**1. Introduction**

Solid electrolytes are one of the functional materials, practically applied in industries because of its high ion conducting property. It provides scientific support for wide variety of advanced electrochemical devices such as fuel cells, batteries, gas separation membranes, chemical sensors and in the last few years, ionic switches. NASICON type ion conductors have been tested widely in energy applications for instance in electric vehicles. High ion conductivity and stability of phosphate units are advantages of NASICON over other electrolyte materials (Hong, 1976). Among the batteries those based on lithium show the best performance.

In NASICON frame-work, AxBy(PO4)3, A is an alkali metal ion and B is a multivalent metal ion. The charge compensating A cations occupy two types of sites, M1 and M2 (1:3 multiplicity), in the interconnected channels formed by corner sharing PO4 tetrahedra and BO6 octahedra. M1 sites are surrounded by six oxygens and located at an inversion center and M2 sites are symmetrically distributed around three-fold axis of the structure with tenfold oxygen coordination. In three-dimensional frame-work of NASICON, numerous ionic substitutions are allowed at various lattice sites. Generally, NASICON structures crystallize in thermally stable rhombohedral symmetry. But, members of A3M2(PO4)3 family (where A=Li, Na and M=Cr, Fe) crystallize in monoclinic modification of Fe2(SO4)3-type structure and show reversible structural phase transitions at high temperatures (d'Yvoire et al.,1983).

NASICON based phosphates are widely studied in past decades. But LiTi2(PO4)3 is an interesting system because of its high ion conductivity at room temperature. The Na3Cr2(PO4)3 and Li3Fe2(PO4)3 are intriguing due to its structural peculiarity. These materials crystallize in structurally unstable phase by conventional synthesis technique. Since, Na3Cr2(PO4)3 and Li3Fe2(PO4) systems are not stable at the room temperature phase, a chemical synthesis technique of solution combustion is explored. In the present work we have achieved a stable phase through solution combustion technique and electrical properties are investigated and results are reported. The LiTi2(PO4)3 and Li3Fe2(PO4)3 systems used as electrolytes in solid state batteries and Na3Cr2(PO4)3 system used in is sodium sensors. High energy ball milling technique can control the crystallite size through milling duration. In LiTi2(PO4)3 system, milling is performed for various duration to study the effect of crystallite size on electrical conductivity.

NASICON Materials: Structure and Electrical Properties 79

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

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:

 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

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

3NaNO3+ 2Cr(NO3)3.9H2O+3NH4H2PO4+8NH2-CH2COOH+5O2 Δ

molar ratios for the synthesis of Na3Cr2(PO4)3.

Li3Fe2(PO4)3 is also prepared using glycine in 1:2 molar ratio.

sintering is applied to maintain the nanocrystalline nature of the samples.

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:


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 technique for the electrical characterization of mobile ions.
