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Stable magnesium peroxide at high pressure.

Lobanov SS, Zhu Q, Holtgrewe N, Prescher C, Prakapenka VB, Oganov AR, Goncharov AF - Sci Rep (2015)

Bottom Line: However, in exoplanets oxygen may be a more abundant constituent.Raman spectroscopy detects the presence of a peroxide ion (O2(2-)) in the synthesized material as well as in the recovered specimen.Likewise, energy-dispersive x-ray spectroscopy confirms that the recovered sample has higher oxygen content than pure MgO.

View Article: PubMed Central - PubMed

Affiliation: Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA.

ABSTRACT
Rocky planets are thought to comprise compounds of Mg and O as these are among the most abundant elements, but knowledge of their stable phases may be incomplete. MgO is known to be remarkably stable to very high pressure and chemically inert under reduced condition of the Earth's lower mantle. However, in exoplanets oxygen may be a more abundant constituent. Here, using synchrotron x-ray diffraction in laser-heated diamond anvil cells, we show that MgO and oxygen react at pressures above 96 GPa and T = 2150 K with the formation of I4/mcm MgO2. Raman spectroscopy detects the presence of a peroxide ion (O2(2-)) in the synthesized material as well as in the recovered specimen. Likewise, energy-dispersive x-ray spectroscopy confirms that the recovered sample has higher oxygen content than pure MgO. Our finding suggests that MgO2 may be present together or instead of MgO in rocky mantles and rocky planetary cores under highly oxidized conditions.

No MeSH data available.


Related in: MedlinePlus

XRD image of I4/mcm MgO2 powder synthesized at 134 GPa (seen as dark grey vertical lines) in rectangular coordinates (cake).Red and violet ticks correspond to the positions of I4/mcm MgO2 and MgO, respectively. Green ticks represent some reflections of ζ-O2 (high angle Bragg reflections are not shown). White labels are Miller indices of the indexed tetragonal phase. Part of this XRD pattern (2θ = 9–13.5) was used to Rietveld refine the predicted structure of I4/mcm MgO2. The x-ray wavelength is 0.3344 Å.
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f2: XRD image of I4/mcm MgO2 powder synthesized at 134 GPa (seen as dark grey vertical lines) in rectangular coordinates (cake).Red and violet ticks correspond to the positions of I4/mcm MgO2 and MgO, respectively. Green ticks represent some reflections of ζ-O2 (high angle Bragg reflections are not shown). White labels are Miller indices of the indexed tetragonal phase. Part of this XRD pattern (2θ = 9–13.5) was used to Rietveld refine the predicted structure of I4/mcm MgO2. The x-ray wavelength is 0.3344 Å.

Mentions: Laser heating of the B2 sample to T > 2000 K at P = 134 GPa provided more information on the high pressure chemistry of the Mg-O system. We were very curious to note that new peaks form a powder-type texture in XRD images (Fig. 2), indicating the presence of a large number of randomly oriented crystallites. Surprisingly, the spotty texture is now built by MgO and ζ-O2. Indexing the most clearly resolved new peaks again yields a tetragonal unit cell with a = 3.925 (1) Å, c = 4.613 (6) Å. Moreover, the obtained Miller indices reproduce that of the tetragonal phase synthesized in the A1 run (Fig. 1) suggesting that the exact same phase has been produced in the A1 and B2 runs. Given the large yields of the new phase as well as the polycrystalline sample texture, Rietveld method can be applied to test and refine the theoretically predicted I4/mcm MgO2. According to the prediction by Zhu et al. (Ref. 12), magnesium occupies a 4a Wyckoff position (0, 0, 0.25) and oxygen is located in 8 h (x, x + 0.5, 0), which leaves only the x fractional coordinate of oxygen to refine. The refined x = 0.1285(13), and the predicted x = 0.126 agrees to within 2σ; thus the refined structural model may be considered identical to the predicted one. Figure 3 compares the experimental XRD pattern with the synthetic XRD of the Rietveld-refined I4/mcm MgO2.


