Modeling Ti/Ge Distribution in LiTi_(2-x)Ge_x(PO_4)_3 NASICON Series by ~(31)P MAS NMR and First-Principles DFT Calculations

摘要

Ti/Ge distribution in rhombohedral Li- Ti_(2-x)Ge_x(PO_4)_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, PO_4 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 R3c NASICON samples. However, ~(31)P MAS NMR spectra of analyzed series display a higher number of signals, suggesting that, although the overall symmetry remains R3c, 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 Ti_(4-n)Ge_n arrangements in the R3 subgroup symmetry. In this assignment, the influence of octahedra of the same or different R_2(PO_4)_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 LiTi_(2-x)Ge_x(PO_4)_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 PO_4 units. This random distribution was supported by XRD data displaying Vegard's law.