NASICON Materials: Structure and Electrical Properties

) 3 and Li 3 Fe 2 (PO 4 ) 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 LiTi 2 (PO 4 ) 3 and Li 3 Fe 2 (PO 4 ) 3 systems used as electrolytes in solid state batteries and Na 3 Cr 2 (PO 4 ) 3 system used in is sodium sensors. High energy ball milling technique can control the crystallite size through milling duration. In LiTi 2 (PO 4 ) 3 system, milling is performed for various duration to study the effect of crystallite size on electrical conductivity. all molar monoclinic and orthorhombic symmetry respectively. The violet colour index shows reflections due to LiFeP 2 O 7 phase. In contradiction with the conventional synthesis process, solution combustion technique crystallized the material as a mixture of room temperature and high temperature phases. In the high temperature orthorhombic phase, alkali ions distribute disorderly in the available sites; hence the structural phase transitions are absent in the investigated Li 3 Fe 2 (PO 4 ) 3 material.


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, A x B y (PO 4 ) 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 PO 4 tetrahedra and BO 6 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 A 3 M 2 (PO 4 ) 3 family (where A=Li, Na and M=Cr, Fe) crystallize in monoclinic modification of Fe 2 (SO 4 ) 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 LiTi 2 (PO 4 ) 3 is an interesting system because of its high ion conductivity at room temperature. The Na 3 Cr 2 (PO 4 ) 3 and Li 3 Fe 2 (PO 4 ) 3 are intriguing due to its structural peculiarity. These materials crystallize in structurally unstable phase by conventional synthesis technique. Since, Na 3 Cr 2 (PO 4 ) 3 and Li 3 Fe 2 (PO 4 ) 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 LiTi 2 (PO 4 ) 3 and Li 3 Fe 2 (PO 4 ) 3 systems used as electrolytes in solid state batteries and Na 3 Cr 2 (PO 4 ) 3 system used in is sodium sensors. High energy ball milling technique can control the crystallite size through milling duration. In LiTi 2 (PO 4 ) 3 system, milling is performed for various duration to study the effect of crystallite size on electrical conductivity.
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. LiTi 2 (PO 4 ) 3 and Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 2.9 (VO 4 ) 0.1 synthesized by high energy ball-milling. ii. A 3 M 2 (PO 4 ) 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 technique for the electrical characterization of mobile ions.

Experimental details
Microcrystalline material is prepared by the conventional solid-state reaction of the stoichiometric mixture of Li 2 CO 3 (Himedia, 99.0%), NH 4 H 2 PO 4 (Himedia, 99.0%), TiO 2 (LR grade, 98.0%), Al 2 O 3 (Himedia, 99.0%) and V 2 O 5 (Himedia, 99.0%). Overall reaction for the formation of LiTi 2 (PO 4 ) 3 and Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 2.9 (VO 4 ) 0.1 [LATPV 0.1  i. Stoichiometric amounts of starting reagents were ground in an agate mortar for 45minutes. 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 in order to ensure the total decomposition of the initial reagents. 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 the systems till this stage. iv. Further, LiTi 2 (PO 4 ) 3 pellets were calcined at 1073K for 36h followed by sintering at 1223K for 2h. In the meanwhile, the pellets of Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 2.9 (VO 4 ) 0.1 is calcined at 1073K for 48h followed by sintering at 1323K for 4h.

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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 Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 2.9 (VO 4 ) 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 LiTi 2 (PO 4 ) 3 , and 923K to 1323K for Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 2.9 (VO 4 ) 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 Na 3 Cr 2 (PO 4 ) 3 using glycine in 1:1fuel ratio (Na 3 Cr 2 (PO 4 ) 3 -G1:1) is prepared from NaNO 3 and Cr(NO 3 ) 3 .9H 2 O. Stoichiometric amount of the metal nitrates and glycine (NH 2 -CH 2 COOH) were mixed with distilled water in 1:1 molar ratio. In the case of glycine-nitrate combustion, primarily N 2 , CO 2 , and H 2 O were evolved as gaseous products. As-prepared material is in amorphous phase and further heating at 800˚C produced the pure Na 3 Cr 2 (PO 4 ) 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 Na 3 Cr 2 (PO 4 ) 3 .
The Fe 3+ based NASICON materials were synthesized using citric acid: ethylene glycol mixture (CA:EG). The metal cations were complexed by citric acid (C 6 H 8 O 7 ) and pH of the resultant solution is adjusted in the range 7-8 using ammonia solution. This solution is kept under constant stirring and NH 4 H 2 PO 4 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 Li 3 Fe 2 (PO 4 ) 3 is also prepared using glycine in 1:2 molar ratio.

