Impact of the Li substructure on the diffusion pathways in alpha and beta Li3PS4: an in situ high temperature neutron diffraction study

Abstract

We report the first in situ variable temperature neutron powder diffraction (NPD) study of the solid-state ion conductor Li₃PS₄ which delineates the transitions between its three polymorphs (γ → β → α) and determines their crystalline Li-sublattices. They are compared to those of nanoporous β-Li₃PS₄, and β′-type Si-substituted Li_3.25Si_0.25P_0.75S₄. Importantly, Rietveld refinement of the NPD data elucidates the crystal structure of the high-temperature α-Li₃PS₄ polymorph for the first time and shows that it crystallizes in the Cmcm space group. The lithium diffusion pathways of both the bulk β and α polymorphs are evaluated using the maximum entropy method and bond valence site energy calculations, revealing that the structure of α-Li₃PS₄ favors facile 3D conduction, supported mainly by its bcc S-sublattice. The variable temperature in situ NPD study of nanoporous β-Li₃PS₄ identifies the presence of an amorphous organic component that is only completely removed upon transformation to the α-phase at elevated temperature, and which may play a role in stabilizing this material at room temperature. Si-substitution of Li₃PS₄ not only stabilizes the β′-Li_3.25Si_0.25P_0.75S₄ phase at room temperature but also prevents phase transformation to the α polymorph upon heating owing to its larger lattice volume.