A First-Principles Exploration of NaxSy Binary Phases at 1 atm and Under Pressure

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 C2/c and Imm2 symmetry, C2-Na2S5 and C2-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, P4/mmm-Na2S8 was predicted to become superconducting below 15.5 K, which is close to results previously obtained for the beta-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.