Comparative structural analysis of LiMPO4 and Li2MPO4F (M = Mn, Fe, Co, Ni) according to XRD, IR, and NMR spectroscopy data

A comparative structural study of LiMPO4 (M = Mn, Fe, Co, Ni) orthophosphates and Li2MPO4F (M = Co, Ni) fluorophosphates obtained by mechanochemically assisted solid-state synthesis is performed using powder XRD, IR, and NMR spectroscopy methods. It is shown that all compounds crystallize in the orthorhombic symmetry (space group Pnma). Lattice parameters decrease on passing from Mn to Ni, which is due to the decrease in the ionic radius of the d metal. According to the IR spectroscopy data, in this series an increase in the covalency of the P–O bond is observed along with a decrease in the covalency of the M–O bond. On passing to fluorophosphates, the symmetry of PO4 tetrahedra increases. 6Li and 31P NMR spectra of all compounds are characterized by the dependence of the contact shift on the nature of metal M and the degree of distortion of the MO6 coordination polyhedron. 6Li MAS NMR line width is noticeably affected by the concentration of structural defects. Unlike orthophosphates with equivalent lithium ions, fluorophosphates contain lithium ions in three different positions.