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2/7/http://www.sciencemag.org/content/early/recent / 22 November 2012 / Page 2/ 10.1126/science.1229450

predicted values for the B1-B2 and melting transitions of MgO (Table 1)

(2). Alternatively, the first transition could be due to melting (3), with

associated superheating upon shock compression (12). In this case, the

first transformation can have negligible volume change (Table 1) (11),

and the second corresponds with a transformation in the fluid.

Our preferred interpretation is that the two transitions are due to B1-

B2 (0.45 TPa) and B2-melt (0.65 TPa), respectively. The second trans-

formation is unlikely to be anything other than melting; there is no theo-retical evidence for a liquid-liquid transition in MgO (8), and known

fluid-fluid transitions in other shock-compressed insulators typically

occur over a much broader pressure and temperature range (12, 13).

Also, the transition from electrically insulating solid to metallic liquid in

this interpretation is consistent with theoretical predictions of electronic

properties for MgO (1, 8, 9) (fig. S4), and reflects the melting behavior

of other nonmetals such as carbon (13), silicon and sulfur (20).

Continuous changes in the reflectivity of the liquid state suggest that

liquid electronic properties are strongly sensitive to temperature and

pressure (8). The large entropy of the first transition relative to typical

solid-solid transitions remains poorly understood, but is consistent with

available theory for MgO (Table 1). The broad region of anomalously

high specific heat beyond ~1 TPa and ~33,000 K is similar to that ob-

served at extreme temperatures in carbon and SiO2 (12, 13), and is simi-

larly identified here as a transformation to a complex bonded fluid. Withthis interpretation, we have a self-consistent picture for the phase dia-

gram of MgO (Fig. 3), involving two crystalline phases (B1, B2) and one

liquid phase (13, 9).

We conclude that MgO is solid at conditions found in the present

Earth (21), in large Earthlike planets (22) andbased on extrapolations

of the melting curvein Jupiter and its core (23) (Fig. 3). The B2 phase

of MgO is likely to be stable in the deep mantles of Earthlike planets of

more than four Earth masses (22) (Fig. 3), and so a B1-B2 transition can

be relevant to planetary iron-bearing magnesiowstite (24). With the

corresponding decrease in volume (up to 7%) and large latent heat of

transformation (Table 1), a B1-B2 transition could influence internal

structure and dynamics (22, 25), orbital evolution (25), and exoplanet

mass-radius relationships (22). This is particularly true for terrestrial

planets exceeding ~ 8 Earth masses (22), where dissociation ofperovskite (1) could make magnesiowstite the most abundant mineral

of the deep mantle.

Major end-member oxides of deep planetary interiors (1)SiO2

(12), MgSiO3 (14), and MgOuniformly exhibit high electrical conduc-

tivities () in their fluid states, consistent with metallic or semi-metallic

behavior: ~ 103 to 105 S/m at T~ 5,000 to 15,000 K and P ~ 0.1 to 0.7

TPa. Planetary magmas comprised of these rocky constituents should

likewise be highly conductive at high pressure, as compared with mag-

mas familiar at Earths surface ( ~ 102 to 102 S/m) (26), and would

thereby resemble liquid iron alloys making up planetary cores (27).

Our experiments therefore reveal a blurring between traditional defi-

nitions of planetary mantle and core material: liquid oxides can be con-

sidered either molten mantle constituents, or electrically conducting core

components. Indeed, for terrestrial planets with extensive melting (i.e.,

magma oceans) and sufficient interior pressure, oxides could contributeto the dynamo process sustaining a planetary magnetic field. The mag-

netic Reynolds number0m

RvL= (28) for such a magma ocean (Rm ~

103 to 107) exceeds the critical value required for a self-sustaining field

(Rm > 101 to 102), considering flow velocities v appropriate for a terres-

trial magma ocean (4 to 40 m/s) (29), and distance scales L appropriate

for terrestrial mantles (106 to 107 m); 0 is the permeability of free space.

While Earth may be too small (P < 0.13 TPa in the mantle) for substan-

tially elevated oxide conductivity, even a modest (order of magnitude)

increase in over typical magma values can shift Rm in an early-Earth

magma ocean from too small (Rm ~ 102 to 102) to large enough to sus-

tain dynamo action. This suggests that an early, short-lived magnetic

field could have existed on Earth, perhaps similar to those inferred f

Mars and the Moon on the basis of remnant crustal magnetism.

