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NATURE CHEMISTRY | www.nature.com/naturechemistry 1

SUPPLEMENTARY INFORMATIONDOI: 10.1038/NCHEM.1497

Supplementary Online Materials: Stability of xenon oxides at high pressuresQiang Zhu,1, a) Daniel Y. Jung,2 Artem R. Oganov,1, 3, b) Colin W. Glass,4 Carlo Gatti,5 and Andriy O. Lyakhov11)Department of Geosciences, Department of Physics and Astronomy, Stony Brook University, Stony Brook,New York 11794, USA2)Laboratory of Crystallography, Department of Materials, ETH Honggerberg, Wolfgang-Pauli-Strasse 10,CH-8093 Zurich, Switzerland3)Geology Department, Moscow State University, 119992, Moscow, Russia4)High Performance Computing Center Stuttgart (HLRS), Germany5)Istituto di Scienze e Tecnologie Molecolari del CNR (CNR-ISTM) e Dipartimento di Chimica, Universita di Milano,via Golgi 19, 20133 Milano, Italy

I. CRYSTALLOGRAPHIC DATA

Structures of the stable xenon oxides, XeO, XeO2, and XeO3 are given in table S1. For XeO, Pbcm phase becomesstable at 83 GPa, then it tranforms to P21/m phase at 145 GPa. For XeO2, P21/c phase becomes stable at 102 GPa,and then a phase transition occurs at 198 GPa (P21/c to Cmcm). For XeO3, the stable phases are P42/mnm-XeO3

(114 - 145 GPa), C2/c (145 - 198 GPa), and Pmmn (> 198 GPa). These crystal structures are shown in Figs. S1,S2, and S3.

II. CHEMICAL REACTIONS FOR XE-O SYSTEM

Fig S4 shows the predicted Gibbs free energy of formation of Xe-O compounds at different temperatures and P =100 GPa. It can be clearly seen that XeO and XeO2 are thermodynamically stable at 100 GPa. The thermal stabilityof XeO2 increases with temperature.

The decomposition/formation reactions and their enthalpies at 120 GPa are listed in Table S2. XeO, XeO2 andXeO3 are thermodynamically stable at this pressure. The formation volumes for xenon oxides are negative, whichindicates that on increasing pressures they will be favoured even more, relative to the elements.

FIG. S1. Crystal structure of XeO-P21/m (stable at P >145 GPa).

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Phase Pressure Space group Lattice parameters Atomic coodinates(GPa)

XeO 100 Pbcm a=2.944 A Xe(4d) 0.468 0.121 0.250b=6.925 A O (4d) 0.893 0.371 0.250c=4.781 A

XeO 200 P21/m a=4.505 A Xe1(2e) 0.313 0.250 0.709b=3.308 A Xe2(2e) 0.168 0.250 0.194c=5.525 A O1 (2e) 0.356 0.750 0.934β=101.83◦ O2 (2e) 0.232 0.750 0.509

XeO2 150 P21/c a=6.452 A Xe1(4e) 0.874 0.588 0.723b=6.536 A Xe2(4e) 0.626 0.370 0.302c=4.948 A O1 (4e) 0.849 0.555 0.198β=92.20◦ O2 (4e) 0.394 0.546 0.164

O3 (4e) 0.362 0.221 0.996O4 (4e) 0.898 0.234 0.436

XeO2 200 Cmcm a=5.943 A Xe (4c) 0.000 0.213 0.250b=4.424 A O (8g) 0.826 0.343 0.750c=3.642 A

XeO3 130 P42/mnm a=6.040 A Xe (4g) 0.737 0.263 0.000c=3.465 A O1 (8i) 0.625 0.881 0.000

O2 (4e) 0.000 0.000 0.198

XeO3 150 C2/c a=9.167 A Xe1(4a) 0.000 0.000 0.000b=5.835 A Xe2(8f) 0.664 0.014 0.176c=6.824 A O1 (8f) 0.681 0.747 0.935β=98.34◦ O2 (8f) 0.791 0.128 0.917

O3 (8f) 0.876 0.872 0.748O4 (8f) 0.458 0.127 0.584O5 (4e) 0.500 0.257 0.250

XeO3 200 Pmmn a=3.443 A Xe (2a) 0.750 0.750 0.807b=4.440 A O1 (2b) 0.250 0.750 0.171b=3.613 A O2 (4e) 0.750 0.000 0.316

TABLE S1. Structures of the stable xenon oxides, XeO, XeO2, and XeO3.

Reactions ∆H [eV] ∆V [A3]Xe + 1/2 O2 → XeO -0.388 -1.46Xe + O2 → XeO2 -0.626 -3.11Xe + 3/2 O2 → XeO3 -0.689 -5.52

XeO + 1/2 O2 → XeO2 -0.239 -1.65XeO + O2 → XeO3 -0.301 -4.06XeO2 + 1/2 O2 → XeO3 -0.062 -2.41

TABLE S2. Investigated reactions of the Xe-O system at 120 GPa.

III. ELECTRONIC STRUCTURE

It is well known that DFT calculations systematically underestimate the band gap. To obtain accurate results, weperformed GW calculations using the VASP code. Pressure dependences of the band gaps are shown in Fig. S5. Allthree compounds are semiconducting.

