Fast Lithium-Ion Conduction in Phosphide Li₉GaP₄

Lithium-ion conductors are currently tested for their possible usage in all-solid-state lithium-ion batteries. In order to design high-performance solid electrolytes, the fundamental understanding of the relationships of the atomic structure and the transport properties such as temperature-dependent ionic conductivity is a basic prerequisite. Therefore, systematic investigations of closely related structures are essential. Phosphide-based materials are promising candidates for solid electrolytes, and recently, we have shown that the superionic conductor Li9AlP4 with an ionic conductivity of 3 mS cm-1 at room temperature can be obtained by the substitution of Si by Al in Li8SiP4. Now, we present the heavier gallium homologue Li9GaP4, which reveals a similarly high superionic conductivity of 1.6 mS cm-1 and a low activation energy. Li9GaP4 is easily accessible via ball milling of the elements and subsequent annealing at quite moderate temperatures. The single-crystal X-ray structure determination reveals that Li9GaP4 is isotypic to Li9AlP4 and crystallizes in the cubic space group P4¯ 3n (no. 218) with a lattice parameter of a = 11.868(1) Å. Temperature-dependent single-crystal X-ray diffraction reveals that lithium is not located at the center of the octahedral voids of the slightly distorted cubic close packing of P atoms but occurs with split positions. Impedance spectroscopy and temperature-dependent static 7Li NMR experiments reveal activation energies of 36 and 25 kJ mol-1, respectively.