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Acknowledgments: We wish to thank N. Gmez-Prez, R. Hemley, D. Swift,

Cohen, D. Dalton, and W. Unites for helpful discussions. This work w

performed under the auspices of the U.S. Department of Energy by Lawren

Livermore National Laboratory in part under Contract No. W-7405-Eng-4

and in part under Contract No. DE-AC52-07NA27344. Financial suppo

from the U.S. Army Research Office (Grant No. 56122-CH-H), a Kre

Institute DOE/NNSA graduate fellowship (Contract No. DE-FC508NA28752), the DOE/NNSA National Laser User Facility Program, th

Miller Institute for Basic Research in Science, and the University

California are also acknowledged. Data are available in this manuscript and

the Supporting Online Material.

Supplementary Materials

www.sciencemag.org/cgi/content/full/science.1229450/DC1

Materials and Methods

Figs. S1 to S6

Tables S1 to S3

References (3853)

29 August 2012; accepted 26 October 2012

10.1126/science.1229450
http://www.sciencemag.org/content/early/recenthttp://www.sciencemag.org/content/early/recenthttp://dx.doi.org/10.1016/j.icarus.2010.12.005http://dx.doi.org/10.1016/j.icarus.2010.12.005http://dx.doi.org/10.1029/2004JC002764http://dx.doi.org/10.1029/2004JC002764http://dx.doi.org/10.1029/GL013i013p01541http://dx.doi.org/10.1029/GL013i013p01541http://dx.doi.org/10.1007/s11214-009-9572-zhttp://dx.doi.org/10.1007/s11214-009-9572-zhttp://dx.doi.org/10.1007/s11214-009-9572-zhttp://dx.doi.org/10.1029/GL008i007p00729http://dx.doi.org/10.1029/GL008i007p00729http://dx.doi.org/10.1029/94JB02065http://dx.doi.org/10.1029/94JB02065http://dx.doi.org/10.1016/j.jpcs.2008.04.006http://dx.doi.org/10.1016/j.jpcs.2008.04.006http://dx.doi.org/10.1103/PhysRevB.78.174102http://dx.doi.org/10.1103/PhysRevLett.103.225501http://dx.doi.org/10.1103/PhysRevLett.103.225501http://dx.doi.org/10.1111/j.1365-246X.1987.tb01664.xhttp://dx.doi.org/10.1111/j.1365-246X.1987.tb01664.xhttp://dx.doi.org/10.1051/0004-6361:20079321http://dx.doi.org/10.1051/0004-6361:20079321http://dx.doi.org/10.6028/jres.049.025http://dx.doi.org/10.6028/jres.049.025http://dx.doi.org/10.1063/1.2009528http://dx.doi.org/10.1063/1.2009528http://dx.doi.org/10.1364/JOSA.57.001140http://dx.doi.org/10.1364/JOSA.57.001140http://dx.doi.org/10.1063/1.1778164http://dx.doi.org/10.1063/1.1778164http://dx.doi.org/10.1063/1.1807008http://dx.doi.org/10.1063/1.1807008http://dx.doi.org/10.1103/PhysRevLett.104.184503http://dx.doi.org/10.1103/PhysRevLett.104.184503http://dx.doi.org/10.1063/1.2712189http://dx.doi.org/10.1063/1.2712189http://dx.doi.org/10.1063/1.1782080http://dx.doi.org/10.1063/1.1782080http://dx.doi.org/10.1063/1.1782080http://dx.doi.org/10.1103/RevModPhys.49.523http://dx.doi.org/10.1103/RevModPhys.49.523http://dx.doi.org/10.1063/1.1732931http://dx.doi.org/10.1063/1.1732931http://dx.doi.org/10.1063/1.1732931http://dx.doi.org/10.1103/RevModPhys.49.523http://dx.doi.org/10.1063/1.1782080http://dx.doi.org/10.1063/1.2712189http://dx.doi.org/10.1063/1.2712189http://dx.doi.org/10.1103/PhysRevLett.104.184503http://dx.doi.org/10.1103/PhysRevLett.104.184503http://dx.doi.org/10.1063/1.1807008http://dx.doi.org/10.1063/1.1778164http://dx.doi.org/10.1364/JOSA.57.001140http://dx.doi.org/10.1063/1.2009528http://dx.doi.org/10.6028/jres.049.025http://dx.doi.org/10.1051/0004-6361:20079321http://dx.doi.org/10.1111/j.1365-246X.1987.tb01664.xhttp://dx.doi.org/10.1103/PhysRevLett.103.225501http://dx.doi.org/10.1103/PhysRevLett.103.225501http://dx.doi.org/10.1103/PhysRevB.78.174102http://dx.doi.org/10.1016/j.jpcs.2008.04.006http://dx.doi.org/10.1029/94JB02065http://dx.doi.org/10.1029/GL008i007p00729http://dx.doi.org/10.1007/s11214-009-9572-zhttp://dx.doi.org/10.1029/GL013i013p01541http://dx.doi.org/10.1029/2004JC002764http://dx.doi.org/10.1016/j.icarus.2010.12.005http://www.sciencemag.org/content/early/recent



