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. 2016 Feb 2:6:18938.
doi: 10.1038/srep18938.

Krypton oxides under pressure

Patryk Zaleski-Ejgierd  1 , Pawel M Lata  1
Affiliations

Krypton oxides under pressure

Patryk Zaleski-Ejgierd et al. Sci Rep. .

Abstract

Under high pressure, krypton, one of the most inert elements is predicted to become sufficiently reactive to form a new class of krypton compounds; krypton oxides. Using modern ab-initio evolutionary algorithms in combination with Density Functional Theory, we predict the existence of several thermodynamically stable Kr/O species at elevated pressures. In particular, our calculations indicate that at approx. 300 GPa the monoxide, KrO, should form spontaneously and remain thermo- and dynamically stable with respect to constituent elements and higher oxides. The monoxide is predicted to form non-molecular crystals with short Kr-O contacts, typical for genuine chemical bonds.

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Figures

Figure 1
Figure 1. Relative enthalpies of formation (per atom) of the KrOx phases (x = 1–4) calculated with respect to elemental krypton and molecular oxygen in their corresponding most stable phases.
Oxides which are stable with respect to disproportionation are those forming a convex hull of enthalpy with respect to composition into selected species. Note the substantial stabilization of the monoxide.
Figure 2
Figure 2. The relative enthalpies of formation, ΔH [eV / f.u.], calculated for selected phases of KrO with respect to pure elements in their thermodynamically most stable states.
For Phase D, the theoretically calculated stabilization pressure is approx. 285 GPa (for details on the structures see Fig. 3 and Table S1).
Figure 3
Figure 3. Schematic representation of the selected most-stable phases of KrO and their corresponding unit cells (large gray spheres – krypton; small orange spheres – oxygen).
For illustration purposes, and clarity, selected Kr-O and O-O contacts are depicted (Kr-O < 2.0 Å and O-O < 1.6 Å). Note the significantly different character of the given phases. For details on bonding see Table S1; for details on the stability see Fig. 2.
Figure 4
Figure 4. Phonon dispersion and the corresponding phonon density of states (PhDOS) calculated at 300 GPa for selected structures: Phase A (C2/m, Z = 1) – top panel, Phase B (C2/m, Z = 2) – middle panel and Phase D (Immm, Z = 2) – bottom panel.
Note the presence of the O-O stretching band due to molecular oxygen (middle panel).
Figure 5
Figure 5. Band structures, total density (DOS) and the partial densities of states (PDOS) [el./eV] calculated for: Phase A (C2/m, Z = 1) – left panel, Phase B (C2/m, Z = 2) – middle panel and Phase D (Immm, Z = 2) – right panel (red: s - character, blue: p - character, black: total DOS).
The Fermi energy (Ef) is set to zero for convenience.
Figure 6
Figure 6. The three enthalpically best structures calculated for KrO at 300 GPa along with the superimposed electron density isosurfaces (we set the cut-off value consistently at 0.15 a0−3, a0 = Bohr radius).
The short Kr-O bonds (R < 1.9 A) and the O-O bonds are also depicted; note the increased electron density distribution precisely along the bonds.
See this image and copyright information in PMC

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