Li₄P₂Se₆ - structure and properties

Non-oxide chalcogenides are among the fastest Li-ion conductors currently available. Here, a new crystalline lithium selenido-phosphate, Li₄P₂Se₆, is reported. Its structure was solved using a combined approach of quantum-chemical structure prediction, powder X-ray diffraction, and solid-state NMR. Li₄P₂Se₆ crystallizes in an orthorhombic unit cell in the space group Pnma and the lattice parameters a = 13.8707(1) Å, b = 11.2115(1) Å, and c = 6.45445(7) Å, representing a novel structure type. ³¹P and ⁷⁷Se magic-angle-spinning NMR spectra are reported and were analyzed for chemical shift and J-couplings. The material's ionic conductivity, characterized by impedance spectroscopy, slightly exceeds that of the non-isostructural crystalline compound Li₄P₂S₆, which contains the homologous complex anion. A computed phase diagram shows that several compounds can be prepared experimentally, which from the computed energy at a temperature of 0 K, feature a positive energy above the convex hull and thus should not exist. This research highlights the influence of configurational and vibrational entropy in stabilizing ionic chalcogenides, emphasizes the complexity of predicting phase stability in related systems by quantum-chemical calculations and contributes to the understanding of non-oxide chalcogenides, including potential fast ionic conductors.