Modeling Ti/Ge Distribution in LiTi_2–xGe_x(PO₄)₃ NASICON Series by ³¹P MAS NMR and First-Principles DFT Calculations

Abstract

Ti/Ge distribution in rhombohedral LiTi2-xGex(PO4)3 NASICON series has been analyzed by (31)P magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and first-principles density functional theory (DFT) calculations. Nuclear magnetic resonance is an excellent probe to follow Ti/Ge disorder, as it is sensitive to the atomic scale environment without long-range periodicity requirements. In the samples considered here, PO4 units are surrounded by four Ti/Ge octahedra, and then, five different components ascribed to P(OTi)4, P(OTi)3(OGe), P(OTi)2(OGe)2, P(OTi)(OGe)3, and P(OGe)4 environments are expected in (31)P MAS NMR spectra of R3̅c NASICON samples. However, (31)P MAS NMR spectra of analyzed series display a higher number of signals, suggesting that, although the overall symmetry remains R3̅c, partial substitution causes a local decrement in symmetry. With the aid of first-principles DFT calculations, 10 detected (31)P NMR signals have been assigned to different Ti4-nGen arrangements in the R3 subgroup symmetry. In this assignment, the influence of octahedra of the same or different R2(PO4)3 structural units has been considered. The influence of bond distances, angles and atom charges on (31)P NMR chemical shieldings has been discussed. Simulation of the LiTi2-xGex(PO4)3 series suggests that detection of 10 P environments is mainly due to the existence of two oxygen types, O1 and O2, whose charges are differently affected by Ge and Ti occupation of octahedra. From the quantitative analysis of detected components, a random Ti/Ge distribution has been deduced in next nearest neighbor (NNN) sites that surround tetrahedral PO4 units. This random distribution was supported by XRD data displaying Vegard's law.

Cation Miscibility and Lithium Mobility in NASICON Li1+xTi2–xScx(PO4)3 (0 ≤ x ≤ 0.5) Series: A Combined NMR and Impedance Study

“…Results deduced in Li 1.2 Ti 1.8 Sc 0.2 (PO 4 ) 3 show reasonable P–O distances between 1.53(3) and 1.55(8) Å and Ti,Sc–O distances between 1.86(5) and 1.95(8) Å. The Ti,Sc–O1 distances are appreciably shorter than Ti,Sc–O2, indicating the existence of some modification on charges of O1 and O2 oxygens . At room temperature, Li–O distances are between 2.22(5) and 2.25(1) Å in M1 sites, and Li3–O between 2.16(4) and 2.18(6) Å in M3 sites (Figure b).…”

“…6 Li/ 7 Li, 45 Sc, and 31 P NMR spectra have been recorded at room temperature in an AVANCE-400 Bruker spectrometer (9.4 T) working at 58.88/155.50, 97.20, and 161.97 MHz, respectively. In magic angle spinning (MAS) experiments, samples were rotated around an axis inclined 54°44′ with respect to the external magnetic field at 2 kHz for 6 Li/ 7 Li and 4 kHz for 31 P and 45 Sc signals. 6 Li/ 7 Li and 31 P NMR signals were recorded after π/2 (3 and 4 μs) and 45 Sc NMR signals after π/8 (1 μs) pulse irradiation.…”

Superionic Halogen-Rich Li-Argyrodites Using In Situ Nanocrystal Nucleation and Rapid Crystal Growth

“…Figure b displays the 31 P NMR spectra of HRLA samples with different Cl contents. NMR spectra were deconvoluted into peaks in four different ppm regions with each deconvoluted region interpreted in conjunction with previous studies ,, and our Rietveld refinement results. The blue and red areas indicate site ordering due to elemental S or Cl that predominantly occupy the 4 a and 4 c sites.…”

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

“…Studies of this type could be carried out on other well characterized materials in which there is confined movement (in the ps range) of an atom. If the system had a moderate degree of positional/occupational disorder, it could be modeled with supercells or a set of simple models . However, the computing effort would be higher than for ordered models.…”

“…If the system had am oderate degree of positional/occupational disorder,i t could be modeled with supercells or as et of simple models. [16] However, the computing effort would be highert han for ordered models. Ab initio molecular dynamics, which has served in this study to analyze the confined mobility of lithium ions, could be used to study the conduction paths of ions in materials of interest in lithium or post-lithium batteries.…”