Stable magnesium peroxide at high pressure.

Lobanov SS, Zhu Q, Holtgrewe N, Prescher C, Prakapenka VB, Oganov AR, Goncharov AF - Sci Rep (2015)

XRD image of I4/mcm MgO2 powder synthesized at 134 GPa (seen as dark grey vertical lines) in rectangular coordinates (cake).Red and violet ticks correspond to the positions of I4/mcm MgO2 and MgO, respectively. Green ticks represent some reflections of ζ-O2 (high angle Bragg reflections are not shown). White labels are Miller indices of the indexed tetragonal phase. Part of this XRD pattern (2θ = 9–13.5) was used to Rietveld refine the predicted structure of I4/mcm MgO2. The x-ray wavelength is 0.3344 Å.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4555032&req=5

f2: XRD image of I4/mcm MgO2 powder synthesized at 134 GPa (seen as dark grey vertical lines) in rectangular coordinates (cake).Red and violet ticks correspond to the positions of I4/mcm MgO2 and MgO, respectively. Green ticks represent some reflections of ζ-O2 (high angle Bragg reflections are not shown). White labels are Miller indices of the indexed tetragonal phase. Part of this XRD pattern (2θ = 9–13.5) was used to Rietveld refine the predicted structure of I4/mcm MgO2. The x-ray wavelength is 0.3344 Å.
Mentions: Laser heating of the B2 sample to T > 2000 K at P = 134 GPa provided more information on the high pressure chemistry of the Mg-O system. We were very curious to note that new peaks form a powder-type texture in XRD images (Fig. 2), indicating the presence of a large number of randomly oriented crystallites. Surprisingly, the spotty texture is now built by MgO and ζ-O2. Indexing the most clearly resolved new peaks again yields a tetragonal unit cell with a = 3.925 (1) Å, c = 4.613 (6) Å. Moreover, the obtained Miller indices reproduce that of the tetragonal phase synthesized in the A1 run (Fig. 1) suggesting that the exact same phase has been produced in the A1 and B2 runs. Given the large yields of the new phase as well as the polycrystalline sample texture, Rietveld method can be applied to test and refine the theoretically predicted I4/mcm MgO2. According to the prediction by Zhu et al. (Ref. 12), magnesium occupies a 4a Wyckoff position (0, 0, 0.25) and oxygen is located in 8 h (x, x + 0.5, 0), which leaves only the x fractional coordinate of oxygen to refine. The refined x = 0.1285(13), and the predicted x = 0.126 agrees to within 2σ; thus the refined structural model may be considered identical to the predicted one. Figure 3 compares the experimental XRD pattern with the synthetic XRD of the Rietveld-refined I4/mcm MgO2.

Bottom Line: However, in exoplanets oxygen may be a more abundant constituent.Raman spectroscopy detects the presence of a peroxide ion (O2(2-)) in the synthesized material as well as in the recovered specimen.Likewise, energy-dispersive x-ray spectroscopy confirms that the recovered sample has higher oxygen content than pure MgO.

View Article: PubMed Central - PubMed

Affiliation: Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA.

ABSTRACT
Rocky planets are thought to comprise compounds of Mg and O as these are among the most abundant elements, but knowledge of their stable phases may be incomplete. MgO is known to be remarkably stable to very high pressure and chemically inert under reduced condition of the Earth's lower mantle. However, in exoplanets oxygen may be a more abundant constituent. Here, using synchrotron x-ray diffraction in laser-heated diamond anvil cells, we show that MgO and oxygen react at pressures above 96 GPa and T = 2150 K with the formation of I4/mcm MgO2. Raman spectroscopy detects the presence of a peroxide ion (O2(2-)) in the synthesized material as well as in the recovered specimen. Likewise, energy-dispersive x-ray spectroscopy confirms that the recovered sample has higher oxygen content than pure MgO. Our finding suggests that MgO2 may be present together or instead of MgO in rocky mantles and rocky planetary cores under highly oxidized conditions.

No MeSH data available.


Related in: MedlinePlus