Synthesis and Characterization of Three New Lithium-Scandium Hexathiohypodiphosphates: Li_4–3xSc_xP₂S₆ (x = 0.358), m-LiScP₂S₆, and t-LiScP₂S₆

We report the first examples of lithium rare earth metal hexathiohypodiphosphates(IV). Three new lithium-scandium hexathiohypodiphosphates(IV) were synthesized and characterized. The compounds crystallize in three different structure types, featuring isolated ethane-like [P₂S₆]^4– hexathiohypodiphosphate(IV) units. Li_2.926Sc_0.358P₂S₆ or Li_4–3xSc_xP₂S₆ (x = 0.358) crystallizes in the space group P31m (no. 162) with a = b = 6.0966(5) Å, c = 6.5866(6) Å, V = 212.02(4) ų, and Z = 1. The compound is isostructural to Li₂FeP₂S₆ as well as the Li ion conductor Li_4–2xMg_xP₂S₆ (x = 1/3, 2/3). Monoclinic LiScP₂S₆ was found to be a layered compound with van der Waals gaps. The layers consist of edge sharing octahedra that are occupied by Li, Sc, and P₂ in an alternating fashion. It is isostructural to LiAlP₂S₆ and crystallizes in the space group C2/c (no. 15) with a = 6.933(1) Å, b = 10.754(2) Å, c = 11.694(2) Å, β = 94.41(3)°, V = 869.3(3) ų, and Z = 4. Trigonal LiScP₂S₆ crystallizes in the space group P31c (no. 163) with a = b = 6.363(1) Å, c = 12.386(3) Å, V = 434.3(2) ų, and Z = 2, and is isostructural to AgInP₂S₆ and AgScP₂S₆. Its structure is closely related to the monoclinic version, except that trigonal LiScP₂S₆ features disordered [P_2/2P_6/6S₆]^4– units. The new phases were investigated by Raman spectroscopy, thermal analysis, and DFT calculations. We further demonstrate that Li_4–3xSc_xP₂S₆ shows significantly enhanced Li ion conductivity compared to the parent compound Li₄P₂S₆.