Results and discussion
3.1 X-ray powder diffraction analysis X-ray patterns are not recorded in very low quality; it is collected using Philips X'pert prodiffractometer with Bragg-Brentano geometry in  configuration. The monochromatic Cu-K  radiation of wavelength, λ = 1.5406Å is used. The pattern is recorded in the 2 range 5º-75º with step size of 0.02º and the step scan of 0.50 seconds. Figs. 1(a)-(b) show XRD patterns of the microcrystalline and 40h ball-milled LiTi 2 (PO 4 ) 3 sintered at 1073K. Peaks in the diffraction pattern correspond to the rhombohedral phase but, minor phase of TiP 2 O 7 are observed due to Li loss in high temperature sintered material [Aono et al.,19984 & Wong et al., 1998]. Fig.1 (Savosta et al., 2004), because high energy ball-milling introduces considerable strain in the material. The strain resulted in broadening the XRD peak and shifting the peak positions towards the higher 2 values. 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 lattice and is the Bragg angle. For Gaussian X-ray profiles, B can be calculated as: where, B m is the FWHM of the material and B s 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 LiTi 2 (PO 4 ) 3 and Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 2.9 (VO 4 ) 0.1 are listed in Table 1.
Ball-milling induces strain in the crystal lattice and decreases the average crystallite size to 70nm for 40h ball-milled LiTi 2 (PO 4 ) 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 Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 2.9 (VO 4 ) 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 intensity as compared to the microcrystalline material. The Na 3 Cr 2 (PO 4 ) 3 is synthesised using glycine, urea and citric acid in 1:1,1:2 and 1:3 molar ratios by solution combustion technique. The Na 3 Cr 2 (PO 4 ) 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 Na 3 Cr 2 (PO 4 ) 3 -G1:1, Na 3 Cr 2 (PO 4 ) 3 -G1:2 and Na 3 Cr 2 (PO 4 ) 3 -G1:3 pellets sintered at 900ºC. The Na 3 Cr 2 (PO 4 ) 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 discussed in this chapter.
d 'Yvoire et al., reported the monoclinic symmetry (-form) of Na 3 Cr 2 (PO 4 ) 3, at the room temperature, where, Na + ions are ordered at M1 site (d' Yvoire et al.,1983). The reversible phase transitions in conventionally synthesized Na 3 Cr 2 (PO 4 ) 3 are: ↔' at 75˚C; '→at 138˚C and ↔at166˚C. In the high temperature -form of rhombohedral symmetry, Na + ions are distributed in M1 and M2 sites in the disordered manner. The temperature dependent XRD studies showed that →phase transitions are associated with slight changes in the crystal lattice. Peaks in the DTA curve are not completely separable for '↔ and ↔ transitions, but it forms relatively broad endo or exothermic effect from 120˚C to 178˚C with two maxima. Change of slope in the Arrhenius plot around 75˚C and increase in conductivity about 140˚C are attributed to ↔' and '↔ transitions respectively. The ↔ transition is associated with the decrease in activation energy (d' Yvoire et al., 1983). While, nanocrystalline Na 3 Cr 2 (PO 4 ) 3 synthesized in the present study, is crystallized in thermally stable rhombohedral symmetry (JCPDS reference code: 01-084-1203). The XRD patterns are indexed and all reflections are from the rhombohedral phase. This type of structural modification is common in materials synthesized by the various chemical routes. In order to confirm the structural stability of Na 3 Cr 2 (PO 4 ) 3 , XRD patterns are recorded at 30˚C, 85˚C, 150˚C and 200˚C. High temperature XRD patterns match well with the room temperature pattern and do not show any structural change with the temperature as shown in Fig. 2(b) for Na 3 Cr 2 (PO 4 ) 3 -G1:1. The Rietveld refinement of room temperature XRD pattern of Na 3 Cr 2 (PO 4 ) 3 -G1:1, is performed using GSAS computer package (Toby, 2001& Larson, 1994 to confirm the crystal system. The Fig. 2(c) shows the Rietveld refinement, where symbol shows the experimental data collected in the slow scan mode, calculated and difference patterns are in solid lines with different colours. Refinement is performed based on rhombohedral crystal system in R3c space group. Initially, the parameters like zero shift, FWHM, background, scale factor and pseudo-Voigt coefficient are refined. Then lattice parameters, atomic positions of Cr, P and O are refined in 12c(0,0,z), 18e(x,0,1/4), and 36f(x,y,z) wyckoff positions respectively. Na + ions are assumed to occupy M1 and M2 sites partially; whose wyckoff positions are 6b(0,0,0) and 18e(x,0,1/4) respectively. The results of Rietveld refinement are given in Table 2. From these results, it is confirmed that in solution combustion synthesised Na 3 Cr 2 (PO 4 ) 3 , Na + ions are distributed in M1 and M2 sites at the room temperature itself. Hence, this material does not show structural changes with temperature.  Table 2. Results of Rietveld refinement of Na 3 Cr 2 (PO 4 ) 3 -G1:1. Atomic and isotropic displacement factors obtained from the refinement are provided below.
Another member of the NASICON family, that shows structural phase transition is Li 3 Fe 2 (PO 4 ) 3 . The Li 3 Fe 2 (PO 4 ) 3 synthesised by ceramic route is crystallized in Fe 2 (SO 4 ) 3 -type monoclinic symmetry and exhibited reversible structural phase transitions below 350˚C, that are not completely separated (d' Yvoire et al.,1983). Its XRD patterns do not show any modifications due to structural phase transitions, implying the Li + ion distribution or ordering, rather than ordering of the networks. d 'Yvoire et al., and Bykov et al., (Bykov,1990) showed that the monoclinic Li 3 Fe 2 (PO 4 ) 3 transforms reversibly to the orthorhombic phase upon heating above 270˚C, due to progressive breaking of long-range ordering of Li + ions in the interstitial space. The Fe 2 (SO 4 ) 3 -type phase generally crystallize in two symmetries: (i) orthorhombic (Pcan) of highest symmetry and (ii) primitive monoclinic (P2 1 /n) symmetry (Mineo, 2002).
In the present study, Li 3 Fe 2 (PO 4 ) 3 is synthesized by solution combustion technique using different fuels i.e., glycine in 1:2 molar ratio and citric acid: ethylene glycol mixture in 1:1 molar ratio. Both of these Li 3 Fe 2 (PO 4 ) 3 is crystallized as mixture of monoclinic (P2 1 /n) and orthorhombic (Pcan) symmetry. Due to sintering in air, XRD patterns showed presence of minor phases of LiFeP 2 O 7 that crystallized in monoclinic symmetry. Fig. 3(a) shows XRD patterns of Li 3 Fe 2 (PO 4 ) 3, sintered at 900˚C, synthesized using glycine. In the Figs. 3(a)-(c), black and red colour indexes are reflections from monoclinic and orthorhombic symmetry respectively. The violet colour index shows reflections due to LiFeP 2 O 7 phase. In contradiction with the conventional synthesis process, solution combustion technique crystallized the material as a mixture of room temperature and high temperature phases. In the high temperature orthorhombic phase, alkali ions distribute disorderly in the available sites; hence the structural phase transitions are absent in the investigated Li 3 Fe 2 (PO 4 ) 3 material. The conventionally synthesised Na 3 Fe 2 (PO 4 ) 3 (d' Yvoire et al.,1983) showed two reversible phase transitions: (i) transition from monoclinic (C2/c) symmetry, ↔ below 368K and (ii) monoclinic to rhombohedral, ↔  at 418K, where, -phase (R3c) is the stable symmetry. The monoclinic symmetry contains two formula units, i.e., Z=2 and in this frame-work, Na + ions occupy three different sites.
In the present study, Na 3 Fe 2 (PO 4 ) 3 is synthesized by solution combustion technique using citric acid: ethylene glycol mixture in 1:1 molar ratio. The material is crystallized in monoclinic symmetry of Cc space group without an impurity phase. Fig. 3 shows the room temperature XRD pattern of Na 3 Fe 2 (PO 4 ) 3 sintered at 910˚C. The high temperature XRD and DTA studies confirmed the structural stability of solution combustion synthesised Na 3 Fe 2 (PO 4 ) 3 . XRD patterns of Na 3 Fe 2 (PO 4 ) 3 is recorded at 30˚C, 110˚C, 300˚C and 500˚C. The high temperature XRD patterns match well with the room temperature pattern and do not show any structural change with temperature. Table 3 provides the crystal system and physical parameters of NASICON materials investigated in the present study.

