Effects of Li Confined Motion on NMR Quadrupolar Interactions: A Combined 7Li NMR and DFT-MD Study of LiR2(PO4)3 (R=Ti or Zr) Phases

Abstract

Ab initio molecular dynamics (MD) simulations and NMR GIPAW (gauge including projector augmented wave) calculations have been used to analyze the coordination and mobility of Li ions in LiTi2(PO4)3 (rhombohedral), LiZr2(PO4)3 (triclinic), and LiZr2(PO4)3 (rhombohedral) phases. Significant discrepancies are observed between static calculations of 7Li quadrupolar parameters and experimental values. The dynamical origin of this disagreement is demonstrated by incorporating in the calculations thermal vibrations and local motion of atoms with MD simulations. For LiTi2(PO4)3, the quadrupolar constant associated with Li ions grows with temperature because the local symmetry of the system decreases, whereas for the Zr phases, the quadrupolar constant decreases because thermal vibrations reduce the anisotropy of the interaction. Finally, for both Zr phases, MD yields Li distributions that compare well with disorder reported from diffraction studies.