xenon oxides under pressure - tcm groupmdt26/tti_talks/qmcitaa_14/needs_tti2014.pdf · xenon oxides...
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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
TTI 2014
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?
TTI 2014
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
TTI 2014
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
TTI 2014
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
TTI 2014
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
TTI 2014
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
TTI 2014
Structure of Xe2O5 with +4 and +6 xenon atoms
TTI 2014
Structure of Xe3O2 with 0 and +4 xenon atoms
TTI 2014
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
TTI 2014
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
TTI 2014
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
TTI 2014
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?
TTI 2014
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?
TTI 2014
Acknowledgements: ESRF, EPSRC, ARCHER, UKCP, CambridgeHigh Performance Computing Service (Darwin)
TTI 2014
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