Chemical and Structural Variety in Sodium Thioarsenate Glasses Studied by Neutron Diffraction and Supported by First-Principles Simulations.

Sodium-conducting sulfide glasses are promising materials for the next generation of solid-state batteries. Deep insight into the glass structure is required to ensure a functional design and tailoring of vitreous alloys for energy applications. Using pulsed neutron diffraction supported by first-principles molecular dynamics, we show a structural diversity of Na ₂ S-As ₂ S ₃ sodium thioarsenate glasses, consisting of long corner-sharing (CS) pyramidal chains CS-(AsSS _2/2 ) _k , small As _p S _q rings ( p + q ≤ 11), mixed corner- and edge-sharing oligomers, edge-sharing (ES) dimers ES-As ₂ S ₄ , and isolated (ISO) pyramids ISO-AsS ₃ , entirely or partially connected by sodium species. Polysulfide S-S bridges and structural units with homopolar As-As bonds complete the glass structure, which is basically different from structural motifs predicted by the equilibrium phase diagram. In contrast to superionic silver and sodium sulfide glasses, characterized by a significant population of isolated sulfur species S _iso (0.20 < S _iso /S _tot < 0.28), that is, sulfur connected to only mobile cations M ⁺ with a usual M/S _iso stoichiometry of 2, poorly conducting Na ₂ S-As ₂ S ₃ alloys exhibit a modest S _iso fraction of 6.2%.