Crystal structure and ion conductivity of a new mixed-anion phosphate LiMg3(PO4)P2O7

Abstract

A new lithium-containing phosphate, LiMg3(PO4)P2O7, was prepared by a solid-state reaction, and it was characterized by an ab initio structure determination method on the basis of synchrotron powder X-ray diffraction data. LiMg3(PO4)P2O7 was found to be orthorhombic (space group Pnma) with lattice parameters a=9.0387(1)Å, b=10.6072(1)Å, c=8.3065(1)Å, and V=796.39(1)Å3. The structure features infinite [Mg3O10]∞ layers that are parallel to the bc plane and that are interconnected along the a axis by PO4 and P2O7 groups. The [Mg3O10]∞ layer contains Mg3O14 trimers that are formed by three edge-shared MgO6 octahedra. The PO4 and P2O7 groups are located alternatively between [Mg3O10]∞ layers. This gives rise to a three-dimensional framework that contains large tunnels along the directions [100] and [010]; the Li+ ions are stabilized in these tunnels. AC impedance spectroscopy shows that LiMg3(PO4)P2O7 has an ionic conductivity of 3.40×10−5Scm−1 at 769K, with an activation energy of 1.17eV.