**2. Experimental details**

Microcrystalline material is prepared by the conventional solid-state reaction of the stoichiometric mixture of Li2CO3 (Himedia, 99.0%), NH4H2PO4 (Himedia, 99.0%), TiO2 (LR grade, 98.0%), Al2O3 (Himedia, 99.0%) and V2O5 (Himedia, 99.0%). Overall reaction for the formation of LiTi2(PO4)3 and Li1.3Ti1.7Al0.3(PO4)2.9(VO4)0.1 [LATPV0.1] are given as:

> 0.5Li2CO3+2TiO2+3NH4H2PO4 ∆ LiTi2(PO4)3+3NH3 +0.5CO2+4.5H2O 0.65Li2CO3+1.7TiO2+0.15Al2O3+2.9NH4H2PO4+0.05V2O5 ∆ Li1.3Ti1.7Al0.3(PO4)2.9(VO4)0.1 + 0.65CO2 + 2.9NH3 + 4.35H2O

Various steps involved in the synthesis of microcrystalline materials are:


To overcome the shortcomings in the conventional synthesis of NASICON, high-energy ball milling and solution combustion technique are explored. Correlation between mobile ion conduction and phase symmetry in NASICONs is explored in this study. Present chapter deals with the structure and electrical properties of important family of NASICONs like: i. LiTi2(PO4)3 and Li1.3Al0.3Ti1.7(PO4)2.9(VO4)0.1 synthesized by high energy ball-milling. ii. A3M2(PO4)3 (A=Li, Na and M=Cr, Fe) synthesised by solution combustion technique. Characterization techniques like X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), thermogravimetry and differential thermal analysis (TG-DTA) *etc* are exploited for structural confirmation of the synthesized material. Microscopy of the surface is analyzed using scanning electron microscope (SEM) and transmission electron microscope (TEM). UV-vis spectroscopy is used for confirmation of the electronic state of the transition elements and Kramers-Kronig test is performed for confirming the quality of measured electrical parameters. Transport number is measured by Wagner polarization technique. The electrical relaxation parameters are investigated in the frequency range 10Hz-25MHz at different temperatures using broadband dielectric spectrometer. Magnetic behavior of the material is investigated by vibrating sample magnetometer (VSM). In general, complex impedance, admittance, permittivity and modulus forms are used for representation of different electrical parameters. Present chapter uses impedance/dielectric spectroscopy

Microcrystalline material is prepared by the conventional solid-state reaction of the stoichiometric mixture of Li2CO3 (Himedia, 99.0%), NH4H2PO4 (Himedia, 99.0%), TiO2 (LR grade, 98.0%), Al2O3 (Himedia, 99.0%) and V2O5 (Himedia, 99.0%). Overall reaction for the

0.5Li2CO3+2TiO2+3NH4H2PO4 ∆ LiTi2(PO4)3+3NH3 +0.5CO2+4.5H2O

0.65Li2CO3+1.7TiO2+0.15Al2O3+2.9NH4H2PO4+0.05V2O5 ∆ Li1.3Ti1.7Al0.3(PO4)2.9(VO4)0.1 + 0.65CO2 + 2.9NH3 + 4.35H2O

i. Stoichiometric amounts of starting reagents were ground in an agate mortar for

ii. The mixture is placed in a silica crucible and slowly heated in an electric furnace up to 523K. Further, the temperature is increased to 623K and held at this temperature for 6h

iii. After cooling the mixture to room temperature, it is again ground for 45min in an agate mortar and pellets of 10mm diameter and 1-1.5mm thickness was formed. Further pellets were heat treated at 923K for 6h. Heating procedure remains the same for both

iv. Further, LiTi2(PO4)3 pellets were calcined at 1073K for 36h followed by sintering at 1223K for 2h. In the meanwhile, the pellets of Li1.3Ti1.7Al0.3(PO4)2.9(VO4)0.1 is calcined at

formation of LiTi2(PO4)3 and Li1.3Ti1.7Al0.3(PO4)2.9(VO4)0.1 [LATPV0.1] are given as:

Various steps involved in the synthesis of microcrystalline materials are:

in order to ensure the total decomposition of the initial reagents.

1073K for 48h followed by sintering at 1323K for 4h.

technique for the electrical characterization of mobile ions.

**2. Experimental details** 

45minutes.

the systems till this stage.

Crystallites of smaller size materials are prepared through conventional solid-state reaction of the ball-milled stoichiometric mixture. The mixture is heated at 623K before ball-milling to remove the gases and water content. This minimizes sticking property of the mixture to the vial and balls. The tungsten carbide vial and balls were used for high energy milling; the typical ball to powder mass ratio is kept at 5:1 throughout the milling. The rotation speed is kept at 300rpm, each cycle comprised of 2h run followed by 30minutes pause, and these cycles were repeated. Milling is carried out in an ethanol medium in case of Li1.3Ti1.7Al0.3(PO4)2.9(VO4)0.1, which acts as a surfactant to decrease the agglomeration and helps to reduce the heat produced while milling. The powder obtained after milling was made into pellets and further heat treatments were applied from 923K to 1223K for LiTi2(PO4)3, and 923K to 1323K for Li1.3Ti1.7Al0.3(PO4)2.9(VO4)0.1 with the same duration as the microcrystalline sample. In this study, material is sintered at a temperature lower than the conventional ceramic route. Even though, the sintering temperature is low, long hours of sintering are performed to obtain the required density for samples. Low temperature sintering is applied to maintain the nanocrystalline nature of the samples.

