Supertetrahedral polyanionic network in the first lithium phosphidoindate LiInP - structural similarity to LiSiP and LiGeP and dissimilarity to LiAlP and LiGaP.

Phosphide-based materials have been investigated as promising candidates for solid electrolytes, among which the recently reported LiAlP displays an ionic conductivity of 3 mS cm. While the phases Li-Al-P and Li-Ga-P have already been investigated, no ternary indium-based phosphide has been reported up to now. Here, we describe the synthesis and characterization of the first lithium phosphidoindate LiInP, which is easily accessible ball milling of the elements and subsequent annealing. LiInP crystallizes in the tetragonal space group 4/ with lattice parameters of = 12.0007(2) and = 23.917(5) Å, featuring a supertetrahedral polyanionic framework of interconnected InP tetrahedra. All lithium atoms occupy tetrahedral voids with no partial occupation. Remarkably, LiInP is not isotypic to the previously reported homologues LiAlP and LiGaP, which both crystallize in the space group and feature 2D layers of connected tetrahedra but no supertetrahedral framework. DFT computations support the observed stability of LiInP. A detailed geometrical analysis leads to a more general insight into the structural factors governing lithium ion mobility in phosphide-based materials: in the non-ionic conducting LiInP the Li ions exclusively occupy tetrahedral voids in the distorted close packing of P atoms, whereas partially filled octahedral voids are present in the moderate ionic conductors LiSiP and LiGeP.