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Interest in Na-S compounds stems from their use in battery materials at 1 atm, as well as the potential for superconductivity under pressure. Evolutionary structure searches coupled with Density Functional Theory calculations were employed to predict stable and low-lying metastable phases of sodium poor and sodium rich sulfides at 1 atm and within 100–200 GPa. At ambient pressures, four new stable or metastable phases with unbranched sulfur motifs were predicted: Na2S3 with C 2 / c and Imm2 symmetry, C 2 -Na2S5 and C 2 -Na2S8. Van der Waals interactions were shown to affect the energy ordering of various polymorphs. At high pressure, several novel phases that contained a wide variety of zero-, one-, and two-dimensional sulfur motifs were predicted, and their electronic structures and bonding were analyzed. At 200 GPa, P 4 / m m m -Na2S8 was predicted to become superconducting below 15.5 K, which is close to results previously obtained for the β -Po phase of elemental sulfur. The structures of the most stable M3S and M4S, M = Na, phases differed from those previously reported for compounds with M = H, Li, K.
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High Pressure-behavior of the Fe-S system and composition of the Earth’s inner core
Using the evolutionary crystal structure predictionalgorithm USPEX, we identify the compositions and crystalstructures of thermodynamically stable compounds in the Fe ±S system at pressures in the range of 100 ± 400 GPa. We findthat at pressures in the Earth's solid inner core (330 ± 364 GPa)two compounds are stable – Fe2S and FeS. In equilibrium withiron, only Fe2S can exist in the inner core. Using the equation ofstate of Fe2S, we find that, in order to reproduce the density ofthe inner core by adding sulfur alone, 10.6 ± 13.7 mol.% (6.4 ±8.4 wt.%) sulfur is needed. An analogous calculation for silicon(where the only stable compound at inner core pressures is FeSi)reproduces the density of the inner core with 9.0 ± 11.8 mol.%(4.8 ± 6.3 wt.%) silicon. In both cases, a virtually identicalmean atomic massMMin the range of 52.6 ± 53.3 results forthe inner core, which is much higher thanMMà49:3 inferred forthe inner core from Birch's law. In the case of oxygen (allowingfor the equilibrium coexistence of suboxide Fe2O with ironunder core conditions), the inner core density can be ex-plained by the oxygen content of 13.2 ± 17.2 mol.% (4.2 ±5.6 wt.%), which corresponds toMMbetween 49.0 and 50.6
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- Award ID(s):
- 1723160
- PAR ID:
- 10172711
- Date Published:
- Journal Name:
- Physics Uspekhi
- Volume:
- 60
- Issue:
- 10
- ISSN:
- 1063-7869
- Page Range / eLocation ID:
- 1025-1032
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3367/UFNe.2017.03.038079
