Ti/Ge distribution in rhombohedral LiTi2-xGex(PO4)3 NASICON series has been analyzed by 31P 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 31P MAS NMR spectra of R3¯c NASICON samples. However, 31P MAS NMR spectra of analyzed series display a higher number of signals, suggesting that, although the overall symmetry remains R3¯c, partial substitution causes a local decrement in symmetry. With the aid of first-principles DFT calculations, 10 detected 31P NMR signals have been assigned to different Ti-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 31P NMR chemical shieldings has been discussed. Simulation of the LiTi-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.
J. Am. Chem. Soc. 138, 9479-9486 (2016)
Figure 1. Detail of NASICON structure with R octahedra denoted in gray, P tetrahedra in orange, O atoms in red, and Li atoms in purple.