Crystal structures of the new fluorophosphates Li9Mg3[PO4]4F3 and Li2Mg[PO4]F and ionic conductivities of selected compositions

Abstract

The new compounds Li₂Mg[PO₄]F and Li₉Mg₃[PO₄]₄F₃ have been synthesized by a solid state reaction route. The crystal structures were determined from single-crystal X-ray diffraction data. Li₂Mg[PO₄]F crystallizes with the orthorhombic Li₂Ni[PO₄]F structure, space group Pnma, a = 10.7874(3), b = 6.2196(5), c = 11.1780(4) Å, V = 721.13(10) ų, and Z = 8, whereas Li₉Mg₃[PO₄]₄F₃ crystallizes with hexagonal symmetry, space group P6₃, with a = 12.6159(6), c = 5.0082(4) Å, V = 690.32(7) ų, and Z = 2. A merohedral twinning was taken into account for its structural refinement. The structure of Li₂Mg[PO₄]F contains MgO₃F chains made up of edge-sharing MgO₄F₂ octahedra. These chains are interlinked by PO₄ tetrahedra forming a 3D-Mg[PO₄]F framework. The lithium atoms occupy mainly three distinct crystallographic sites. The structure of Li₉Mg₃[PO₄]₄F₃ consists of corner-sharing MgO₄F₂ octahedra forming MgO₄F chains running along the c axis. These chains are interlinked by PO₄ tetrahedra forming a 3D-Mg₃[PO₄]₄F₃ framework with hexagonal and pentagonal tunnels, in which are located the Li atoms. This study reveals also a strong relationship between Li₂Mg[PO₄]F-, Mg_1−xFe_xAl₃[BO₃][SiO₄]O₂- and P2₁/c-Li₅V[PO₄]₂F₂- structures; and between P6₃-Li₉Mg₃[PO₄]₄F₃ and P2₁/c-Na₂Mn[PO₄]F. The ionic conductivities σ of the composite material Li₆Mg₄[PO₄]₃[SO₄]F₃ and Li₉Mg₃[PO₄]₄F₃, estimated using electrochemical impedance spectroscopic analyses at 300 °C, are 3.9 × 10⁻⁵ and 10⁻⁴ S cm⁻¹ with activation energies of 0.524 eV and 0.835 eV, respectively.