FT-IR analysis
FT-IR is one of the most general spectroscopic techniques used to identify the functional groups in materials. It is an important and popular tool for structural exposition and compound identification. The FT-IR spectra of NASICON materials are dominated by intense, overlapping intramolecular PO 4 3-stretching modes (ν 1 and ν 3 ) that range from 1300 to 700cm −1 (Corbridge and Lowe, 1954) . In most of the cases, experimentally measured vibrations are divided into internal and external modes. The internal vibrations consist predominantly of intramolecular stretching and bending motions of the PO 4 3-anions and are usually described in terms of the fundamental vibrations of the free anion (i.e., ν 1 -ν 4 ). Bands between 650 and 400cm −1 are attributed to the harmonics of deformation of O-P-O angle (ν 2 and ν 4 modes) (Rao, 2001). Bands in the region 580cm −1 are attributed to the asymmetric bending vibrational modes of O-P-O units (Sayer & Mansingh, 1972). The region 931-870cm -1 is assigned to PO 4 3ionic group vibration (Rulmont, 1991). The entire region down to 400cm -1 is dominated by vibrations of PO 4 tetrahedra group. Stretching vibrations of P-O-P bond are identified in the region 700-758cm -1 (Alamo & Roy, 1998;Kravchenko et al. 1992;Rougier, 1997). Further, FT-IR spectra show weak peak of carbonates in the region 1400-1600cm -1 .
The FT-IR absorption bands of ball-milled Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 2.9 (VO 4 ) 0.1 in the range 1600cm -1 -400cm -1 are shown in Fig. 4. The asymmetric stretching vibration of VO 4 tetrahedra is observed at 810-850cm -1 as broad band (Benmokhtar, 2007). In addition, oxygen atom in the VO 4 tetrahedra can form bond with Al atom which can lead to some asymmetry. The stretching modes of VO 4 in the IR spectra confirm the substitution of vanadium for phosphorus in PO 4 tetrahedra. The external modes are composed of Li + /Na + , Fe 3+ , Cr 3+ , Mg 2+ , PO 4 3-translations and pseudorotations. Separation of internal and external modes is justified as because the intramolecular PO 4 3-vibrations have much larger force constants than the external modes. The Li + translatory vibrations (Li + ion "cage modes") often occur at relatively high frequencies and mix with PO 4 3bending modes of identical symmetry (Rulmont, et al. ,1997). In these vibrations, Li + ions undergo translatory motions in a potential energy environment, that is determined by the nearest neighbour oxygen atoms. Bands in the region 1227-185cm -1 of Na 3 Cr 2 (PO 4 ) 3 correspond to the interaction of P-O bond and adjacent Cr-O bond (Alamo & Roy, 1986).

SEM-EDS and TEM analysis
The crystallites in nanocrystalline LiTi 2 (PO 4 ) 3 are agglomerated and its size distribution is not uniform due to dry milling (Puclin,1995). The quantitative chemical analysis is performed through EDS, but it cannot detect elements with atomic number less than four and hence Li metal cannot be detected by this technique. Surface morphology of Li 3 Ti 1.7 Al 0.3 (PO 4 ) 2.9 (VO 4 ) 0.1 material also shows agglomeration of crystallites in the ballmilled samples and the particle size decreases with milling duration as shown in Figs. 5(a)-(c). X-ray mapping is an imaging technique performed using X-ray. This analytical technique provides a high magnification image related to the distribution and relative abundance of elements within a given specimen. This technique is useful for: (i) identifying the location of individual elements and (ii) mapping the spatial distribution of specific elements and phases in the material surface. Figs. 5(d(ii)-(vi)) show X-ray dot mapping of the SEM image of the 55h ball-milled Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 2.9 (VO 4 ) 0.1 material shown in d(i). Elemental analysis shows peaks corresponding to Ti, Al, P and O elements present in the material. The inset table in Fig. 6(e) give weight and atomic percentage of elements present in 55h ball-milled Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 2.9 (VO 4 ) 0.1 material. The percentages of elements that are detected by instrument and calculated from molecular formula fall within the error. Figs. 6(a)-(c) are SEM images of sintered pellets of Na 3 Cr 2 (PO 4 ) 3 , synthesized using glycine in different fuel/complexing agent molar ratios. The molar ratios affect product morphology and sinterability. It is evident from SEM images that, the crystallite's density decreases and agglomeration increases with the molar ratios. The surface morphology reveals that the particles are of submicron size. Fig. 6. The SEM images of sintered pellet of (a) Na 3 Cr 2 (PO 4 ) 3 -G1:1 (b) Na 3 Cr 2 (PO 4 ) 3 -G1:2 and (c) Na 3 Cr 2 (PO 4 ) 3 -G1:3.
The nanocrystalline nature of the samples is confirmed from TEM images. The Figs. 7(a)-(b) show TEM images of sintered Na 3 Cr 2 (PO 4 ) 3 -G1:1 and Na 3 Cr 2 (PO 4 ) 3 -G1:3 materials. The agglomeration of nanometer sized crystallites is seen in TEM images. Fig. 7(c) is the diffraction pattern of the selected area from the microscopic image of Na 3 Cr 2 (PO 4 ) 3 -G1:3 in Fig. 7(b). Due to strong association, the individually well separated microcrystals are not observable in the TEM images.  TEM image of Li 3 Fe 2 (PO 4 ) 3 -G in Fig. 8(a) shows lattice fringes with different orientation. The lattice fringes with d-spacing of 1.98Aº are identified in the images. In comparison with the XRD pattern, these fringes correspond to (-1, 2, 4) plane. Figs. 8(b)-(c) show the diffraction patterns from different planes of Li 3 Fe 2 (PO 4 ) 3 -G. TEM image in Fig. 8(c) corresponds to 800°C sintered Li 3 Fe 2 (PO 4 ) 3 and the material is not well sintered at this temperature. The amorphous regions in the TEM image are due to less sintering.

