Cation distribution in LiMgVO4 and LiZnVO4: structural and spectroscopic study

The room temperature cation occupancy in LiMgVO4 and LiZnVO4 crystallographic sites is obtained by means of the combined use of X-ray powder diffraction (XRPD), Li-7 and V-51 magic angle spinning nuclear magnetic resonance (MAS NMR), and micro-Raman measurements. In the LiMgVO4 Cmcm orthorhombic structure, the 4c (C-2v symmetry) tetrahedral vanadium site is fully ordered; on the contrary, the Li 4c tetrahedral site and the 4b (C-2h symmetry) Mg octahedral site display about 22% of reciprocal cationic exchange. Higher cationic disorder is observed in LiZnVO4: the three cations can distribute on the three tetrahedral and distinct sites of the R-3 structure. XRPD and MAS NMR analysis results highly agree for what concerns vanadium ion distribution on the three cationic sites (about 25, 26, and 47%). From the full profile fitting of XRPD patterns with the Rietveld method, it is also obtained that Li+ displays a slightly preferred occupation of the T1 position (similar to 55%) and Zn2+ of the T2 position (similar to 46%). The vibrational spectra of the two compounds are characterized by different peak positions and broadening of the Raman modes, reflecting the cation distribution and the local vibrational unit distortion. A comparison is also made with recent Raman results on Li3VO4. High temperature XRPD measurements rule out possible structural transitions up to 673 K for both compounds