2446 ISSN 1063-7834, Physics of the Solid State, 2019, Vol. 61, No. 12, pp. 2446–2450. © Pleiades Publishing, Ltd., 2019. The Origin of Phase Transition and the Usual Evolutions of the Unit-Cell Constants of the NASICON Structures of the Solid Solution LiTi 2– x Ge x (PO 4 ) 3 Nedjemeddine Bounar* LIME Laboratory, University of Jijel, Jijel, Algeria *e-mail: nedjmbounar@yahoo.fr Received July 8, 2019; revised July 8, 2019; accepted July 15, 2019 Abstract—Ge-doped LiTi 2 (PO 4 ) 3 has been synthesized by a conventional solid-state reaction. Compounds LiM (PO 4 ) 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 LiTi 2 (PO 4 ) 3 –LiGe 2 (PO 4 ) 3 solid solutions are discussed in relation to the com- position. Their structures LiTi 2– x Ge x (PO 4 ) 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. Keywords: NASICON, origin of unusual evolutions of lattice parameters, phase transition, scanning electron microscopy SEM, X-ray diffraction DRX, Rietveld refinements DOI: 10.1134/S1063783419120072 1. INTRODUCTION The NASICON (Na Super Ionic CONductors)- type materials are good ionic conductors when serving as solid electrolytes for Li-ion batteries. Lithium tita- nium phosphate LiTi 2 (PO 4 ) 3 with NASICON-type structure has been well known as a solid electrolyte material for Li-ion batteries [1–5]. This ceramic structure is known as high ionic conducting materials and is therefore potential solid electrolyte that could be used in batteries with high energy density ranging from portable electronic devices such as laptop and calculators to electrical vehicles [1, 6, 7]. Properties of any materials generally depend on its purity and com- position [8]. Several researchers synthesized NASI- CON-type systems [9] by conventional solid state method which required high temperatures [1, 10]. The conventional method of heating the stoichiometric amounts of reactants at high temperatures leads to inhomogeneity, impurities, defects, and large disper- sion of the particle size [11–14]. The NASICON structure [15] is built up of corner-sharing BO 6 octa- hedra and PO 4 tetrahedra leading to a framework of B 2 P 3 O 12 formulas with interconnected channels where cations can be inserted in two types of sites usually noted M 1 at 6b (0, 0, 0) and M 2 at 18e (2/3, 0, 1/4) Wyckoff positions (Fig. 1). The great flexibility of this structure allows large chemical substitutions [16] and makes it possible that the sites M 1 and M 2 may be empty as in Nb 2 (PO 4 ) 3 [17], partially occupied as in [18–20] or completely full as in Na 4 Zr 2 (SiO 4 ) 3 [21]. The study of solid solutions NaTi 2 (PO 4 ) 3 – NaGe 2 (PO 4 ) 3 was carried out by Carrasco et al. [22], who observed a phase transition, as a function of the degree of substitution of Ge. In this paper, we report the structural characterisation of the phases in the solid solution LiTi 2– x Ge x (PO 4 ) 3 (0 ≤ x < 2) using the powder X-ray diffraction and the Rietveld refinements and their microstructures using Scanning Electronic Microscopy. A comparative study is carried out with the phases of the Na analogue to explain the evolution of the NASICON compounds lattice parameters and the phase transition with the nature of the substituted cations. 2. EXPERIMENT DESCRIPTION Syntheses of LiTi 2– x Ge x (PO 4 ) 3 (x = 0, 0.2–1.8) were carried out using conventional solid state reac- tion techniques. First, stoichiometric mixtures of Li 2 CO 3 , (NH 4 ) 2 HPO 4 , TiO 2 , and GeO 2 were heated at 673 K for 6 h in order to decompose the ammonium phosphate and the lithium carbonate. In a second IV 2 PHASE TRANSITIONS