Thermal analysis
Thermal studies include measurement of time dependence of material's temperature, while it is subjected to temperature-time variation. DSC measurements are also carried out for the phase transition analysis. But, DSC measurements are performed up to 500°C due to instrument limitation. In this range of temperature, investigated systems of Na 3 Cr 2 (PO 4 ) 3 and Li 3 Fe 2 (PO 4 ) 3 are stable at room temperature phase. To confirm the phase stability at higher temperature, DTA measurement is carried out.
Thermal and gravimetric analyses of as-prepared materials are carried out in the temperature range 40˚C to 1000˚C. Thermal study confirms the structural phase transition in the material and change in enthalpy of the products is calculated from the area of crystallization peak. TG-DTA curves of as-prepared Na 3 Cr 2 (PO 4 ) 3 in different fuel molar ratios are shown in Figs. 9(a)-(c). Out of the two exothermic peaks observed in DTA, the broad peak around 200-400˚C corresponds to the decomposition of organic fuel/complexing agent and nitrates. The sharp peak between 740-780˚C represents the crystallization process. The gravimetric plot shows significant weight loss in the temperature range 300 • C to 740 • C, that is due to the decomposition of organic intermediate and the crystallization process. Further weight loss between 740 • C and 800 • C is due to the formation of NASICON phase www.intechopen.com that is articulated in the DTA plot as sharp exothermic peak. The weight loss curve follows the same path for all materials, but the percentage of weight loss is more for higher molar ratios, due to the presence of more amount of carbonaceous residue. Absence of any additional peaks, in the DTA plot of the as-prepared material, ruled out the possibility of thermodynamical changes due to structural transition. Fig. 9. Thermal and gravimetric plots of as-prepared (a) Na 3 Cr 2 (PO 4 ) 3 -G1:1 (b) Na 3 Cr 2 (PO 4 ) 3 -G1:2 and (c) Na 3 Cr 2 (PO 4 ) 3 -G1:3.
The surface area and crystallite size are primarily decided by enthalpy or flame temperature of combustion process. The flame temperature depends on the nature of fuel/complexing agent and its molar ratio. Rapid evolution of large volume of gaseous products during combustion process dissipates heat, whereby limits the increase of temperature. This reduces the possibility of premature local partial sintering among the primary particles and helps in limiting the inter-particle contact. The crystallite size is decided mainly by two factors i.e., adiabatic flame temperature and number of moles of gases released during combustion process. These two factors are more for higher fuel/complexing agent molar ratios. Higher values of flame temperature result in the formation of dense agglomerates that are disintegrated by the release of more amounts of gases (Hahn, 1990). The competition between flame temperature and number of moles of gases released decides the crystallite size. Crystallites of Na 3 Cr 2 (PO 4 ) 3 -G1:1 are the smallest among the three fuel/ complexing agent ratios. Table 3 gives variation of crystallite size with molar ratios. In the present study, flame temperature has a major role than the number of moles of gases released, on controlling the crystallite size. DTA curves of Li 3 Fe 2 (PO 4 ) 3 prepared using different fuels/complexing agents are different due to the difference in the chemical decomposition of organic components. Table 4 provides crystallization temperature of investigated NASICONs obtained from the DTA plot. The crystallization temperature depends on nature of the fuel/complexing agent and its molar ratio.

Ultraviolet and visible absorption spectroscopy analysis
UV-vis spectroscopy is a tool for identifying valency (electronic) state of transition metals. The transition metals like Fe and Cr show variable valencies and can co-ordinate tetrahedrally and octahedrally. Each co-ordination state produces its own set of characteristic absorption bands in the visible and near UV range. These characteristic absorption bands are used to find skeleton co-valency of the material, which is related to the electronic contribution.
The UV-vis spectra of Na 3 Cr 2 (PO 4 ) 3 in three different glycine molar ratios has two absorption peaks as shown in Fig. 11(a). The 3d 3 configuration of Cr 3+ has a 4 F fundamental state with 4 P as the first excited state. The spin allowed transitions appeared at 670, 468 and 300nm are:  1 : 4 A 2g (F)→ 4 T 2g (F),  2 : 4 A 2g (F)→ 4 T 1g (F),  3 : 4 A 2g (F)→ 4 T 1g (P). Out of these three bands, 3 band appears occasionally [Stalhandske, 2000]. In Na 3 Cr 2 (PO 4 ) 3 material, Cr 3+ does not show variable valency state and its contribution to the electronic part is negligible. The Fe 3+ ions reveal absorption bands in the visible and near UV range. Both Fe 2+ and Fe 3+ ions can exist in tetrahedral and octahedral s i t e s , a n d m a j o r i t y o f F e 3+ ions are believed to occupy the tetrahedral network. The double absorption band at 340 and 380nm may be attributed to 4 D 5 for ferric ion in tetrahedral state and absorption at 440nm is due to 4 G 5 for ferric ion mostly in www.intechopen.com tetrahedral form. In addition to that, the absorption band at 560-580nm may be due to the presence of ferric ion in octahedral site (ElBatal et al., 1988;Bates & Mackenzie, 1962;Kurkjian & Sigety, 1968;Steele & Douglas, 1965;Edwards & Paul, 1972). The absorption band at 410-420nm of transition, 3 F 2 → 3 P 4 , is related to the ferric ion in octahedral symmetry. Li 3 Fe 2 (PO 4 ) 3 -CA:EG, Li 3 Fe 2 (PO 4 ) 3 -G and Na 3 Fe 2 (PO 4 ) 3 -CA:EG show absorption bands in the region of 420nm, 550nm and 720nm as in Fig. 11(b). These absorption peaks correspond to Fe 3+ ions in octahedral state, this ruled out the presence of Fe 2+ ions in the material. The present study concluded that, the dominating contribution to the total conductivity is from ions and the electronic part is negligible. The broad band in the region 200-400nm is due to the phosphate group and its location is independent of the nature of the cation. Fig. 11. UV-vis spectrum of (a) Na 3 Cr 2 (PO 4 ) 3 -G1:1, G1:2 and G1:3 and (b) Li 3 Fe 2 (PO 4 ) 3 -G, Li 3 Fe 2 (PO 4 ) 3 -CA:EG and Na 3 Fe 2 (PO 4 ) 3 .

