Structural Aspects of Lithium‐Ion Conduction in the Phosphidotitanate Li 8 TiP 4 and Its Comparison With Li 7+5 x Ta 1− x P 4 and Li 8− x Ti 1− x Ta x P 4

ABSTRACT 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 Li 8 TiP 4 . Phase pure Li 8 TiP 4 was obtained, which crystallizes in the tetragonal space group P 4 2 mc (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, Li 8 TiP 4 is closely related to known Li 8 Tt P 4 with Tt = Si, Ge, and Sn, but adapts a different structure type. Basic structural findings are confirmed by solid state 6 Li and 31 P 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 Li 8 TiP 4 , 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 Li 7+5 x Ta x P 4 and Li 8− x Ti 1− x Ta x P 4 and traced back to the difference in crystal symmetry. Possible diffusion pathways of the Li + ions were approached by BVSE calculations.