Li2.5V2(PO4)3: A Room-Temperature Analogue to the Fast-Ion Conducting High-Temperature γ-Phase of Li3V2(PO4)3

The structure of the NASICON-like phase Li2.5V2(PO4)3 has been determined by X-ray and neutron powder diffraction, demonstrating that it crystallizes in the space group P21/n (a = 8.5539(4) A; b = 8.6099(4) A; c = 11.9825(5) A; β = 90.002(7)°). Combined studies using 6,7Li MAS NMR, EPR, and ac impedance spectroscopy show that this pseudo-orthorhombic lattice represents a room-temperature analogue of the high-temperature orthorhombic phase γ-Li3Sc2(PO4)3. Lithium disorder over partially vacant sites leads to enhanced ion mobility. EPR spectroscopy confirms the presence of localized V4+ in the lattice, whose short-range order with V3+ gives rise to five resolvable Li shifts in the 6,7Li MAS NMR spectrum. These shifts can be rationalized by consideration of the Fermi-contact interactions arising from the difference between V3+ and V4+ coupling in the three lithium sites.