The Origin of Phase Transition and the Usual Evolutions of the Unit-Cell Constants of the NASICON Structures of the Solid Solution LiTi2 – xGex(PO4)3

Abstract

Ge-doped LiTi₂(PO₄)₃ has been synthesized by a conventional solid-state reaction. Compounds LiM\(_{2}^{{{\text{IV}}}}\)(PO₄)₃ with LTP-type structure present a different behaviour depending on nature of M^(IV). For M^(IV) = Ti and Ge, the structure shows the space group R3c, whereas for M^(IV) = Ge the space group is R3. Differences in behaviour of LiTi₂(PO₄)₃–LiGe₂(PO₄)₃ solid solutions are discussed in relation to the composition. Their structures LiTi_2 – xGe_x(PO₄)₃ (0 ≤ x < 2) were determined from X-ray powder diffraction method (XRD) using Rietveld analysis. A sharp change in the lattice parameter a is observed between the compositions with x = 1. The lattice parameter c increases as the Ge content increases in the whole range of composition. The space group R3c becomes R3 for the composition with x > 1. The SEM micrographs of the samples show relative porous microstructures due to the effect of the substitution.