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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. [ABSTRACT FROM AUTHOR]

The increasing demand for a high-performance and low-cost battery technology promotes the search for Li+-conducting materials. Recently, phosphidotetrelates and -aluminates were introduced as an innovative class of phosphide-based Li+-conducting materials featuring ionic conductivities of up to 3 mS cm−1 at ambient temperature. In order to get a deeper understanding in structure–property relationship of lithium ion conductors closely related structures that differ in their ionic conductivity are of special interest. Here, we report on the two polymorphs α- and β-Li8SnP4, which show ionic conductivities of up to 0.7 mS cm−1 and low activation energies EA of about 28 kJ mol−1 (0.29 eV) at 298 K. The structures of the two phases are determined by single crystal X-ray and powder neutron diffraction experiments at different temperatures, and their significantly different ionic conductivities allow for a detailed insight into the structure–property relationship. The investigations are completed by 6Li, 31P and 119Sn solid state magic angle spinning NMR, temperature-dependent 7Li NMR experiments and electrochemical impedance spectroscopy. Negative nuclear density maps reconstructed from experimental structure factors were analyzed by the maximum entropy method (MEM) and the one-particle-potential (OPP) formalism. Distinct Li+ migration pathways including divergent activation barriers have been identified, which allow to interpret the different conductivities of the two modifications. The importance of partially occupied octahedral sites in the β-phase is ascertained to cause considerably lower energy barriers to adjacent tetrahedral voids, which promote the higher conductivity in comparison to the α-phase. The title compounds complete the series of three phosphidotetrelates (α-)Li8SiP4, α-Li8GeP4 and β-Li8GeP4, and allow a detailed investigation of the structure–property relationships for further tailoring of the material properties. [ABSTRACT FROM AUTHOR]