Supertetrahedral polyanionic network in the first lithium phosphidoindate Li(3)InP(2) – structural similarity to Li(2)SiP(2) and Li(2)GeP(2) and dissimilarity to Li(3)AlP(2) and Li(3)GaP(2)

Phosphide-based materials have been investigated as promising candidates for solid electrolytes, among which the recently reported Li(9)AlP(4) displays an ionic conductivity of 3 mS cm(−1). 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 Li(3)InP(2), which is easily accessible via ball milling of the elements and subsequent annealing. Li(3)InP(2) crystallizes in the tetragonal space group I4(1)/acd with lattice parameters of a = 12.0007(2) and c = 23.917(5) Å, featuring a supertetrahedral polyanionic framework of interconnected InP(4) tetrahedra. All lithium atoms occupy tetrahedral voids with no partial occupation. Remarkably, Li(3)InP(2) is not isotypic to the previously reported homologues Li(3)AlP(2) and Li(3)GaP(2), which both crystallize in the space group Cmce and feature 2D layers of connected tetrahedra but no supertetrahedral framework. DFT computations support the observed stability of Li(3)InP(2). 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 Li(3)InP(2) 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 Li(2)SiP(2) and Li(2)GeP(2).