Fig. 1. Schematic of laser-shock experiments (A) showingtarget configuration with drive laser impinging on an Al bufferplate, to which the sample (MgO) or a combination ofstandard (SiO2) and sample were attached (11). Shockvelocity, temperature, and reflectivity were determined usingvelocity interferometry (B) and streaked optical pyrometry (C)that document intensity (of interference fringes or self-emission, respectively) as a function of distance across thesample (vertical axis, ~500 m fullscale) and time (horizontalaxis, ~25 ns fullscale) (1114). Reflectivity and velocity aredetermined, respectively, from the observed intensity and

fringe shift in (B), with shock velocities extrapolated below US= 17.3 km/s where reflection from the shock was notdetectable (11). In the shock-velocity (black dots, dashed linewhere extrapolated) and emission-intensity (blue dots)records (D), events are labeled e1 to e6: entry of the shockinto the MgO sample (e1) and a period of nearly steady shockpropagation are followed by steady decay of velocity andemission until e2, where the rate of emission decaydecreases, then increases (e3), while velocity continues todecay steadily; then emission increases (e4 to e5) anddecays again before the shock exits the MgO (e6).



Fig. 2. Shock temperature T (A), specific heat CV (B) and shock-front reflectivity R (C)measured for MgO in two experiments [blue and red solid curves, systematic uncertaintybounds (11) shown by dotted curves (mean SD)] are shown as a function of shock-wavevelocity (bottom scale) and pressure (top scale), determined from a linearUS-UP equation-of-state (D) based on a fit (11) to data at low-pressure data (3033) (off scale, fig. S1) andhigh-pressure (open symbols, table S1) measured (34) relative to SiO2 (35). A Drude-semiconductor model fit to the reflectivity data are shown by the green curve in C (11);solid and dashed black lines respectively indicate the band gap (Eg) for modelsconstrained to the zero-pressure value for MgO, or not. Events, labeled as in Fig. 1,correspond to transitions between anomalous (grey shading) and normal (6Rgas) values ofCV.



Fig. 3. Phase diagram of MgO constructed from shock temperature data [solid circles:this study, red and blue for two shots; open black circles (36)] with B1-phasetemperature model (dot-dashed black line) (36). Our proposed phase diagram (heavyblack lines) is a modification of the recent theoretical phase diagram of Refs (2, 4), inwhich the melt curve is identical but the B1-B2 transition has moved to higher pressure.Zero-temperature B1-B2 transition pressures from theory (excluding outliers) are givenby the gray bracket (10). Experimental melting temperatures (6, 7) are open triangles.

Also shown are the conditions of planetary interiors for Earth (21), a theoretical Earth-likeplanet of 5 earth masses (Super Earth) (22), Jupiter (23), and a hot Jupiter-mass planet(average prediction) (37); temperature discontinuities at 1.3 and 6.5 Mbar in theterrestrial planets correspond to the base of the oxide mantle.


7/7

Table 1. Comparison between experimental and theoretical properties of phase

transitions in MgO. Experimental ranges include thermodynamic equilibrium and

nonequilibrium [e.g., superheating (12)] interpretations of the transitions (11). The

Clapeyron slope is ( )B

TP / . Uncertainties are mean SD.

P(TPa)

T(10

3K)

( )BTP / (10

4TPa/K)

Str/Rgas Vtr/ V(%)

1stTransition

Experiment

(this study)

0.44 0.08 9.0 0.7 3.9 3.0 3.9 0.6 3.8 3.1

B1-B2

(theory)

0.33 0.04

(2)

8.1 0.7

(2)

0.60 0.18

(2)

1.9 0.9

4.0 0.8

(9)

2ndTransition

Experiment

(this study)

0.65 0.05 14.0 1.1 1.2 0.8 1.6 0.1 4.1 2.9

Melt

(theory)

0.59 0.05

(2)

13.6 0.6

(2)

0.77

(2)

0.5 0.1 0.9 0.2

(3, 8)

Upper bound, assuming VTr/Vto be constant along the phase boundary, and calculatingStrfrom the Clausius-Clapeyron relation ( ) tr tr / /BP T S V = ; lower bounds are Str

~ 0 and Vtr~ 0, obtained ifStris presumed to be constant along the boundary above

zero temperature, and infinitesimal (11).Estimated assuming that Vtr/Von the melting curve is similar at high pressure for the

B1 and B2 phases, and using the Clapeyron relation to calculate Str.

Phase transformations and metallization of magnesium oxide at high pressure and temperature

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