Crystal Structures of Li4ZrF8 and Li3Zr4F19 and Reinvestigation of the LiF-ZrF4 Phase Diagram

The crystal structure of Li4ZrF8 has been refined from X-ray powder data using the Rietveld profile analysis. This compound is isostructural with Li4UF8 and crystallizes in the orthorhomic system (space group Pnma, no. 62) with cell parameters a = 9.581(1) Å, b = 9.611(1) Å, c = 5.663(1) Å, Z = 4. The crystal structure of Li3Zr4F19 has been solved from single-crystal X-ray diffraction data and refined to a conventional R = 0.028 (Rw = 0.032) for 4078 unique reflections with I > 3σ(I). This fluoride crystallizes in the triclinic system (P1̄ space group no. 2) with unit cell dimensions a = 5.418(2) Å, b = 10.822(2) Å, c = 12.708(2) Å, α = 107.7°(1), β = 92.0°(1), γ = 103.4°(1). The 3D structure is built of corner- and edge-shared pentagonal bipyramids [ZrF7]. This three-dimensional framework delimits channels parallel to the a direction, with an S-like shape, within which lie the Li+ ions in octahedral coordination. These (LiF6)5- octahedra are linked by corners or edges in hexameric units. Since a controversial statement about the LiF-ZrF4 system exists in the literature, these published inconsistencies prompt a reinvestigation of the system. In the light of our structural studies a revised phase diagram of this system is presented.