An Uncommon Hypervalent Fluorooxosilicophosphate

The species diversity of silicon (including traditional tetrahedral coordinated silicon and hypervalent penta- and hexa-coordinate silicon) gives rise to the structural richness and diverse properties of silicates. Among these silicon species, hypervalent silicon is very rare, not to mention almost unexplored mixed-anion hypervalent fluoroxosilicate species. In this work, we successfully obtained a mixed-anion fluorooxosilicophosphate Na4Si2PO4F9 consisting of two uncommon hypervalent fluoroxosilicate species, namely, trans-SiO2F4 species and SiOF₅ species. To the best of our knowledge, such hypervalent silicon species are reported for the first time in inorganic compounds. Remarkably, the coexistence of two distinct hypervalent fluoroxosilicate species in one compound is somewhat conflicted with Pauling's parsimony rule, but it indeed achieves an unlikely connection by PO₄ and our phonon dispersion calculation confirms the structure stability of Na₄ Si₂ PO₄ F₉ . Temperature-dependent conductivity measurements show that Na₄ Si₂ PO₄ F₉ is a promising solid ionic conductor with a high conductivity of 4.0×10^-5  S⋅cm^-1 at 700 K and a low active energy of about 53.1 KJ⋅mol^-1 . This work will enrich the structure chemistry of silicates and may provide a new platform for solid ionic batteries.