Structure and vibrational analyses of LiP₁₅

LiP₁₅ was synthesized by reaction of the elements in the presence of CuCl₂ as a mineralizer additive in sealed silica ampoules at 823 K. The product was characterized by powder and single-crystal X-ray diffraction experiments, quantum chemical calculations and IR and Raman spectroscopy. It crystallizes in the triclinic crystal system in space group P1¯ with the lattice parameters a = 6.974(1) Å, b = 9.008(1) Å, c = 11.294(2) Å, α = 104.51(1)°, β = 93.78(1)°, γ = 106.330(10)° and V = 652.1 ų. The structure of the phosphorus-rich polyphosphide consists of tubular polyphosphide strands that are coordinated by lithium atoms to form double strands with an antiparallel arrangement. Quantum chemical investigations and structure optimizations were applied to localize the Li positions within the polyphosphide framework, and a full factor group analysis was done to understand the complex IR and Raman spectra of the title compound. LiP₁₅ does not crystallize isotypically to compounds with the higher homologues of lithium but forms its own structure type, in contrast to previous reports. On the basis of our results, a LiP₁₅ polymorph isotypic to KP₁₅ might exist energetically separated by a few kJ/mol. We also predict that a second KP₁₅ polymorph with the reported LiP₁₅ structure would be energetically as favourable as the known one.