Self propagating solution combustion synthesis is a rapid and energy saving technique that works on the principle of decomposition of an oxidizer, metal nitrate, in the presence of fuel/complexing agent . The Na3Cr2(PO4)3 using glycine in 1:1fuel ratio (Na3Cr2(PO4)3- G1:1) is prepared from NaNO3 and Cr(NO3)3.9H2O. Stoichiometric amount of the metal nitrates and glycine (NH2-CH2COOH) were mixed with distilled water in 1:1 molar ratio. The NH4H2PO4 dissolved in distilled water is added to this mixture to form homogenous solution. Slow evaporation of the homogenous solution produced thick viscous gel. Further heating resulted in flame, producing voluminous powder named as-prepared material. Over all reaction for the formation of Na3Cr2(PO4)3-G1:1 is calculated as:

3NaNO3+ 2Cr(NO3)3.9H2O+3NH4H2PO4+8NH2-CH2COOH+5O2 Δ Na3Cr2(PO4)3+10N2+16CO2+47H2O

In the case of glycine-nitrate combustion, primarily N2, CO2, and H2O were evolved as gaseous products. As-prepared material is in amorphous phase and further heating at 800C produced the pure Na3Cr2(PO4)3 phase. To understand the effect of fuel molar ratio on physical and electrical properties; glycine, urea and citric acid were used in 1:1, 1:2 and 1:3 molar ratios for the synthesis of Na3Cr2(PO4)3.

The Fe3+ based NASICON materials were synthesized using citric acid: ethylene glycol mixture (CA:EG). The metal cations were complexed by citric acid (C6H8O7) and pH of the resultant solution is adjusted in the range 7-8 using ammonia solution. This solution is kept under constant stirring and NH4H2PO4 is added to it. After proper stirring, ethylene glycol is added to this solution by maintaining the molar ratio with citric acid at 1:1. The homogenous solution is heated further and the as-prepared material is formed. Further calcination at 800◦C resulted in pure phase. Objective of the present investigation is to synthesize nanocrystalline materials by a unique combination of citric acid (as complexing agent) and ethylene glycol (as polymerizing agent). In the presence of ethylene glycol, esterification (reaction between alcohol and acid) resulted in the formation of gel. The Li3Fe2(PO4)3 is also prepared using glycine in 1:2 molar ratio.

NASICON Materials: Structure and Electrical Properties 81

where, B is the full width at half maximum (FWHM) of XRD peaks, K is the Scherrer constant, D is the crystallite size, is the wavelength of X-ray, is the micro-strain in the

where, Bm is the FWHM of the material and Bs is the FWHM of a standard sample; silicon is chosen as the standard for calculation of instrumental parameters. Linear extrapolation of the plot of Bcos *vs* 4sin gives average crystallite size from the intercept, K/D and the slope gives micro-strain. The strain contribution in Eq. (1) is negligible for the crystallite size calculation of microcrystalline materials. Micro-strain and average crystallite size of

Ball-milling induces strain in the crystal lattice and decreases the average crystallite size to 70nm for 40h ball-milled LiTi2(PO4)3 material. Milling reduces the average size of crystallites to nanometer range and long hours of ball-milling lead to the formation of an amorphous state (Yamamoto et al., 2004 & Nobuya et al., 2005). Hence, sintering at high temperature after ball-milling resulted in the formation of nanocrystallites instead of microcrystalline material. XRD pattern gradually broadens and the particle size decreases with milling duration, which is clear from the FWHM of highest intensity peaks of ball-milled Li1.3Ti1.7Al0.3(PO4)2.9(VO4)0.1 as given in Fig. 1(d). The nanocrystalline nature of the ballmilled materials is evident from the broadened XRD peak and there is decrease in peak

LiTi2(PO4)3

 Micro-crystalline (0.23±0.01)m (0.05±0.001)% 8.514(9) 20.857(2) 1309.633(0) Nano-crystalline (70.14±0.07)nm (0.36±0.05)% 8.495(9) 20.719(5) 1295.156(6) Li1.3Ti1.7Al0.3(PO4)2.9(VO4)0.1 Micro-crystalline (1.60 ±0.49)m (0.02±0.003)% 8.500(9) 20.819(6) 1302.958(1) 22h ball-milled (86.62±0.27)nm (0.29±0.04)% 8.504(1) 20.825(2) 1304.303(6) 55h ball-milled (60.86±0.34)nm (0.62±0.06)% 8.512(9) 20.845(0) 1308.254(0) Table 1. Average crystallite size, micro-strain and unit cell parameters of microcrystalline

The Na3Cr2(PO4)3 is synthesised using glycine, urea and citric acid in 1:1,1:2 and 1:3 molar ratios by solution combustion technique. The Na3Cr2(PO4)3 synthesized through conventional ceramic route is reported to exhibit two main structural phase transitions at 138ºC and 166ºC, before the stable rhombohedral symmetry is attained at high temperature (d'Yvoire et al.,1983). Fig. 2(a) shows the powder XRD patterns of Na3Cr2(PO4)3-G1:1, Na3Cr2(PO4)3-G1:2 and Na3Cr2(PO4)3-G1:3 pellets sintered at 900ºC. The Na3Cr2(PO4)3, that are synthesised using citric acid in all molar ratios and urea in 1:3 molar ratio, are crystallized in mixed phase. Hence, further studies related to these compositions are not

crystallite size Micro-strain Unit cell parameters

2 (2)

a[A] c[A] V[A]3

lattice and is the Bragg angle. For Gaussian X-ray profiles, B can be calculated as:

B2=Bm2-Bs

LiTi2(PO4)3 and Li1.3Ti1.7Al0.3(PO4)2.9(VO4)0.1 are listed in Table 1.

intensity as compared to the microcrystalline material.

discussed in this chapter.

Average

and nanocrystalline LiTi2(PO4)3 and Li1.3Ti1.7Al0.3(PO4)2.9(VO4)0.1 materials.
