Structural and electrochemical studies of rhombohedral Na2TiM(PO4)3 and Li1.6Na0.4TiM(PO4)3 (M = Fe, Cr) phosphates

The crystal structures of NASICON compositions Z(2)TiM(PO4)(3) (Z = Na, Li; M Cr, Fe) were investigated through powder neutron diffraction on the beam lines D1A (lambda = 1.9104 Angstrom) and D2B (lambda = 1.5924 Angstrom) at ILL, Grenoble, France. Na2TiCr(PO4)(3) and Na2TiFe(PO4)(3) are isostructural. They crystallize in the space group R (3) over barc, with the sodium ions being located in the M1 (tau similar to 1) and M2 sites (tau similar to 1/3). NASICON Li1.6Na0.4TiCr(PO4)(3) and Li1.6Na0.4TiFe(PO4)(3) were prepared through ion exchange from the sodium analogues. They crystallize in the space group R (3) over bar, with the lithium ions being located in the M3 tetrahedral site. The remaining, unexchanged, sodium ions are still located on the M1 site. Electrochemical insertion of sodium or lithium into these structures was followed through slow intermittent titration techniques coupled with in situ X-ray diffraction. We show that for both Na2+xTiM(PO4)(3) and Li1.6+xNa0.4TiM(PO4)(3) (0 less than or equal to X less than or equal to 1 for M = Cr, 0 less than or equal to x less than or equal to 2 for M = Fe), the insertion proceeds through a single-phase mechanism.