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 P2(1)/n (a = 8.5539(4) Angstrom; b = 8.6099(4) Angstrom; c = 11.9825(5) Angstrom; beta = 90.002(7)degrees). Combined studies using Li-6,Li-7 MAS NMR, EPR, and ac impedance spectroscopy show that this pseudo-orthorhombic lattice represents a room-temperature analogue of the high-temperature orthorhombic phase gamma-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 Li-6,Li-7 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.