The atomic orbital contributions plotted in Fig. 3 exactly sum up to the total DOS and thus, when integrated upto the Fermi level, represent the fraction of valence electrons that occupy the corresponding set of atomic orbitals.Note that the contribution of Xe s and p set of orbitals strongly decreases on going from left to right in Fig. 3 because




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FIG. S2. Crystal structure of XeO2-Cmcm (stable at P > 198 GPa).

FIG. S3. Crystal structures of XeO3 polymorphs. a), top view of two sublattices: XeO2 chain + O2 chain; b) front view of-O2-O2- chain in P42/mnm XeO3 (stable from 114 GPa to 145 GPa); c) front view of -O2-O- chain in C2/c XeO3 (stabel from145 GPa to 198 GPa).

of two concurring effects, the change in stoichiometry (from a Xe:O ratio of 1 down to 1/3) and the parallel enhancedelectron transfer from Xe to O (Xe Bader charges being +1.01 in XeO, +1.995 on average in XeO2 and +2.754 inXeO3). To help the reader to disentangle the effect of stoichiometry from that of the increasing charge transfer, thesame DOS shown in Fig. 3, but with contributions from Xe orbitals multiplied as to counterbalance the stoichiometrychange, are shown in Fig. S6, which clearly displays a trend that the ratio of O p orbitals’ contribution to Xe porbitals’ contribution below Fermi energy increases as stoichiometry changes from XeO to XeO3. For comparison, theGW DOS plot is also shown in Fig. S7.




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FIG. S4. Predicted Gibbs free energy of formation of Xe-O compounds at different temperatures and P = 100 GPa. Forclarity, the Gibbs free energies of formation were shifted by -0.1, -0.2, and -0.3 eV/atom at 1000, 2000, and 3000 K, respectively.

FIG. S5. Pressure dependence of the GW band gap for the selected xenon oxides.

FIG. S6. Total density of states for a) XeO-Pbcm at 100 GPa, b) XeO2-P21/c at 150 GPa, c) XeO3-Pmmn at 200 GPatogether with their projections on the set of orbitals of the atoms. The contributions from Xe orbitals have been multiplied bytwo in XeO2 and by three in XeO3 to show the relative contribution from each kind of atoms. At variance with Fig. 3 in themain text, the sum over the plotted projections does not yield the total DOS.




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FIG. S7. Total GW density of states for a) XeO-Pbcm at 100 GPa, b) XeO2-P21/c at 150 GPa, c) XeO3-Pmmn at 200 GPa.

IV. DYNAMICAL STABILITY

We have checked the dynamical stability of these proposed structures from phonon calculations (Fig. S8). All ofthem have no imaginary phonon frequencies in the entire pressure ranges where they are predicted to be stable basedon enthalpy calculations. However, none of these phases remain dynamically stable at P = 1 atm, and therefore noneof them are quenchable to ambient conditions and can only be studied in situ.

V. XENON IN THE EARTH’S LOWER MANTLE

Table S3 shows the representative chemical reactions involving xenon oxides and silicates in the Earth’s lower mantleat 100 GPa. Xe oxides are only stable above 83 GPa, i.e. at pressures corresponding to the lower mantle. All of thepredicted xenon oxides are very strong oxidants and our calculations show that they will oxidize Fe, producing ironoxide and free xenon (FeO + Xe). Therefore, Xe oxides cannot be present in the lower mantle, where free Fe shouldexist.Since xenon oxides are not stable in coexistence with metallic Fe, we investigated the formation of stable xenon

silicates under pressure, focusing on XeSiO3 and Xe2SiO4, which contain the least oxidized divalent xenon. All of theinvestigated compositions were unstable towards decomposition into XeO, XeO2, SiO2 and the elemental Xe; Xe2SiO4

(Fig. S9) proved to be one of the least unstable silicates, but still is unstable. In this structure, Xe atoms tend toterminate the silicate perovskite layers, suggesting that xenon could also be stored in perovskite/post-perovskitedefects, or be incorporated in microscopic defects, such as stacking faults.

1J. Lin, et al, Proc. Nat. Aca. Sci. 100, 4405 (2003).2I. I. Mazin, et al, American Mineralogist 83, 451 (1998).




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FIG. S8. Phonon dispersion curves of a) XeO at 100 GPa; b) XeO2 at 150 GPa; c)XeO3 at 220 GPa.

Reaction ∆H [eV] ∆V [A3]FeO+Xe → XeO+Fe 2.170 -1.35FeO+Xe+1/2 O2 → XeO2+Fe 2.203 -3.09Fe+2Xe+SiO2 → FeSi+2XeO 8.540 -1.88Fe+Xe+SiO2 → FeSi+XeO2 8.687 -2.034XeO + 2SiO2 → 2Xe2SiO4 0.910 -0.614XeO + 2MgSiO3 → 2Xe2SiO4 + 2MgO 1.490 0.274XeO + 2CaSiO3 → 2Xe2SiO4 + 2CaO 4.409 -0.33Xe+SiO2 → Si+XeO2 12.205 0.332Xe+SiO2 → Si+2XeO 12.057 0.48

TABLE S3. Representative chemical reactions involving xenon oxides and silicates in Earth’s lower mantle at 100 GPa, Fe inhcp and FeO in the antiferromagnetic inverse NiAs structure1,2.




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FIG. S9. Crystal structure of Cmc21 Xe2SiO4.


SUPPLEMENTARY INFORMATION - images.nature.com · Artem R. Oganov, 1,3, b) Colin W. Glass, 4. ......

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