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The 6th School on Simulation and Modeling Physics "Ab-initio methods and their applications" Hanoi, 27 - 29 November 2007. Exploring planetary interiors by ab-initio molecular dynamics. 6 th SMP, Hanoi, Nov 27-29, 2007 Sandro Scandolo (the Abdus Salam ICTP, Trieste, Italy).

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slide1

The 6th School on Simulation and Modeling Physics"Ab-initio methods and their applications"

Hanoi, 27 - 29 November 2007

slide2

Exploring planetary interiors by

ab-initio molecular dynamics

6th SMP, Hanoi, Nov 27-29, 2007

Sandro Scandolo

(the Abdus Salam ICTP, Trieste, Italy)

slide3

The Abdus Salam International Centre for Theoretical Physics

(under UNESCO / IAEA), Trieste, Italy

www.ictp.it

ICTP’s mission:

To foster the growth of advanced studies and research in physics and mathematics, especially among researchers from developing countries

Opportunities

- Conferences / Schools throughout the year

- Associates (our faculty “in the field”)

- Visitors (junior, STEP, TRIL, etc)

- Diploma Course

slide4

High pressure in

Materials Science

Planetary Science

1991: Earth’s core conditions

reproduced in the laboratory

1951: the first man-made diamonds

slide5

Shock waves

Diamond anvil cell

slide6

from “first principles”

Approximations

- Born-Oppenheimer

- Classical nuclei

- Density functional theory

Variable-cell molecular

dynamics

[Parrinello & Rahman, 1981]

slide7

Outline

High Pressure in Materials Science:

+ “Carbonia”: a silica-like CO2 amorph

+ Novel transition metal nitrides

High pressure in Planetary Science:

+ Carbon in planetary interiors

+ Melting of iron

slide8

Experimental confirmation of

silica-like CO2

Yoo et al, Science 283, 1510 (1999)

Silica-like CO2 : the crystal phases

Serra, Cavazzoni, Chiarotti, Scandolo, Tosatti,

Science 284, 788 (1999)

Molecular CO2

Candidate theoretical structures

for silica-like CO2

Crystal structure

of silica-like CO2

not yet determined

Is there a glass

analog?

+ Molecular CO2 predicted to tranform into a silica-like crystal at high pressure

+ Silica-like phases of CO2 predicted to be ultrahard

slide9

“Carbonia”: the silica-like CO2 glass

Experimental evidence for amorphous CO2

Santoro, Gorelli, Bini, Ruocco, Scandolo, Crichton,

Nature, in press (2006)

Tetrahedral CO4

Octahedral CO6

Compression by ab-initio

molecular dynamics gives:

At 1000 K and 100 GPa:

a crystalline phase

At 2000 K and 80 GPa:

an amorphous phase

slide10

Platinum nitride: PtN or PtN2?

Polymeric N

Molecular N2

Raman shift (cm-1)

Single-bonded polymeric

nitrogen

Gregoryanz et al, Nature Mat. (2004)

Crowhurst et al, Science (2006)

Eremets et al, Nature Mat. (2004)

Ab-initio calculations on structures with 1:1

stoichiometry fail to reproduce the experimental

Raman spectra

slide11

Platinum nitride:

PtN2 pyrite

Young, Montoya, Lazzeri, Sanloup,

Gregoryanz, Scandolo, PRB 2006

Raman spectra

X-ray diffraction

slide12

MN2 with interstitial N2 units:

a new class of transition metal nitrides?

New “heavy” metal nitrides are

superhard

Bulk modulus:

428 GPa (IrN2), 358 GPa (OsN2)

PtN2 pyrite

IrN2

Single bonded N-N

Young, Sanloup, Gregoryanz, Scandolo,

Hemley, Mao, PRL 96, 155501 (2006)

slide13

Scandolo & Jeanloz, American Scientist (2003)

H2O+CH4+NH3

Marvin Ross, “Diamonds in the sky”

Nature (1981)

Methane was found to

dissociate under a shock wave

slide14

Methane dissociation: what do simulations say

Compressed methane

Compressed methane

heated up to 4000 K

F. Ancilotto et al., Science 275, 1288 (1997)

slide15

Methane dissociation: the experimental confirmation

L.R. Benedetti et al., Science 283, 100 (1999)

Before compression

After compression

slide16

Melting of Carbon

X. Wang, S. Scandolo, R. Car, Phys. Rev. Lett 2005

Coordination number

BC8, SC ??

5.3-5.8

5.2

diamond

liquid

4.7

4.3

Neptune and Uranus isentrope

3.8

slide17

Accuracy of DFT?: Melting of silicon

Liquid

Solid

Tm(LDA) = 1380 K

Tm(GGA) = 1520 K

Tm(meta-GGA) = 1700 K

Tm(experiment) = 1687 K

X. Wang, S. Scandolo, R. Car

slide18

How hot is the centre of the Earth?

liquid

solid

The temperature at the inner core boundary coincides with the melting temperature of Fe at 330 GPa

Inner core

(solid Fe)

Mantle

Outer core

(liquid Fe)

slide19

The “optimized” potential method

“Optimized”

potential at P,T

A. Laio et al, Science 287, 1027 (2000)

slide20

Melting of Iron at Earth’s core pressures

A. Laio et al, Science 287, 1027 (2000)

Shock wave

experiments

DFT-based

calculation by

Alfe’ et al

Our results

Inner-Outer

core boundary

Static experiments

slide22

The crew:

J. Montoya

A. Young

R. Rousseau

C. Cavazzoni

Y. Liang

X. Wang

A. Laio

R. Car

….

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