Xenon oxides under pressure

Richard Needs and Nicholas Worth (Uni of Cambridge, UK)

Agnes Dewaele (CEA, DAM, DIF, France)

Chris Pickard (UCL, UK)

Sakura Pascarelli (ESRF, France)

Olivier Mathon (ESRF, France)

Mohamed Mezouar (ESRF, France)

Tetsuo Irifune (Ehime University, Japan)

TTI 2014

Xenon oxides

I Xenon is not very reactive at low pressures

I Xenon can be made to form metastable oxides at lowpressures which readily decompose

I DFT study by Zhu, Jung, Oganov, Glass, Gatti and Lyakhov[Nature Chem 5, 61 (2013)] considered XeO2, XeO3 and XeO4

I Zhu et al. predicted stable xenon oxides above 83 GPa withoxidation states of +2, +4, +6

I No experimental evidence for the formation of xenon oxides athigh pressures has been reported

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Questions about xenon oxides

I What are the stable stoichiometries of Xe/O mixtures at highpressures?

I What are the stable structures of xenon oxides?

I What are the oxidation states of xenon atoms in the oxides?

I At what pressure do xenon oxides become thermodynamicallystable?

I Metals or insulators?

I Role of Xe 4d electrons?

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Methodology

I Diamond anvil cell experiments at ESRF

I Use laser heating to try and synthesise equilibrium phases

I Characterise xenon oxides using X-ray diffraction (XRD),X-ray absorption, and Raman spectroscopy

I Use Ab initio random structure searching (AIRSS) withinDFT to identify stable stoichiometries and structures

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The “missing xenon” paradox

I Chondrite: a stony meteorite unmodified by melting ordifferentiation of the parent body

I Xenon depletion of >90% in the Earth’s atmosphere,compared with its abundance in chondritic meteorites

I Possible resolutions of the paradox include:Xe could have escaped into spaceXe could be stored within Earth’s mantleXe could be stored within Earth’s core

I Solving the paradox will require improved understanding ofthe high-pressure chemistry of xenon

I We have studied the stability of compounds of xenon with themost abundant element in the Earth’s mantle, oxygen

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XRD data

x1

06

15105

2q (deg.)

P~84 GPa, before heating

P~84 GPa, after heating

Xe Xe(O2)2

ReXe2O5

151052q (deg.)

XeXe(O2)2

ReXe2O5

Xe3O2

P~95 GPa, before heating

P~97 GPa, after heating

before heating after heatingbefore heating after heating

33% Xe+67% O2 89% Xe+11% O233% Xe+67% O2

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Convex hull at 83 GPa

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1x in XexO1-x

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

For

mat

ion

Ent

halp

y (e

V/a

tom

)

Structures on the convex hull are stable against decompositionStructures above the convex hull are unstable to decomposition

Blue: Structures of Zhu et al. recalculated at high accuracy

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Combining theory and experiment

I The Xe2O5 phase was discovered in XRD experiments andidentified using AIRSS

I The Xe3O2 phase was predicted using AIRSS andsubsequently verified by XRD experiments

I Fruitful “dialogue” between theory and experiment

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Structure of Xe2O5 with +4 and +6 xenon atoms

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Structure of Xe3O2 with 0 and +4 xenon atoms

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Cell parameters

5.6

5.4

5.2

5.0

d (

Å)

100806040200P (GPa)

a K0=176, K'0=4 c/2 K0=120, K'0=4

PathH:Agnes:XeO2:NewPhaseXeO2:XeO2_8_EoS; file: cc1 lun. 24 févr. 2014

180

160

140

V/3

f.u

. (Å

3)

100806040P (GPa)

0.37

0.36

0.35

b/a

100806040P (GPa)

0.58

0.57

0.56

c/a

100806040

Xe2O5 Xe3O2

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Raman Data

x1

03

800700600500400300200Raman shift (cm

-1)

Xe2O5 after decompression

XeO4

XeO3

XeO2

Inte

nsity

800600400200

Raman shift (cm-1

)

Xe3O2, P=97 GPa

Xe2O5, P=88 GPa

PathH:Agnes:XeO2:NewPhaseXeO2:Raman_78_reaction_EHPRG; file: lun. 3 févr. 2014 800

600

400

200

Ra

ma

n f

req

ue

ncy (

cm

-1)

1.00.90.8V/V0

XeO2 XeO3 XeO4

PathH:Agnes:XeO2:NewPhaseXeO2:Raman_78_reaction_EHPRG; file: LambdamaxSauv jeu. 6 févr. 2014

Xe2O5 and Xe3O2 Xe2O5

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Bandstructures of Xe2O5 and Xe3O2 at 83 GPa

0,0,1/2

0,0,01/2,1/2,0

1/2,1/2,1/2

0,1/2,1/2

0,1/2,0

0,0,0

-40

-20

0

20

40

ε (e

V)

Xe2O5-P4nnc-83GPa.BU

0,1/2,0

0,0,00,1/4,1/4

1/4,0,1/4

1/4,1/4,0

1/4,1/4,1/4

0,0,0

-40

-20

0

20

40

ε (e

V)

Xe3O2-Immm-83GPa

Xe2O5 Xe3O2

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Influence of the Xe 4d electrons

I Xe 4d orbitals lie 45 eV below the HOMO and 30 eV belowthe bottom of the sp bands

I Surely they contribute very little to the bonding and they canbe replaced by the pseudopotential?

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Influence of the Xe 4d electrons

I The Xe 4d orbitals have a strong influence on the energies ofstructures

I Surely they contribute very little to the bonding and they canbe replaced by the pseudopotential?

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Acknowledgements: ESRF, EPSRC, ARCHER, UKCP, CambridgeHigh Performance Computing Service (Darwin)

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