Phase Diagram of the Li 4 GeS 4 –Li 3 PS 4 Quasi‐Binary System Containing the Superionic Conductor Li 10 GeP 2 S 12

A phase diagram is constructed for the quasi‐binary Li 4 GeS 4 –Li 3 PS 4 system containing the lithium superionic conductor Li 10 GeP 2 S 12 ( LGPS ), having an LGPS ‐type structure, and the β‐Li 3 PS 4 ‐type phase, having a thio‐ LISICON structure. The LGPS and thio‐ LISICON intermediate compounds are found to exhibit solid solution ranges and show incongruent melting at 650°C and 560°C, respectively. The end‐member Li 4 GeS 4 has a compositional range of 0 <  k  <   0.3 in [(1− k ) Li 4 GeS 4  +  k Li 3 PS 4 ], while the other end‐member γ‐Li 3 PS 4 has no solid solution range. The crystal structures appearing in the binary systems are the α‐, β‐, and γ‐type Li 3 PS 4 structures and the LGPS ‐type structure, and these are formed by PS 4 tetrahedra with different arrangements in each structure. The phase and structural relationships between the compounds appearing in the Li 4 GeS 4 –Li 3 PS 4 system are thus clarified.