Wagner polarization technique
The NASICON materials investigated in the present study contain transition elements like Cr and Fe. Due to the presence of variable valency states, these elements may contribute to the electronic part in the total conductivity. The electronic contribution is determined quantitatively by transport number measurement through Wagner polarization technique. The transport number is calculated from the instantaneous and steady state values of current obtained by dc polarization technique. The transport number of investigated materials in the present study is found to be approximately equal to one. Hence, the contribution of electronic part to total conductivity is negligibly small and this corroborates the results from VSM data.

K-K transformations
To validate the electrical microstructure of the material, like grain, grain-boundary and other external parameters such as electrode polarization, ac electrical parameters are plotted in the complex impedance formalism. Kramers-Kronig (K-K) relation is used to evaluate the quality of the measured impedance data. The K-K relations are true for complex impedance spectroscopic data that are linear, causal, and stable. Fig. 12(a) shows K-K fit to Na 3 Cr 2 (PO 4 ) 3 -G1:3 at different temperatures and (b) shows K-K fit to Na 3 Cr 2 (PO 4 ) 3 in different glycine molar ratios at 393K. All these fits match well with the experimental data, implying good quality of the measured data. Kramers-Kronig fit to the complex impedance data is achieved through the software K-K test. Solid line shows K-K fit to the experimental data at different temperatures.

Vibrating sample magnetometer analysis
In the present study, magnetization of NASICON materials are recorded over a range of field, using VSM at room temperature. Electronic contribution to the total conducivity is related to the co-existence of different electronic states. Generally, exchange interaction between equal valence ions is antiferromagnetic and interaction between ions with different valence states like Fe 3+ (3d 5 ) and Fe 2+ (3d 6 ) is ferromagnetic (Takano, 1981;Li, 1997). VSM measurement of the investigated NASICON materials as in Fig.13 show antiferromagnetic behaviour. This indicates that, the contribution to electronic conductivity is negligible in these materials. Fig. 13. Magnetization versus applied magnetic field at room temperature for Na 3 Cr 2 (PO 4 ) 3 ,Na 3 Fe 2 (PO 4 ) 3 , Li 3 Fe 2 (PO 4 ) 3 -EG:CA and Li 3 Fe 2 (PO 4 ) 3 -G.

Impedance spectroscopy analysis
The real part, Z'() and the imaginary part, Z"() of the complex impedance, Z*()= Z'()-iZ"() are calculated from the measured G and C values as: where, f, f being the frequency in Hertz. The elements of an equivalent circuit model represent various (macroscopic) processes involved in the transport of mass and charge. Using NLLS techniques, all the parameters in the equivalent circuit are adjusted simultaneously, thus obtaining the optimum fit to the measured dispersion data. A more general NLLS-fit program based on the Marquardt algorithm has been used. The impedance parameters are obtained by fitting the data to an equivalent circuit using NLLS fitting procedure due to Boukamp [Boukamp, 1989;Mariappan & G. Govindaraj, 2004.
Figs. 14 (a) and (b) show the complex impedance plane plot of microcrystalline LiTi 2 (PO 4 ) 3 material at 368K and nanocrystalline LiTi 2 (PO 4 ) 3 material at 348K. For both of these materials equivalent circuit model is the same throughout the temperature range from 309K to 388K. Equivalent circuit model consists of two depressed semi-circles, where the high frequency semi-circle is displaced from the origin. Since, the high frequency semi-circle is impeded by the low frequency one and effectively only one semi-circle can be visible in the complex impedance plane plot. The ratio of grain capacitance to the grain-boundary capacitance should be less than 10 -3 for the appearance of two separate semi-circles in the complex impedance plane plot (Barsoukov & Macdonald, 2005;Mariappan & Govindaraj, 2005). Inset of Figs. 14(a) and (b) show the high frequency part in the complex impedance plane plot, where continuous line is the simulation result. Simulation clearly shows the grain semi-circle, which is not seen explicitly in the complex impedance representation of the equivalent circuit. www.intechopen.com The impedance plane plots are depressed due to the distribution of relaxation times; a nonideal capacitor or the CPE, Q, is used to explain the depressed semi-circle ) Equivalent circuit of the impedance plane plots obtained using the Boukamp equivalent circuit analysis is found to be R c (R g Q g )(R gb Q gb ). Resistance of the electrolyteelectrode contact is R c , which is characterized by the shift of the impedance arc from the origin. Constant phase elements, Q g and Q gb represent the grain and grain-boundary property of the sample. Grain resistance, R g and the grain-boundary resistance, R gb of the sample are obtained by right and left intercepts of the semi-circles with the real axis. R g and R gb are used to calculate the corresponding grain conductivity,  dcg and grain-boundary conductivity, dcgb . The obtained equivalent circuit is the same for both the LiTi 2 (PO 4 ) 3 samples, but with the different magnitudes of circuit parameters.
For both the samples, R c variation is not consistent with temperature, R g and R gb of both the samples decrease with increase in temperature, Q gb values increases with temperature, while, Q g decreases. The grain conductivity at 309K ( dcg309K 1.82x10 -6 Scm -1 ) of the microcrystalline material is consistent with the reported room temperature value of 10 -7 Scm -1 (Palani Balaya, 2006). At 388K, grain conductivity ( dcg388K =8.57x10 -4 Scm -1 ) of nanocrystalline material shows an order of magnitude jump (Lakshmi et al, 2009 compared to the microcrystalline material ( dcg388K =7.74x10 -5 Scm -1 ). This significant increase in the grain conduction resulted from the reduced crystallite size. High energy ball-milling introduces grain-boundaries in the material and its volume fraction is more in nanocrystalline material. The diffusion through grain-boundaries is much faster than the grain diffusion; hence large volume fractions of grain-boundaries play a dominant role in the ion conduction (Schoonman, 2003).

