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.

A Review on Combination of Ab Initio Molecular Dynamics and NMR Parameters Calculations

“…A similar experiment refers to the mobility of 7 Li ions in LiTi 2 (PO 4 ) 3 and LiZr 2 (PO 4 ) 3 [ 36 ]. A stimulus for this work was huge discrepancies between the results of a static NMR calculations and experimental data previously obtained for the above-mentioned systems.…”

Lithium-Ion Diffusion in Perovskite-Type Solid Electrolyte Lithium Lanthanum Titanate Revealed by Pulsed-Field Gradient Nuclear Magnetic Resonance

“…The decrease in C Q with increasing temperature is not a general behavior in crystalline solid electrolytes. Indeed, the opposite trend is observed in other crystalline solid electrolytes, such as LiTi 2 (PO 4 ) 3 , and Li-β-alumina. , Recent DFT–MD calculations have shown that framework vibrations and Li + -ion motion around the Li cavities lower the local symmetry around the Li + ions to increase C Q . In the case of LLTO, as the temperature increases, the TiO 6 tilt angle decreases and a phase transition to the cubic phases occurs. , These changes increase the local symmetry of the Li sites and consequently decrease C Q .…”

“…75,76 Recent DFT−MD calculations have shown that framework vibrations and Li + -ion motion around the Li cavities lower the local symmetry around the Li + ions to increase C Q . 77 In the case of LLTO, as the temperature increases, the TiO 6 tilt angle decreases 8 and a phase transition to the cubic phases occurs. 78,79 These changes increase the local symmetry of the Li sites and consequently decrease C Q .…”

Self-diffusion in polycrystalline Li1+xTi2−xAlx(PO4)3 (0.2 ≤ x ≤ 0.4) samples followed by 7Li PFG (pulse field gradient) NMR spectroscopy

“…To deduce structural reasons that increase Li mobility, Density Functional Theory (DFT) and Molecular Dynamics (MD) simulations should be used. 28,29,38…”