Super-Ionic Conductivity in ω-Li₉TrP₄ (Tr = Al, Ga, In) and Lithium Diffusion Pathways in Li₉AlP₄ Polymorphs

Phosphide-based compounds are promising materials for solid electrolytes. In recent times, a multiplicity of compounds featuring isolated MP₄ (M = Si,Ge,Sn,Al,Ga) tetrahedra as structural building units in different arrangements with superionic lithium conductivity have been discovered. ω-Li₉AlP₄, ω-Li₉GaP₄, and ω-Li₉InP₄ are presented as new high-temperature modifications with superionic lithium conductivity reaching 4.5 mS cm⁻¹ at room temperature. Impedance spectroscopy and static temperature-dependent ⁷Li NMR experiments reveal conductivity values in the range of 0.2 to 4.5 mS cm⁻¹ at room temperature and low activation energies for the title compounds. X-ray and neutron diffraction methods disclose that the phosphorus atoms form a cubic-close packing. The triel element and Li atoms are located in tetrahedral voids, further Li atoms partially fill the octahedral voids. Temperature-dependent neutron diffraction shows for Li₉AlP₄ a phase transition at 573 K that influences the occupation of voids with Li and significantly affects the Li-ion mobility. The evaluation of nuclear scattering densities by the maximum-entropy approach and application of the one-particle-potential formalism reveal 3D lithium diffusion with a low activation energy preferentially on paths of adjacent tetrahedral and octahedral voids. The investigation of different polymorphs suggests that the equilibrated filling of tetrahedral and octahedral voids is a crucial parameter for the enhancement of superionic lithium conductivity.