Figs. 15(a) and (b)
show Arrhenius plot of grain and grain-boundary conductivity of the microcrystalline and nanocrystalline LiTi 2 (PO 4 ) 3 material. The Arrhenius equation is given by: where,  dc is the dc conductivity,  0 is the pre-exponential factor, T is the temperature in Kelvin,   is the activation energy for dc conduction and k  is the Boltzmann's constant.  From the slope of the Arrhenius plots, grain and grain-boundary activation energies  g and   gb are calculated and are shown in Table 5. Increase in the grain conductivity of nanocrystalline material is due to the feasible conduction through grain-boundaries as its activation energy for grain-boundary conduction is less compared to the grain conduction (Mouahid, 2001). Even though the ball-milling decreases the crystallite size, its distribution is not uniform due to dry milling. The non-uniform size distribution and agglomeration are the causes of higher activation energy in nanocrystalline LiTi 2 (PO 4 ) 3 material, in spite of its higher conductivity (Lakshmi et. al, 2009. These agglomerated crystallites are seen clearly in SEM images. Table 6 provides the charge carrier concentration, n c , of microcrystalline and nanocrystalline LiTi 2 (PO 4 ) 3 material, which authenticate that the ballmilling does not increase the carrier concentration.   show impedance plots of microcrystalline and ball-milled materials. Equivalent circuit consists of series combination of a semi-circle associated to grain-boundary contribution and spike characterizing the electrode disparity at the low frequency part. The equivalent circuit representation is (R gb Q gb C gb ) up to 85°C and at higher temperatures it becomes (R gb Q gb )(Q e C e ) for the microcrystalline material. In the case of 22h ball-milled material, the equivalent circuit representation is (R gb Q gb )(Q e C e ) in the whole temperature range. Impedance plane plots of 55h ball-milled material show overlapped semicircles; in which the high frequency arc is attributed to the grain contribution.
Inset of Fig. 16(c) shows the high frequency part in the complex impedance plane plot where continuous line is the simulation result. Simulation clearly shows the grain semi-circle, -Z"()x10 5 www.intechopen.com which is not seen explicitly in the complex impedance plane representation. The high frequency studies are requisite to obtain the grain contribution of microcrystalline and 22h ball-milled material. Mechanical milling changes the capacitive contribution in such a way that in 55h ball-milled material, grain contribution is substantial within the frequency window (Lakshmi et al, 2009. Mechanical milling decreases the difference between the grain and grain-boundary capacitance values; which indicates relatively good connectivity between the grains.  Table 6. Carrier concentartion of microcrystalline and nanocrystalline LiTi 2 (PO 4 ) 3 materials over the temperature range 308K to 398K. Table 7 provides the dc conductivity values and activation energies of microcrystalline and ball-milled Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 2.9 (VO 4 ) 0.1 materials. Grain-boundary conductivity of 55h ballmilled material at 65ºC illustrates an order of magnitude increase in comparison to the microcrystalline counterpart. High frequency investigation is needed to explore the grain characteristics of the microcrystalline and 22h ball-milled material. Micro-strain induced by the milling creates defects like grain-boundaries and its volume fraction is much more in ball-milled samples. Ions can diffuse faster through grain-boundaries and it is reflected in the observed jump in the conductivity in the 55h ball-milled material (Lakshmi et al, 2009.The ease of ion diffusion through grain-boundary is reflected in the values of activation energy as given in Table 7. With the milling duration activation energy decreases since the ion diffusion become easier as the volume fraction of the grain-boundaries increases. Activation energy E g [eV] Microcrystalline 3.75x10 -8 (0.73±0.090) -------22h ball-milled 1.28x10 -7 (0.65±0.007) -------55h ball-milled 3.13x10 -7 (0.26±0.040) 5.32x10 -5 (0.300.01) Table 7. Conductivity and activation energy of the grain-boundary and grain conduction in microcrystalline and ball-milled Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 2.9 (VO 4 ) 0.1 materials.

