Structural Factors That Enhance Lithium Mobility in Fast-Ion Li1+xTi2-xAlx(PO4)(3) (0 <= x <= 0.4) Conductors Investigated by Neutron Diffraction in the Temperature Range 100-500 K

Structural features responsible for lithium conductivity in Li1+xTi2-xAlx(PO4)(3) (x = 0, 0.2, and 0.4) samples have been investigated by Rietveld analysis of high-resolution neutron diffraction (ND) patterns. From structural analysis, variation of the Li site occupancies and atomic thermal factors have been deduced as a function of aluminum doping in the temperature range 100-500 K. Fourier map differences deduced from ND patterns revealed that Li ions occupy M1 sites and, to a lower extent, M3 sites, disposed around ternary axes. The occupation of M1 sites by Li ions is responsible for the preferential expansion of the rhombohedral R (3) over barc unit cell along the c axis with temperature. The occupation of less symmetric M3 sites decreases electrostatic repulsions among Li cations, favoring ion conductivity in Li1+xTi2-xAlx(PO4)(3) compounds. The variations detected on long-range lithium motions have. been related to variations of the oxygen thermal factors with temperature. The information deduced by ND explains two lithium motion regimes deduced previously by Li-7 NMR and impedance spectroscopy.