Structural Aspects of Lithium-Ion Conduction in the Phosphidotitanate Li8TiP4 and Its Comparison With Li7+5 xTa1- xP4 and Li8- xTi1- xTaxP4.

The chemical system Li/Ti/P has previously been subject to intensive investigation. However, reliable structural data for the reported phases have remained elusive. Motivated by the growing interest in phosphorus-based lithium-ion conductors, we have reinvestigated the synthesis, crystal structure, and physical properties of Li8TiP4. Phase pure Li8TiP4 was obtained, which crystallizes in the tetragonal space group P42mc (no. 105) with a = 8.37581(2) Å and c = 5.90489(2) Å. According to a structure determination from X-ray diffraction powder data, Li8TiP4 is closely related to known Li8TtP4 with Tt = Si, Ge, and Sn, but adapts a different structure type. Basic structural findings are confirmed by solid state 6Li and 31P NMR spectroscopy. DFT calculations reveal a band gap of 2.5 eV and a good correlation between theoretical and experimental Raman spectra. From potentiostatic impedance spectroscopy an ion conductivity of (4.3 ± 0.6) × 10-6 S∙cm-1 at 298 K was found. In addition to the investigation of ternary Li8TiP4, an isotypic quaternary Ta-containing phase is observed and studied by single crystal structure determination. Special emphasis in this study is placed on the role of Li occupancy in the voids of the cubic close-packed (ccp) P atom arrangement and its impact on the ionic conductivity, in comparison to the known compounds Li7+5 xTaxP4 and Li8- xTi1- xTaxP4 and traced back to the difference in crystal symmetry. Possible diffusion pathways of the Li+ ions were approached by BVSE calculations.