Abstract
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.
Highlights
At atmospheric pressures, the size of the metal atom is thought to be important in determining the alkali metal polysulfide stoichiometries that are stable
The phases whose ∆HF lie on the convex hull are thermodynamically stable, while those whose ∆HF are not too far from the hull may be metastable stable provided their phonon modes are all real
Calculations carried out with both the PBE and optB88-van der Waals (vdW) functionals showed that the Na2 S, Na2 S2, Na2 S4 and Na2 S5 stoichiometries lay on the hull
Summary
The size of the metal atom is thought to be important in determining the alkali metal polysulfide stoichiometries that are stable. Crystal structure prediction (CSP) techniques coupled with DFT calculations have been employed to search for new ambient pressure phases [20,21]. A novel low energy polymorph of Na2 S5 was predicted, and a higher energy e polymorph was synthesized Both of these were computed to be more stable than the known α phase. We carry out a comprehensive theoretical investigation that employs an evolutionary structure search to predict the most stable, and low-lying metastable phases in the metal rich and metal poor regions of the Na-S phase diagram at 1 atm, as well as 100–200 GPa. In addition to identifying many of the known or previously predicted ambient pressure phases, novel polymorphs with the. We hope the structural diversity of the phases predicted inspires the directed synthesis of new sulfides of sodium
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