Structural Aspects of Lithium-Ion Conduction in the Phosphidotitanate Li₈TiP₄ and Its Comparison With Li_7+5xTa_1−xP₄ and Li_8−xTi_1−xTa_xP₄

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 Li_7+5xTa_xP₄ and Li_8−xTi_1−xTa_xP₄ and traced back to the difference in crystal symmetry. Possible diffusion pathways of the Li⁺ ions were approached by BVSE calculations.