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The spectroscopic plot of real part of the complex permittivity,  * () of Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 2.9 (VO 4 ) 0.1 shows relaxation at the high frequency. This results from the constriction effect at the grain-boundaries (Mouahid et al., 2001) and is explicit in the impedance representation. This relaxation is prominent in the samples milled for longer times since the grain-boundaries are more significant in those samples . The ' of Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 2.9 (VO 4 ) 0.1 material shows a prominent increase at low frequency which is associated with charges accumulating at the blocking electrode. Permittivity loss in the 55h ball-milled material shows an order of magnitude increase in comparison to the microcrystalline material and the augmented permittivity loss may be due to the ease of diffusion through the grain-boundaries that is reflected in the total conductivity hike of the 55h ball-milled material.
The complex impedance plane plots of Na 3 Cr 2 (PO 4 ) 3 -G1:3 at 373K and 323K are given in Fig.  17(a). The equivalent circuit, (R g Q g )(R e Q e ), at 373K consists of a depressed semi-circle and part of a semi-circle. The impedance plane plots are depressed due to the distribution of relaxation times; a non-ideal capacitor or constant phase element, Q, is used to explain the depression (Barsoukov & Macdonald, 2005;Mariappan & Govindaraj, 2005). The high frequency part, (R g Q g ), corresponds to grain contribution and the part of a semi-circle, (R e Q e ) in the low frequency represents the electrode polarization [32]. Exponent n g =(0.93±0.01), R e =(3.92±0.42)x10 5 , Q e =(6.34±0.82)x10 -7 S.s n and n e =(0.69±0.02). The magnitude of chi-square is found to be 9.02x10 -3 . The magnitude of Q g confirms that the high frequency contribution is from grain and not from the grain-boundary. Fig. 17. (a) Complex impedance plane plot for Na 3 Cr 2 (PO 4 ) 3 -G1:3 at 373K and 323K and the solid line represents NLLS fit to equivalent circuit (b) Complex impedance plane plot for G1:1 and G1:2 molar ratios at 323K and the solid line represents NLLS fit (c) Arrhenius plot of dc conductivity values,  dcg and  dcgb, of the three fuel molar ratios.
The R g and R gb values are obtained by intercept of high frequency and low frequency semicircles with the real axis and are used to calculate the dc conductivity values,  dcg and  dcgb using the cell constant. The parameters,  dcg and  dcgb are thermally activated and show Arrhenius dependence on temperature. The dc conductivity values and the activation energy, obtained from the slope of Arrhenius plot, are given in Table 8. Complex impedance plane plots for 1:1 and 1:2 glycine fuel molar ratios at 323K are shown in Fig. 17(b) and the solid line represents NLLS fit to the equivalent circuit. The highest dc conductivity value, (2.35±0.25)x10 -6 Scm -1 at 323K, is obtained for Na 3 Cr 2 (PO 4 ) 3 -G1:1 among the different glycine molar ratios. This magnitude is one order higher than the reported value, 1.1x10 -7 Scm -1 , for conventionally synthesized Na 3 Cr 2 (PO 4 ) 3 (d' Yvoire et al.,1983.) The increase in the conductivity of Na 3 Cr 2 (PO 4 ) 3 -G1:1 is explained through its dense sintering  (93.25% of theoretical density) and the smallest crystallite size, (31.29±3.91)nm, among the series . The present study evidenced that the grain and grain-boundary conductivity values decreases with fuel/complexing agent ratio in glycine assisted synthesis. Arrhenius plot of dc conductivity values,  dcg and  dcgb , for three glycine molar ratios are shown in Fig. 17(c). Agglomeration increases with fuel molar ratio, due to hike in the flame temperature. Agglomeration decreases the density owing to less packing of larger crystallites, which affects the electrical properties adversely. This study concluded that, the fuel molar ratio play a major role in deciding the physical and electrical properties and 1:1 fuel molar ratio is found to be the optimized value to obtain the highest electrical conductivity.  Table 8. The dc conductivity values and activation energy of Na 3 Cr 2 (PO 4 ) 3 synthesized using different fuels/complexing agents. Fig. 18(a) shows the complex impedance plot of Na 3 Cr 2 (PO 4 ) 3 -U1:1 and Na 3 Cr 2 (PO 4 ) 3 -U1:2 at 383K. In urea assisted Na 3 Cr 2 (PO 4 ) 3 series, 1:2 molar ratio showed improved conductivity due to less activation energy compared to 1:1 molar ratio, as shown in Fig. 18(b). Among the different fuels used, Na 3 Cr 2 (PO 4 ) 3 -U1:2 showed the highest conductivity due to lower grainboundary activation energy of (0.73±0.02)eV. The volume fraction of grain-boundary is more in nanocrystalline materials and it enhances the diffusion of ions. Table 8 gives dc conductivity and activation energy values of Na 3 Cr 2 (PO 4 ) 3 synthesized using different fuels. www.intechopen.com The rhombohedral symmetry of combustion synthesised Na 3 Cr 2 (PO 4 ) 3 is a disordered phase. It shows higher conductivity compared to the conventionally synthesised material, may be due to the enhanced mobility owing to increase in unit cell volume compared to the microcrystalline material. Fig. 18. (a) Complex impedance plane plot at 383K for Na 3 Cr 2 (PO 4 ) 3 -U1:1 and Na 3 Cr 2 (PO 4 ) 3 -U1:2. The solid line represents NLLS fit to equivalent circuit R c (R g Q g )(R gb Q gb ) (b) Arrhenius plots of grain dc conductivity values of Na 3 Cr 2 (PO 4 ) 3 -U1:1 and Na 3 Cr 2 (PO 4 ) 3 -U1:2.
The characteristic frequency of electrical relaxation in grain is obtained from the maximum of imaginary part of electric modulus or impedance spectrum. Characteristic relaxation frequencies ( R ) obtained from ″() curve shift towards high frequency with increase in temperature. Figs. 19(a) and (b) show the spectroscopic plot of imaginary part of impedance. The Na 3 Cr 2 (PO 4 ) 3 -G1:1 contains both grain and grain-boundary contributions at high temperatures, while Na 3 Cr 2 (PO 4 ) 3 -G1:3 contains only grain contribution. The characteristic relaxation frequencies for grain are obtained by NLLS fitting of ″() plot. Relaxation frequency exponentially increases with temperature and its activation energy, E h , is obtained from the Arrhenius plot, as shown in Fig. 19(c). The activation energy for electrical relaxation is given in Table 8 for Na 3 Cr 2 (PO 4 ) 3 material synthesized using different fuels. Such ion transport peculiarities are dominant in compounds with lithium or sodium as well as in oxygen solid electrolytes. The hopping polarization loss is responsible for the peak in the dispersive plot of Z″() (Losila et al., 1998;Elliot, 1994)]. This illustrates that, while relaxing ions have to overcome less energy barrier compared to the conduction process. Fig. 19. Dispersion of Z"() at different temperatures of (a) Na 3 Cr 2 (PO 4 ) 3 -G1:1 (b) Na 3 Cr 2 (PO 4 ) 3 -G1:3 and (c) Arrhenius plot of dispersion peak frequency ( p ) of Na 3 Cr 2 (PO 4 ) 3 -G1:1, Na 3 Cr 2 (PO 4 ) 3 -G1:2 and Na 3 Cr 2 (PO 4 ) 3 -G1:3. www.intechopen.com The Li 3 Fe 2 (PO 4 ) 3 is synthesized using different fuels i.e., glycine (Li 3 Fe 2 (PO 4 ) 3 -G) in 1:2 molar ratio and citric acid: ethylene glycol mixture in 1:1 molar ratio (Li 3 Fe 2 (PO 4 ) 3 -CA:EG). The complex impedance spectra of Li 3 Fe 2 (PO 4 ) 3 -CA:EG at 373K is shown in Fig. 20(a). The equivalent circuit consists of a depressed semi-circle, R c (R g Q g )(Q e ), corresponding to the grain contribution and the spike, (Q e ), represents the electrode polarization at the low frequency region. R c is the resistance of electrolyte-electrode contact, that is characterized by the shift of impedance arc from the origin. Grain-boundary contribution is observed at higher temperatures in addition to the grain contribution in this material. Fig. 20(b), shows Arrhenius temperature dependence of dc conductivity and hopping frequency of Li 3 Fe 2 (PO 4 ) 3 materials. Li 3 Fe 2 (PO 4 ) 3 -G shows higher conductivity values and its dc conductivity value at 323K is (1.14±0.05)x10 -7 Scm -1 . This value is around one order higher than the reported value, 5.6x10 -8 Scm -1 , for the microcrystalline Li 3 Fe 2 (PO 4 ) 3 . The dc conductivity in ion conducting materials mainly depends on charge carrier density and mobility; but the carrier density is almost same for both Li 3 Fe 2 (PO 4 ) 3 materials (CA:EG and G) as shown in Fig. 21(a). The hopping rate and unit cell volume of Li 3 Fe 2 (PO 4 ) 3 -G is higher than Li 3 Fe 2 (PO 4 ) 3 -CA:EG. The improved conduction in Li 3 Fe 2 (PO 4 ) 3 -G is resulted from the enhanced mobility and the frame-work volume (Miyajima et al., 1996). The spectroscopic plot of ″() for Li 3 Fe 2 (PO 4 ) 3 -G is shown in Fig. 21(b). The relaxation frequencies show Arrhenius dependence on temperature and its activation energy, E h , for investigated samples are given in Table 9. The activation energy for electrical relaxation is almost same for both Li 3 Fe 2 (PO 4 ) 3 prepared using different fuels/complexing agents. While relaxing, ions have to overcome less energy barrier compared to the conduction process.  Complex impedance plot of Na 3 Fe 2 (PO 4 ) 3 contains depressed semi-circle and the low frequency electrode effect is as shown in Fig. 22(a). The circuit description is R c (R g Q g )(Q e C e ), where R c is the contact resistance, that is characterized by the shift of impedance arc from the origin. The (R g Q g ) and (Q e C e ) correspond to grain and electrode contribution. Typical value of Q e and Q g are of the order of 10 -7 and 10 -12 S.s n respectively. Arrhenius plot of grain dc conductivity is shown in Fig. 22(b) and the activation energies for conduction and relaxation are given in Table 9. The dc conductivity value of Na 3 Fe 2 (PO 4 ) 3 is higher than Li 3 Fe 2 (PO 4 ) 3 due to the rattling of Li + ions within the large interstitial space available (Shannon et al. ,1977). Na 3 Fe 2 (PO 4 ) 3 synthesized by the present technique show one order increase in conductivity compared to the conventional microcrystalline material. Solution combustion synthesized Li 3 Fe 2 (PO 4 ) 3 and Na 3 Fe 2 (PO 4 ) 3 are crystallized in monoclinic symmetry i.e., -Fe 2 (SO 4 ) 3 type structure. In this structure, mobile ion occupies a four coordination site in contradiction with the NASICON structure, where it occupies six coordination sites. The four co-ordination site of monoclinic structure is preferable to the six co-ordination site of NASICON for ion conduction (Nomura et al., 1993).  (1.52±0.03)x10 -6 (0.63±0.04) ---- Table 9. The grain dc conductivity values at 323K and grain activation energies for dc conduction and electrical relaxation of Li 3 Fe 2 (PO 4 ) 3 and Na 3 Fe 2 (PO 4 ) 3 .

