xenon oxides under pressure - tcm mdt26/tti_talks/qmcitaa_14/needs_  · xenon oxides i xenon

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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 Earths 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 Earths mantleXe could be stored within Earths 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 Earths mantle, oxygen

    TTI 2014

  • XRD data

    x1

    06

    15105

    2q (deg.)

    P~84 GPa, before heating

    P~84 GPa, after heating

    Xe Xe(O2)2ReXe2O5

    151052q (deg.)

    XeXe(O2)2ReXe2O5Xe3O2

    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 (

    )

    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 fvr. 2014

    180

    160

    140

    V/3

    f.u

    . (

    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 fvr. 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 fvr. 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

    (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

    (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