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

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 Li3.25Si0.25P0.75S4. 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′-Li3.25Si0.25P0.75S4 phase at room temperature but also prevents phase transformation to the α polymorph upon heating owing to its larger lattice volume.