Modulus representation and scaling analysis
Macedo et al., (Macedo et al., 1972& Moynihan et al., 1974  www.intechopen.com extracted. The maximum value of M p ″ is found at  p =1/ p and  p shifts to higher frequencies with increase in temperature. The charge carriers are mobile over long distances in the region left to peak; while right to the peak ions are spatially confined to the potential wells. The frequency of relaxation,  p , where M p ″ () is an indicative of transition from short-range to long-range mobility at the decreasing frequency. The  p exponentially increases with temperature and the activation energy for relaxation is calculated from the Arrhenius behaviour. The scaling of modulus spectra is shown in Fig. 23(a), for Na 3 Cr 2 (PO 4 ) 3 -G1:3 and inset shows the Arrhenius plot of  p . Grain contribution is dominant in Na 3 Cr 2 (PO 4 ) 3 -G1:3, over the frequency and temperature range of the experiment. Further, in Fig. 23(b), Z) and Mpeaks are almost coincident and there is no additional peak in these representations. The single relaxation peak in the modulus representation of Na 3 Cr 2 (PO 4 ) 3 -G1:3 is contributed from the grain part since the electrode contribution is suppressed. The Z) and Mpeaks are almost coincident, which implies that the grain contributes for impedance relaxation. The small separation in the modulus and impedance peak positions points to the good grain connectivity.

Conclusion
In the present study, NASICON materials of two different symmetry, i.e. rhombohedral (NASICON type) and modifications of monoclinic (Fe 2 (SO 4 ) 3 -type), are investigated. Different characterization techniques are used for the confirmation of structural, magnetic and electrical properties. The main initiative of the present study is to correlate the ion mobility with the symmetry.
Out of these, LiTi 2 (PO 4 ) 3 family based on rhombohedral symmetry is synthesized by high energy ball-milling. Due to strain effect, defects like grain-boundaries are introduced in these materials. These grain-boundaries are less activation energy path for mobile ions and thus enhancing the electrical conductivity. The A 3 M 2 (PO 4 ) 3 (where A=Li, Na and M=Fe, Cr) family is prepared by solution combustion technique. By solution combustion synthesis technique, thermal stability is achieved for room temperature phase of Na 3 Cr 2 (PO 4 ) 3 and Li 3 Fe 2 (PO 4 ) 3 materials. The fuels/complexing agents played a major role in controlling the physical and electrical properties in these materials. This study concluded that, the fuel molar ratio play a major role in deciding the physical and electrical properties and 1:1 glycine molar ratio is found to be the optimized value to obtain the highest electrical conductivity in Na 3 Cr 2 (PO 4 ) 3 materials. While, the charge carrier density in Na 3 Cr 2 (PO 4 ) 3 and Li 3 Fe 2 (PO 4 ) 3 was independent of the fuels/complexing agents.
Structural distortions, involving a symmetry lowering to orthorhombic, monoclinic or triclinic, are possible and that may affect the disorder state and mobility of lithium/sodium substantially. Mobile cation occupies a six coordination site in the NASICON-type structure and a four coordination site in the Fe 2 (SO 4 ) 3 -type compounds. The activation energies for ionic conduction of Fe 2 (SO 4 ) 3 -type structure is a little lower than that of the NASICON. This indicates that the four coordination site of the Fe 2 (SO 4 ) 3 -type structure is preferable to the six coordination of the NASICON-type structure for ionic conduction. This is the reason behind the enhanced conduction in combustion synthesized Li 3 Fe 2 (PO 4 ) 3 and Na 3 Fe 2 (PO 4 ) 3 materials.
The scaling of ac conductivity and modulus spectra provided time-temperature superposition principle of ion dynamics in these materials. The ability to scale different data sets to one common curve indicated that the common physical mechanism in a process can be separated by thermodynamic scales. These materials find application in sensors, rechargeable batteries etc.