Extensive lithium disorder in Li1.5Fe0.5Ti1.5(PO4)3 Nasicon by neutron diffraction, and the Li1+xFexTi2−x(PO4)3 phase diagram

Abstract

Phases with 0 < x ≤ 1.5 were synthesized within the system Li_1+xFe_xTi_2−x(PO₄)₃, of interest for applications as high Li⁺ mobility material. By powder X-ray diffractometry, three modifications were found to be stable for x ≤ 0.6 (Nasicon-type Rc), 0.6 < x < 1.1 (orthorhombic Pbca), and x ≥ 1.1 (orthorhombic Pbna). The compound Li_1.5Fe_0.5Ti_1.5(PO₄)₃ was employed for a detailed structural investigation by high resolution powder neutron diffraction (HRPD time-of-flight diffractometer, ISIS spallation source, UK) at 298 and 673 K. Using Rietveld and Fourier difference techniques, lithium atoms were located and the two structures refined to wR_p = 0.053 and 0.035, respectively. Lithium was found to be highly disordered, and distributed in similar amounts between the M1 and M2 cavities of the Nasicon framework, unlike what is usually observed in lithium Nasicon phases. A particularly large ion mobility should thus be expected. The configuration of Li sites within the M1 hollow is strongly affected on heating, so as to displace lithium towards the periphery of the cavity. The effects on the mobility pathways of Li⁺ ions are analyzed and discussed.